Related
Here's a bare minimum pseodo-code of what I use:
class A{
//other variables
B b;
void delayedPartnerInit(B b){
this.b=b;
}
}
class B{
//some other variables
A a;
void delayedPartnerInit(A a){
this.a=a;
}
}
I could make it into a single class but certain members(not shown here) of A are needed to exist before data about B. In other words, objects of A and B are instanced at different times but need reference of each other's variables once both set of variables are available.
The question is there a better way to do this? Am I missing some basic concept of programming?
Though I am currently working on C#, I have had this thought many times before when working with other languages too.
Update: I am using this in Unity game engine where B is Unity C# script. Since Unity doesn't allow us create scripts without adding it to something I need 2 classes. I get certain data(A's data) earliar which needs processing.
Didn't mention this earlier because I asked it as a generic question.
Note before closing as duplicate: I checked similar questions but found only specific questions that caused issues to authors who tried to do what I am doing. My question is whether it is a bad practice.
Tightly coupled classes are generally bad practice:
Changes in one class lead to changes in another.
You cannot test one of classes without creating (or mocking) another one. Which in your case creates circular dependency.
Both classes depend on each other's implementations, not abstractions.
Harder for other persons (or yourself half a year later) to understand and reason about first class logic without inspecting second class as well.
Since Unity doesn't allow us create scripts without adding it to something I need 2 classes. I get certain data(A's data) earliar which needs processing
MVC pattern for Unity provides useful trick for decoupling monobehaviours:
interface ISomeObjectView {
event Action OnUpdate;
event Action Destroyed;
event Action TriggerEntered;
void SetTransform(Vector3 position, Quaternion rotation);
void AddForce(Vector3 force);
// Other methods or events you need to expose:
// MouseOver, OnFixedUpdate, Move() or SetScale(), ...
}
MonoBehaviour itself does not contain any logic, it simply invokes events and uses incoming values:
public void SetTransform(Vector3 position, Quaternion rotation)
{
// Params validation
transform.rotation = rotation;
transform.position = position;
}
private void Update()
=> OnUpdate?.Invoke();
MonoBehaviour logic must be moved to your data class or new controller class. Now your data class simply links itself to provided interface events and methods without circular dependencies. Monobehaviour does not require any references to other classes, it simply provides methods to manipulate itself and events to catch input.
This trick helps in several ways:
MonoBehaviour doesn't depend on anything and doesn't require any references to other classes.
Your data/logic classes doesn't require ant special knowledge about monobehaviours, only provided interface.
You can have several implementations for interface, switching different views depending on situation.
Easy to test, easy to mock.
You can move all the "Unity stuff" inside MonoBehaviour and write all related classes on pure C#. If you want to.
Please note that using event Action is not conventional way to deal with events! I think it's very convenient, but I'd suggest to use conventional EventHandler (UnityEvents is another option that might suit your needs).
Update: an example of simple MVC.
Consider the following Player controller:
[Serializable]
public class PlayerInfo {
// Values configurable via inspector
[SerializeField] private float speed = 1.5f;
[SerializeField] private float jumpHeight = 5;
[SerializeField] private float damage = 15;
[SerializeField] private Player playerPrefab;
public float Speed => speed;
public float JumpHeight => jumpHeight ;
public float Damage => damage;
private Player playerInstance;
public void InitializePlayer() {
playerInstance = Instantiate(playerPrefab);
playerInstance.Info = this;
}
public void TeleportTo(Vector3 newPosition) {
playerInstance.transform.position = newPosition;
}
}
public class Player : MonoBehaviour {
public PlayerInfo Info { get; set; }
private Rigidbody rb;
private void Awake() {
rb = GetComponent<Rigidbody>();
}
private void Update() {
if (Input.GetButtonDown("Jump")
rb.AddForce(Vector3.up * info.JumpHeight);
Vector3 movement = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
rb.AddForce(movement * info.Speed);
}
private void OnTriggerEnter(Collider other) {
var enemy = other.GetComponent<Enemy>();
if (enemy != null)
enemy.TakeHit(info.Damage);
}
}
There you go. PlayerInfo is created before Player. PlayerInfo references Player and Player references PlayerInfo. Player is used to move gameobject and attack enemies, PlayerInfo contains required info. What can we do here?
First, Rewrite MonoBehaviour without any logic:
public class PlayerView : MonoBehaviour {
private Rigidbody rb;
// Events for future subscription.
public event Action OnUpdate;
public event TriggerEntered<Collider>;
// Simple initialization of required components.
private void Awake() {
rb = GetComponent<Rigidbody>();
}
// Unity methods doing nothing but invoking events.
private void Update() {
OnUpdate?.Invoke();
}
private void OnTriggerEnter(Collider other) {
TriggerEntered?.Invoke(other);
}
// We still need a method to move our player, right?
public void Move(Vector3 direction) {
rb.AddForce(direction);
}
public void SetPosition(Vector3 position) {
transform.position = position;
}
}
Now you need class containing data about player:
[Serializable]
public class PlayerModel {
[SerializeField] private float speed = 1.5f;
[SerializeField] private float jumpHeight = 5;
[SerializeField] private float damage = 15;
public float Speed => speed;
public float JumpHeight => jumpHeight ;
public float Damage => damage;
}
Now we need a way to tie those two together:
public class PlayerController {
private readonly PlayerModel model;
private readonly PlayerView view;
public PlayerController(PlayerModel model, PlayerView view) {
// Validate values here.
this.model = model;
this.view = view;
// Linking logic to events.
view.OnUpdate += Move;
view.TriggerEntered += Attack;
}
// Actual logic moved here.
private void Move() {
Vector3 movement = Vector3.zero;
if (Input.GetButtonDown("Jump")
movement += Vector3.up * model.JumpHeight;
movement += new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical")) * model.Speed;
view.Move(movement);
}
private void Attack(Collider other) {
var enemy = other.GetComponent<Enemy>();
if (enemy != null)
enemy.TakeHit(model.Damage);
}
// Method from PlayerInfo to set player position without actual movements.
public void MoveTo(Vector3 position) {
view.SetPosition(position);
}
}
Now you have 3 classes instead of 2, but model and view classes are very simple and do not require any dependencies. All the work is done by Controller class, which receives other two pieces and ties them together.
It becomes even better when interfaces are introduced in addition to classes: IPlayerModel, IPlayerView, IPlayerController.
You might think that it would be easier to create single class instead of three linked classes, but in the long run you will thank yourself for using this pattern: view and model are extremely simple, easy to read, easy to check for errors, easy to expand with new functionality. Single class, on the other hand, will quickly grow to hundreds of lines and become a nightmare to test and expand.
I strongly recommend this article with more complicated examples of MVC.
In a Sitecore project I've integrated Simple Injector using this article
It uses sitecore pipelines and then uses a method in App_start
namespace BBC.App_Start
{
public class SimpleInjector : IPackage
{
public void RegisterServices(Container container)
{
GetContainer.RegisterServices(container);
container.Register(() => new SitecoreContext(), Lifestyle.Scoped);
container.Register(() => new Container(), Lifestyle.Singleton);
}
}
}
Simply I can inject container into controller constructor but can't have container in View files.
I tried to declare a static property in App-start and save container to it. but still I'm getting no registration type in Views
What is the best way to have container object in views?
As Stephen suggests in his comment, the literal answer to your question is "you shouldn't do that - because it's not really the way MVC and DI are supposed to work". The more detailed answer goes something like this:
The job of your view is to present data that it has been passed via the Model. Views should not really contain logic. Very simple stuff like "if flag is false, hide this block of mark-up" is ok, but the more complex code to work out what the value of the flag is shouldn't be in the view.
MVC tries to make our website code better by encouraging you to separate presentation (the View) from data (the Model) and logic (the Controller). This should make our code easier to work with - So if you have processing that needs doing, then it should really be happening when your controller method runs.
If your view requires some special data, best practice suggests it should work it out in the controller method and pass it to the view in the model. The code might look more like this:
public class MyModel
{
public string SpecialData { get; set; }
}
public class MyController : Controller
{
public ActionResult DoSomething()
{
// do whatever processing is needed
var somethingCalculate = resultFromYourOtherObject();
// do other stuff
var model = new MyModel() { SpecialData = somethingCalculated };
return View(model);
}
}
And then the View just needs to accept the MyModel class as its model, and render the SpecialData property - no logic required.
I think also it's considered a bad idea to have calls to fetch objects from your DI container spread about your codebase. For MVC apps, generally your DI container gets wired in to the process of creating a controller for a request when the app starts up. Rather than passing about a DI Container into your controllers, the DI framework extends the Controller-creation process, and the container isn't exposed outside of this. When the MVC runtime needs to create a controller, the controller-creation logic uses the DI framework to fetch objects for all the controller's dependencies.
Without more detail about what you actually want to achieve, it's difficult to say what the "right" approach to creating your object(s) here is, but the two most common patterns are probably:
1) Constructor injection: Your controller has a parameter which accepts the object required. The DI container creates this object for you at the point where it creates the controller, so your controller gets all its dependencies when it is created. Good for: scenarios where you know how to create the object at the beginning of the request.
public interface IMySpecialObject
{
string DoSomething();
}
public class MyController : Controller
{
private IMySpecialObject _specialObject;
public MyController(IMySpecialObject specialObject)
{
_specialObject = specialObject;
}
public ActionResult RenderAView()
{
// do some stuff
var data = _specialObject.DoSomething();
return View(data);
}
}
As long as IMySpecialObject and a concrete implementation for it are registered with your DI container when your app starts up, all is well.
2) Factory classes: Sometimes, however, the object in question might be optional, or it might require data that's not available at controller-creation time to create it. In that case, your DI framework could pass in a Factory object to your controller, and this is used to do the construction of the special object later.
public interface ISpecialFactory
{
ISpecialObject CreateSpecialObject(object data);
}
public class MyController : Controller
{
private IMySpecialFactory _specialFactory;
public MyController(IMySpecialFactory specialFactory)
{
_specialFactory = specialFactory;
}
public ActionResult RenderAView()
{
// do some stuff
if( requireSpecialObject )
{
var data = getSomeData();
var specialObject = _specialFactory.CreateSpecialObject(data);
var data = _specialObject.DoSomething();
return View(data);
}
return View("someOtherView");
}
}
But a good book on using DI may suggest other approaches that fit your specific problem better.
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I want to demonstrate use of Adapter Pattern to my team. I've read many books and articles online. Everyone is citing an example which are useful to understand the concept (Shape, Memory Card, Electronic Adapter etc.), but there is no real case study.
Can you please share any case study of Adapter Pattern?
p.s. I tried searching existing questions on stackoverflow, but did not find the answer so posting it as a new question. If you know there's already an answer for this, then please redirect.
Many examples of Adapter are trivial or unrealistic (Rectangle vs. LegacyRectangle, Ratchet vs. Socket, SquarePeg vs RoundPeg, Duck vs. Turkey). Worse, many don't show multiple Adapters for different Adaptees (someone cited Java's Arrays.asList as an example of the adapter pattern). Adapting an interface of only one class to work with another seems a weak example of the GoF Adapter pattern. This pattern uses inheritance and polymorphism, so one would expect a good example to show multiple implementations of adapters for different adaptees.
The best example I found is in Chapter 26 of Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and Iterative Development (3rd Edition). The following images are from the instructor material provided on an FTP site for the book.
The first one shows how an application can use multiple implementations (adaptees) that are functionally similar (e.g., tax calculators, accounting modules, credit authorization services, etc.) but have different APIs. We want to avoid hard-coding our domain-layer code to handle the different possible ways to calculate tax, post sales, authorize credit card requests, etc. Those are all external modules that might vary, and for which we can't modify the code. The adapter allows us to do the hard-coding in the adapter, whereas our domain-layer code always uses the same interface (the IWhateverAdapter interface).
We don't see in the above figure the actual adaptees. However, the following figure shows how a polymorphic call to postSale(...) in the IAccountingAdapter interface is made, which results in a posting of the sale via SOAP to an SAP system.
How to turn a french person into a normal person...
public interface IPerson
{
string Name { get; set; }
}
public interface IFrenchPerson
{
string Nom { get; set; }
}
public class Person : IPerson
{
public string Name { get; set; }
}
public class FrenchPerson : IFrenchPerson
{
public string Nom { get; set; }
}
// that is a service that we want to use with our French person
// we cannot or don't want to change the service contract
// therefore we need 'l'Adaptateur'
public class PersonService
{
public void PrintName(IPerson person)
{
Debug.Write(person.Name);
}
}
public class FrenchPersonAdapter : IPerson
{
private readonly IFrenchPerson frenchPerson;
public FrenchPersonAdapter(IFrenchPerson frenchPerson)
{
this.frenchPerson = frenchPerson;
}
public string Name
{
get { return frenchPerson.Nom; }
set { frenchPerson.Nom = value; }
}
}
Example
var service = new PersonService();
var person = new Person();
var frenchPerson = new FrenchPerson();
service.PrintName(person);
service.PrintName(new FrenchPersonAdapter(frenchPerson));
Convert an Interface into another Interface.
Any real example of Adapter Pattern
In order to connect power, we have different interfaces all over the world.
Using Adapter we can connect easily like wise.
Here is an example that simulates converting analog data to digit data.
It provides an adapter that converts float digit data to binary data, it's probably not useful in real world, it just helps to explain the concept of adapter pattern.
Code
AnalogSignal.java
package eric.designpattern.adapter;
public interface AnalogSignal {
float[] getAnalog();
void setAnalog(float[] analogData);
void printAnalog();
}
DigitSignal.java
package eric.designpattern.adapter;
public interface DigitSignal {
byte[] getDigit();
void setDigit(byte[] digitData);
void printDigit();
}
FloatAnalogSignal.java
package eric.designpattern.adapter;
import java.util.Arrays;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class FloatAnalogSignal implements AnalogSignal {
private Logger logger = LoggerFactory.getLogger(this.getClass());
private float[] data;
public FloatAnalogSignal(float[] data) {
this.data = data;
}
#Override
public float[] getAnalog() {
return data;
}
#Override
public void setAnalog(float[] analogData) {
this.data = analogData;
}
#Override
public void printAnalog() {
logger.info("{}", Arrays.toString(getAnalog()));
}
}
BinDigitSignal.java
package eric.designpattern.adapter;
import java.util.Arrays;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class BinDigitSignal implements DigitSignal {
private Logger logger = LoggerFactory.getLogger(this.getClass());
private byte[] data;
public BinDigitSignal(byte[] data) {
this.data = data;
}
#Override
public byte[] getDigit() {
return data;
}
#Override
public void setDigit(byte[] digitData) {
this.data = digitData;
}
#Override
public void printDigit() {
logger.info("{}", Arrays.toString(getDigit()));
}
}
AnalogToDigitAdapter.java
package eric.designpattern.adapter;
import java.util.Arrays;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* <p>
* Adapter - convert analog data to digit data.
* </p>
*
* #author eric
* #date Mar 8, 2016 1:07:00 PM
*/
public class AnalogToDigitAdapter implements DigitSignal {
public static final float DEFAULT_THRESHOLD_FLOAT_TO_BIN = 1.0f; // default threshold,
private Logger logger = LoggerFactory.getLogger(this.getClass());
private AnalogSignal analogSignal;
private byte[] digitData;
private float threshold;
private boolean cached;
public AnalogToDigitAdapter(AnalogSignal analogSignal) {
this(analogSignal, DEFAULT_THRESHOLD_FLOAT_TO_BIN);
}
public AnalogToDigitAdapter(AnalogSignal analogSignal, float threshold) {
this.analogSignal = analogSignal;
this.threshold = threshold;
this.cached = false;
}
#Override
public synchronized byte[] getDigit() {
if (!cached) {
float[] analogData = analogSignal.getAnalog();
int len = analogData.length;
digitData = new byte[len];
for (int i = 0; i < len; i++) {
digitData[i] = floatToByte(analogData[i]);
}
}
return digitData;
}
// not supported, should set the inner analog data instead,
#Override
public void setDigit(byte[] digitData) {
throw new UnsupportedOperationException();
}
public synchronized void setAnalogData(float[] analogData) {
invalidCache();
this.analogSignal.setAnalog(analogData);
}
public synchronized void invalidCache() {
cached = false;
digitData = null;
}
#Override
public void printDigit() {
logger.info("{}", Arrays.toString(getDigit()));
}
// float -> byte convert,
private byte floatToByte(float f) {
return (byte) (f >= threshold ? 1 : 0);
}
}
Code - Test case
AdapterTest.java
package eric.designpattern.adapter.test;
import java.util.Arrays;
import junit.framework.TestCase;
import org.junit.Test;
import eric.designpattern.adapter.AnalogSignal;
import eric.designpattern.adapter.AnalogToDigitAdapter;
import eric.designpattern.adapter.BinDigitSignal;
import eric.designpattern.adapter.DigitSignal;
import eric.designpattern.adapter.FloatAnalogSignal;
public class AdapterTest extends TestCase {
private float[] analogData = { 0.2f, 1.4f, 3.12f, 0.9f };
private byte[] binData = { 0, 1, 1, 0 };
private float[] analogData2 = { 1.2f, 1.4f, 0.12f, 0.9f };
#Test
public void testAdapter() {
AnalogSignal analogSignal = new FloatAnalogSignal(analogData);
analogSignal.printAnalog();
DigitSignal digitSignal = new BinDigitSignal(binData);
digitSignal.printDigit();
// adapter
AnalogToDigitAdapter adAdapter = new AnalogToDigitAdapter(analogSignal);
adAdapter.printDigit();
assertTrue(Arrays.equals(digitSignal.getDigit(), adAdapter.getDigit()));
adAdapter.setAnalogData(analogData2);
adAdapter.printDigit();
assertFalse(Arrays.equals(digitSignal.getDigit(), adAdapter.getDigit()));
}
}
Dependence - via maven
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.8.2</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.13</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
<version>1.7.13</version>
</dependency>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.16</version>
</dependency>
How to test
Just run the unit test.
Adapter pattern works as a bridge between two incompatible interfaces.
This pattern involves a single class called adapter which is
responsible for communication between two independent or incompatible
interfaces.
Real-world examples might be a language translator or a mobile charger. More here in this youtube video:
Youtube - Adapter Design pattern: Introduction
You can use the Adapter design pattern when you have to deal with different interfaces with similar behavior (which usually means classes with similar behavior but with different methods). An example of it would be a class to connect to a Samsung TV and another one to connect to a Sony TV. They will share common behavior like open menu, start playback, connect to a network and etc but each library will have a different implementation of it (with different method names and signatures). These different vendor specific implementations are called Adaptee in the UML diagrams.
So, in your code (called Client in the UML diagrams), instead of hard code the method calls of each vendor (or Adaptee), you could then create a generic interface (called Target in UML diagrams) to wrap these similar behaviors and work with only one type of object.
The Adapters will then implement the Target interface delegating its method calls to the Adaptees that are passed to the Adapters via constructor.
For you to realize this in Java code, I wrote a very simple project using exactly the same example mentioned above using adapters to deal with multiple smart TV interfaces. The code is small, well documented and self explanatory so dig on it to see how a real world implementation would look like.
Just download the code and import it to Eclipse (or your favorite IDE) as a Maven project. You can execute the code by running org.example.Main.java. Remember that the important thing here is to understand how classes and interfaces are assembled together to design the pattern. I also created some fake Adaptees in the package com.thirdparty.libs. Hope it helps!
https://github.com/Dannemann/java-design-patterns
Adapter design patterns helps in converting interface of one class into interface of client expects.
Example:
You have a service which returns weather (in celsius) by passing city name as a input value. Now, assume that your client wants to pass zipcode as input and expecting the temperature of the city in return. Here you need an adaptor to achieve this.
public interface IWetherFinder {
public double getTemperature(String cityName);
}
class WeatherFinder implements IWetherFinder{
#Override
public double getTemperature(String cityName){
return 40;
}
}
interface IWeatherFinderClient
{
public double getTemperature(String zipcode);
}
public class WeatherAdapter implements IWeatherFinderClient {
#Override
public double getTemperature(String zipcode) {
//method to get cityname by zipcode
String cityName = getCityName(zipcode);
//invoke actual service
IWetherFinder wetherFinder = new WeatherFinder();
return wetherFinder.getTemperature(cityName);
}
private String getCityName(String zipCode) {
return "Banaglore";
}
}
One Real example is Qt-Dbus.
The qt-dbus has a utility to generate the adaptor and interface code from the xml file provided. Here are the steps to do so.
1. Create the xml file - this xml file should have the interfaces
that can be viewed by the qdbus-view in the system either on
the system or session bus.
2.With the utility - qdbusxml2cpp , you generate the interface adaptor code.
This interface adaptor does the demarshalling of the data that is
received from the client. After demarshalling, it invokes the
user defined - custom methods ( we can say as adaptee).
3. At the client side, we generate the interface from the xml file.
This interface is invoked by the client. The interface does the
marshalling of the data and invokes the adaptor interface. As told
in the point number 2, the adaptor interface does the demarshalling
and calls the adaptee - user defined methods.
You can see the complete example of Qt-Dbus over here -
http://www.tune2wizard.com/linux-qt-signals-and-slots-qt-d-bus/
Use Adapter when you have an interface you cannot change, but which you need to use. See it as you're the new guy in an office and you can't make the gray-hairs follow your rules - you must adapt to theirs. Here is a real example from a real project I worked on sometime where the user interface is a given.
You have an application that read all the lines in a file into a List data structure and displayed them in a grid (let's call the underlying data store interface IDataStore). The user can navigate through these data by clicking the buttons "First page", "Previous page", "Next page", "Last Page". Everything works fine.
Now the application needs to be used with production logs which are too big to read into memory but the user still needs to navigate through it! One solution would be to implement a Cache that stores the first page, next, previous and last pages. What we want is when the user clicks "Next page", we return the page from the cache and update the cache; when they click last page, we return last page from cache. In the background we have a filestream doing all the magic. By so doing we only have four pages in memory as opposed to the entire file.
You can use an adapter to add this new cache feature to your application without the user noticing it. We extend the current IDataStore and call it CacheDataStore. If the file to load is big, we use CacheDataStore. When we make a request for First, Next, Previous and Last pages, the information is routed to our Cache.
And who knows, tomorrow the boss wants to start reading the files from a database table. All you do is still extend IDataStore to SQLDataStore as you did for Cache, setup the connection in the background. When they click Next page, you generate the necessary sql query to fetch the next couple hundred rows from the database.
Essentially, the original interface of the application did not change. We simply adapted modern and cool features to work it while preserving the legacy interface.
You can find a PHP implementation of the Adapter pattern used as a defense against injection attacks here:
http://www.php5dp.com/category/design-patterns/adapter-composition/
One of the interesting aspects of the Adapter pattern is that it comes in two flavors: A class adapter relying on multiple inheritance and an object adapter relying on composition. The above example relies on composition.
#Justice o's example does not talk about adapter pattern clearly. Extending his answer -
We have existing interface IDataStore that our consumer code uses and we cannot change it. Now we are asked to use a cool new class from XYZ library that does what we want to implement, but but but, we cannot change that class to extend our IDataStore, seen the problem already ?
Creating a new class - ADAPTER, that implements interface our consumer code expects, i.e. IDataStore and by using class from the library whose features we need to have - ADAPTEE, as a member in our ADAPTER, we can achieve what we wanted to.
As per “C# 3.0 Design Patterns” book by Judith Bishop, Apple used Adapter pattern to adapt Mac OS to work with Intel products (explained in Chapter # 4, excerpt here2)
C# 3.0 Design Patterns
Structural Patterns: Adapter and Façade
An example from Yii framework would be: Yii uses internally cache utilizing an interface
ICache.
https://www.yiiframework.com/doc/api/1.1/ICache
whose signature is like : -
abstract public boolean set(string $id, mixed $value, integer $expire=0, ICacheDependency $dependency=NULL)
abstract public mixed get(string $id)
Let's say, you would like to use inside a Yii project the symfony cache library
https://packagist.org/packages/symfony/cache with it's cache interface, by defining this service in Yii services components (service locator) configuration
https://github.com/symfony/cache-contracts/blob/master/CacheInterface.php
public function get(string $key, callable $callback, float $beta = null, array &$metadata = null);
We see, symfony cache has an interface with only a get method, missing a set method and a different signature for a get method, as Symfony uses the get method also as a setter when supplying the second callable parameter.
As Yii core internally uses this Yii cache/interface, it's difficult (extending Yii/YiiBase) if not impossible at places , to rewrite the calls to that interface.
Plus Symfony cache is nor our class, so we can't rewrite it's interface to fit with the Yii cache interface.
So here comes the adapter pattern to rescue. We will write a mapping = an intermediate adapter which will map the Yii cache interface calls to Symfony cache interface
Would look like this
class YiiToSymfonyCacheAdapter implements \Yii\system\caching\ICache
{
private \Symfony\Contracts\Cache\CacheInterface $symfonyCache;
public function __construct(\Symfony\Contracts\Cache\CacheInterface $symfonyCache)
{
$this->symfonyCache = $symfonyCache;
}
public boolean set(string $id, mixed $value, integer $expire=0, ICacheDependency
$dependency=NULL)
{
// https://symfony.com/doc/current/cache.html
return $this->symfonyCache->get(
$id,
function($item) {
// some logic ..
return $value;
}
);
// https://github.com/symfony/cache/blob/master/Adapter/MemcachedAdapter.php
// if a class could be called statically, the adapter could call statically also eg. like this
// return \Symfony\Component\Cache\Adapter\MemcacheAdapter::get(
// $id,
// function($item) {
// // some logic ..
// return $value;
// }
);
}
public mixed get(string $id)
{
// https://github.com/symfony/cache/blob/master/Adapter/FilesystemAdapter.php
// if a class could be called statically, the adapter could call statically also eg. like this
// \Symfony\Component\Cache\Adapter\FileSystemAdapter::get($id)
return $this->symfonyCache->get($id)
}
}
A real example can be reporting documents in an application. Simple code as here.
Adapters i think are very useful for programming structure.
class WordAdaptee implements IReport{
public void report(String s) {
System.out.println(s +" Word");
}
}
class ExcellAdaptee implements IReport{
public void report(String s) {
System.out.println(s +" Excel");
}
}
class ReportAdapter implements IReport{
WordAdaptee wordAdaptee=new WordAdaptee();
#Override
public void report(String s) {
wordAdaptee.report(s);
}
}
interface IReport {
public void report(String s);
}
public class Main {
public static void main(String[] args) {
//create the interface that client wants
IReport iReport=new ReportAdapter();
//we want to write a report both from excel and world
iReport.report("Trial report1 with one adaptee"); //we can directly write the report if one adaptee is avaliable
//assume there are N adaptees so it is like in our example
IReport[] iReport2={new ExcellAdaptee(),new WordAdaptee()};
//here we can use Polymorphism here
for (int i = 0; i < iReport2.length; i++) {
iReport2[i].report("Trial report 2");
}
}
}
Results will be:
Trial report1 with one adaptee Word
Trial report 2 Excel
Trial report 2 Word
This is an example of adapter implementation:
interface NokiaInterface {
chargementNokia(x:boolean):void
}
class SamsungAdapter implements NokiaInterface {
//nokia chargement adapted to samsung
chargementNokia(x:boolean){
const old= new SamsungCharger();
let y:number = x ? 20 : 1;
old.charge(y);
}
}
class SamsungCharger {
charge(x:number){
console.log("chrgement x ==>", x);
}
}
function main() {
//charge samsung with nokia charger
const adapter = new SamsungAdapter();
adapter.chargementNokia(true);
}
Referring to the below link:
http://www.javaworld.com/javaworld/jw-11-1998/jw-11-techniques.html?page=2
The composition approach to code reuse provides stronger encapsulation
than inheritance, because a change to a back-end class needn't break
any code that relies only on the front-end class. For example,
changing the return type of Fruit's peel() method from the previous
example doesn't force a change in Apple's interface and therefore
needn't break Example2's code.
Surely if you change the return type of peel() (see code below) this means getPeelCount() wouldn't be able to return an int any more? Wouldn't you have to change the interface, or get a compiler error otherwise?
class Fruit {
// Return int number of pieces of peel that
// resulted from the peeling activity.
public int peel() {
System.out.println("Peeling is appealing.");
return 1;
}
}
class Apple {
private Fruit fruit = new Fruit();
public int peel() {
return fruit.peel();
}
}
class Example2 {
public static void main(String[] args) {
Apple apple = new Apple();
int pieces = apple.peel();
}
}
With a composition, changing the class Fruit doesn't necessary require you to change Apple, for example, let's change peel to return a double instead :
class Fruit {
// Return String number of pieces of peel that
// resulted from the peeling activity.
public double peel() {
System.out.println("Peeling is appealing.");
return 1.0;
}
}
Now, the class Apple will warn about a lost of precision, but your Example2 class will be just fine, because a composition is more "loose" and a change in a composed element does not break the composing class API. In our case example, just change Apple like so :
class Apple {
private Fruit fruit = new Fruit();
public int peel() {
return (int) fruit.peel();
}
}
Whereas if Apple inherited from Fruit (class Apple extends Fruit), you would not only get an error about an incompatible return type method, but you'd also get a compilation error in Example2.
** Edit **
Lets start this over and give a "real world" example of composition vs inheritance. Note that a composition is not limited to this example and there are more use case where you can use the pattern.
Example 1 : inheritance
An application draw shapes into a canvas. The application does not need to know which shapes it has to draw and the implementation lies in the concrete class inheriting the abstract class or interface. However, the application knows what and how many different concrete shapes it can create, thus adding or removing concrete shapes requires some refactoring in the application.
interface Shape {
public void draw(Graphics g);
}
class Box implement Shape {
...
public void draw(Graphics g) { ... }
}
class Ellipse implements Shape {
...
public void draw(Graphics g) { ... }
}
class ShapeCanvas extends JPanel {
private List<Shape> shapes;
...
protected void paintComponent(Graphics g) {
for (Shape s : shapes) { s.draw(g); }
}
}
Example 2 : Composition
An application is using a native library to process some data. The actual library implementation may or may not be known, and may or may not change in the future. A public interface is thus created and the actual implementation is determined at run-time. For example :
interface DataProcessorAdapter {
...
public Result process(Data data);
}
class DataProcessor {
private DataProcessorAdapter adapter;
public DataProcessor() {
try {
adapter = DataProcessorManager.createAdapter();
} catch (Exception e) {
throw new RuntimeException("Could not load processor adapter");
}
}
public Object process(Object data) {
return adapter.process(data);
}
}
static class DataProcessorManager {
static public DataProcessorAdapter createAdapter() throws ClassNotFoundException, InstantiationException, IllegalAccessException {
String adapterClassName = /* load class name from resource bundle */;
Class<?> adapterClass = Class.forName(adapterClassName);
DataProcessorAdapter adapter = (DataProcessorAdapter) adapterClass.newInstance();
//...
return adapter;
}
}
So, as you can see, the composition may offer some advantage over inheritance in the sense that it allows more flexibility in the code. It allows the application to have a solid API while the underlaying implementation may still change during it's life cycle. Composition can significantly reduce the cost of maintenance if properly used.
For example, when implementing test cases with JUnit for Exemple 2, you may want to use a dummy processor and would setup the DataProcessorManager to return such adapter, while using a "real" adapter (perhaps OS dependent) in production without changing the application source code. Using inheritance, you would most likely hack something up, or perhaps write a lot more initialization test code.
As you can see, compisition and inheritance differ in many aspects and are not preferred over another; each depend on the problem at hand. You could even mix inheritance and composition, for example :
static interface IShape {
public void draw(Graphics g);
}
static class Shape implements IShape {
private IShape shape;
public Shape(Class<? extends IShape> shape) throws InstantiationException, IllegalAccessException {
this.shape = (IShape) shape.newInstance();
}
public void draw(Graphics g) {
System.out.print("Drawing shape : ");
shape.draw(g);
}
}
static class Box implements IShape {
#Override
public void draw(Graphics g) {
System.out.println("Box");
}
}
static class Ellipse implements IShape {
#Override
public void draw(Graphics g) {
System.out.println("Ellipse");
}
}
static public void main(String...args) throws InstantiationException, IllegalAccessException {
IShape box = new Shape(Box.class);
IShape ellipse = new Shape(Ellipse.class);
box.draw(null);
ellipse.draw(null);
}
Granted, this last example is not clean (meaning, avoid it), but it shows how composition can be used.
Bottom line is that both examples, DataProcessor and Shape are "solid" classes, and their API should not change. However, the adapter classes may change and if they do, these changes should only affect their composing container, thus limit the maintenance to only these classes and not the entire application, as opposed to Example 1 where any change require more changes throughout the application. It all depends how flexible your application needs to be.
If you would change Fruit.peel()'s return type, you would have to modify Apple.peel() as well. But you don't have to change Apple's interface.
Remember: The interface are only the method names and their signatures, NOT the implementation.
Say you'd change Fruit.peel() to return a boolean instead of a int. Then, you could still let Apple.peel() return an int. So: The interface of Apple stays the same but Fruit's changed.
If you would have use inheritance, that would not be possible: Since Fruit.peel() now returns a boolean, Apple.peel() has to return an boolean, too. So: All code that uses Apple.peel() has to be changed, too. In the composition example, ONLY Apple.peel()'s code has to be changed.
The key word in the sentence is "interface".
You'll almost always need to change the Apple class in some way to accomodate the new return type of Fruit.peel, but you don't need to change its public interface if you use composition rather than inheritance.
If Apple is a Fruit (ie, inheritance) then any change to the public interface of Fruit necessitates a change to the public interface of Apple too. If Apple has a Fruit (ie, composition) then you get to decide how to accomodate any changes to the Fruit class; you're not forced to change your public interface if you don't want to.
Return type of Fruit.peel() is being changed from int to Peel. This doesn't meant that the return type of Apple.peel() is being forced to change to Peel as well. In case of inheritance, it is forced and any client using Apple has to be changed. In case of composition, Apple.peel() still returns an integer, by calling the Peel.getPeelCount() getter and hence the client need not be changed and hence Apple's interface is not changed ( or being forced to be changed)
Well, in the composition case, Apple.peel()'s implementation needs to be updated, but its method signature can stay the same. And that means the client code (which uses Apple) does not have to be modified, retested, and redeployed.
This is in contrast to inheritance, where a change in Fruit.peel()'s method signature would require changes all way into the client code.
I am looking for an alternative to the visitor pattern. Let me just focus on a couple of pertinent aspects of the pattern, while skipping over unimportant details. I'll use a Shape example (sorry!):
You have a hierarchy of objects that implement the IShape interface
You have a number of global operations that are to be performed on all objects in the hierarchy, e.g. Draw, WriteToXml etc...
It is tempting to dive straight in and add a Draw() and WriteToXml() method to the IShape interface. This is not necessarily a good thing - whenever you wish to add a new operation that is to be performed on all shapes, each IShape-derived class must be changed
Implementing a visitor for each operation i.e. a Draw visitor or a WirteToXml visitor encapsulates all the code for that operation in one class. Adding a new operation is then a matter of creating a new visitor class that performs the operation on all types of IShape
When you need to add a new IShape-derived class, you essentially have the same problem as you did in 3 - all visitor classes must be changed to add a method to handle the new IShape-derived type
Most places where you read about the visitor pattern state that point 5 is pretty much the main criteria for the pattern to work and I totally agree. If the number of IShape-derived classes is fixed, then this can be a quite elegant approach.
So, the problem is when a new IShape-derived class is added - each visitor implementation needs to add a new method to handle that class. This is, at best, unpleasant and, at worst, not possible and shows that this pattern is not really designed to cope with such changes.
So, the question is has anybody come across alterative approaches to handling this situation?
You might want to have a look at the Strategy pattern. This still gives you a separation of concerns while still being able to add new functionality without having to change each class in your hierarchy.
class AbstractShape
{
IXmlWriter _xmlWriter = null;
IShapeDrawer _shapeDrawer = null;
public AbstractShape(IXmlWriter xmlWriter,
IShapeDrawer drawer)
{
_xmlWriter = xmlWriter;
_shapeDrawer = drawer;
}
//...
public void WriteToXml(IStream stream)
{
_xmlWriter.Write(this, stream);
}
public void Draw()
{
_drawer.Draw(this);
}
// any operation could easily be injected and executed
// on this object at run-time
public void Execute(IGeneralStrategy generalOperation)
{
generalOperation.Execute(this);
}
}
More information is in this related discussion:
Should an object write itself out to a file, or should another object act on it to perform I/O?
There is the "Visitor Pattern With Default", in which you do the visitor pattern as normal but then define an abstract class that implements your IShapeVisitor class by delegating everything to an abstract method with the signature visitDefault(IShape).
Then, when you define a visitor, extend this abstract class instead of implementing the interface directly. You can override the visit* methods you know about at that time, and provide for a sensible default. However, if there really isn't any way to figure out sensible default behavior ahead of time, you should just implement the interface directly.
When you add a new IShape subclass, then, you fix the abstract class to delegate to its visitDefault method, and every visitor that specified a default behavior gets that behavior for the new IShape.
A variation on this if your IShape classes fall naturally into a hierarchy is to make the abstract class delegate through several different methods; for example, an DefaultAnimalVisitor might do:
public abstract class DefaultAnimalVisitor implements IAnimalVisitor {
// The concrete animal classes we have so far: Lion, Tiger, Bear, Snake
public void visitLion(Lion l) { visitFeline(l); }
public void visitTiger(Tiger t) { visitFeline(t); }
public void visitBear(Bear b) { visitMammal(b); }
public void visitSnake(Snake s) { visitDefault(s); }
// Up the class hierarchy
public void visitFeline(Feline f) { visitMammal(f); }
public void visitMammal(Mammal m) { visitDefault(m); }
public abstract void visitDefault(Animal a);
}
This lets you define visitors that specify their behavior at whatever level of specificity you wish.
Unfortunately, there is no way to avoid doing something to specify how visitors will behave with a new class - either you can set up a default ahead of time, or you can't. (See also the second panel of this cartoon )
I maintain a CAD/CAM software for metal cutting machine. So I have some experience with this issues.
When we first converted our software (it was first released in 1985!) to a object oriented designed I did just what you don't like. Objects and Interfaces had Draw, WriteToFile, etc. Discovering and reading about Design Patterns midway through the conversion helped a lot but there were still a lot of bad code smells.
Eventually I realized that none of these types of operations were really the concern of the object. But rather the various subsystems that needed to do the various operations. I handled this by using what is now called a Passive View Command object, and well defined Interface between the layers of software.
Our software is structured basically like this
The Forms implementing various Form
Interface. These forms are a thing shell passing events to the UI Layer.
UI layer that receives Events and manipulate forms through the Form interface.
The UI Layer will execute commands that all implement the Command interface
The UI Object have interfaces of their own that the command can interact with.
The Commands get the information they need, process it, manipulates the model and then report back to the UI Objects which then does anything needed with the forms.
Finally the models which contains the various objects of our system. Like Shape Programs, Cutting Paths, Cutting Table, and Metal Sheets.
So Drawing is handled in the UI Layer. We have different software for different machines. So while all of our software share the same model and reuse many of the same commands. They handle things like drawing very different. For a example a cutting table is draw different for a router machine versus a machine using a plasma torch despite them both being esstentially a giant X-Y flat table. This because like cars the two machines are built differently enough so that there is a visual difference to the customer.
As for shapes what we do is as follows
We have shape programs that produce cutting paths through the entered parameters. The cutting path knows which shape program produced. However a cutting path isn't a shape. It just the information needed to draw on the screen and to cut the shape. One reason for this design is that cutting paths can be created without a shape program when they are imported from a external app.
This design allows us to separate the design of the cutting path from the design of the shape which are not always the same thing. In your case likely all you need to package is the information needed to draw the shape.
Each shape program has a number of views implementing a IShapeView Interface. Through the IShapeView interface the shape program can tell the generic shape form we have how to setup itself up to show the parameters of that shape. The generic shape form implements a IShapeForm interface and registers itself with the ShapeScreen Object. The ShapeScreen Object registers itself with our application object. The shape views use whatever shapescreen that registers itself with the application.
The reason for the multiple views that we have customers that like to enter shapes in different ways. Our customer base is split in half between those who like to enter shape parameters in a table form and those who like to enter with a graphical representation of the shape in front of them. We also need to access the parameters at times through a minimal dialog rather than our full shape entry screen. Hence the multiple views.
Commands that manipulate shapes fall in one of two catagories. Either they manipulate the cutting path or they manipulate the shape parameters. To manipulate the shape parameters generally we either throw them back into the shape entry screen or show the minimal dialog. Recalculate the shape, and display it in the same location.
For the cutting path we bundled up each operation in a separate command object. For example we have command objects
ResizePath
RotatePath
MovePath
SplitPath
and so on.
When we need to add new functionality we add another command object, find a menu, keyboard short or toolbar button slot in the right UI screen and setup the UI object to ececute that command.
For example
CuttingTableScreen.KeyRoute.Add vbShift+vbKeyF1, New MirrorPath
or
CuttingTableScreen.Toolbar("Edit Path").AddButton Application.Icons("MirrorPath"),"Mirror Path", New MirrorPath
In both instances the Command object MirrorPath is being associated with a desired UI element. In the execute method of MirrorPath is all the code needed to mirror the path in a particular axis. Likely the command will have it's own dialog or use one of the UI elements to ask the user which axis to mirror. None of this is making a visitor, or adding a method to the path.
You will find that a lot can be handled through bundling actions into commands. However I caution that is not a black or white situation. You will still find that certain things work better as methods on the original object. In may experience I found that perhaps 80% of what I used to do in methods were able to be moved into the command. The last 20% just plain work better on the object.
Now some may not like this because it seems to violate encapsulations. From maintaining our software as a object oriented system for the last decade I have to say the MOST important long term thing you can do is clearly document the interactions between the different layers of your software and between the different objects.
Bundling actions into Command objects helps with this goal way better than a slavish devotion to the ideals of encapsulation. Everything that is needs to be done to Mirror a Path is bundled in the Mirror Path Command Object.
Visitor design pattern is a workaround, not a solution to the problem. Short answer would be pattern matching.
Regardless of what path you take, the implementation of alternate functionality that is currently provided by the Visitor pattern will have to 'know' something about the concrete implementation of the interface that it is working on. So there is no getting around the fact that you are going to have to write addition 'visitor' functionality for each additional implementation. That said what you are looking for is a more flexible and structured approach to creating this functionality.
You need to separate out the visitor functionality from the interface of the shape.
What I would propose is a creationist approach via an abstract factory to create replacement implementations for visitor functionality.
public interface IShape {
// .. common shape interfaces
}
//
// This is an interface of a factory product that performs 'work' on the shape.
//
public interface IShapeWorker {
void process(IShape shape);
}
//
// This is the abstract factory that caters for all implementations of
// shape.
//
public interface IShapeWorkerFactory {
IShapeWorker build(IShape shape);
...
}
//
// In order to assemble a correct worker we need to create
// and implementation of the factory that links the Class of
// shape to an IShapeWorker implementation.
// To do this we implement an abstract class that implements IShapeWorkerFactory
//
public AbsractWorkerFactory implements IShapeWorkerFactory {
protected Hashtable map_ = null;
protected AbstractWorkerFactory() {
map_ = new Hashtable();
CreateWorkerMappings();
}
protected void AddMapping(Class c, IShapeWorker worker) {
map_.put(c, worker);
}
//
// Implement this method to add IShape implementations to IShapeWorker
// implementations.
//
protected abstract void CreateWorkerMappings();
public IShapeWorker build(IShape shape) {
return (IShapeWorker)map_.get(shape.getClass())
}
}
//
// An implementation that draws circles on graphics
//
public GraphicsCircleWorker implements IShapeWorker {
Graphics graphics_ = null;
public GraphicsCircleWorker(Graphics g) {
graphics_ = g;
}
public void process(IShape s) {
Circle circle = (Circle)s;
if( circle != null) {
// do something with it.
graphics_.doSomething();
}
}
}
//
// To replace the previous graphics visitor you create
// a GraphicsWorkderFactory that implements AbstractShapeFactory
// Adding mappings for those implementations of IShape that you are interested in.
//
public class GraphicsWorkerFactory implements AbstractShapeFactory {
Graphics graphics_ = null;
public GraphicsWorkerFactory(Graphics g) {
graphics_ = g;
}
protected void CreateWorkerMappings() {
AddMapping(Circle.class, new GraphicCircleWorker(graphics_));
}
}
//
// Now in your code you could do the following.
//
IShapeWorkerFactory factory = SelectAppropriateFactory();
//
// for each IShape in the heirarchy
//
for(IShape shape : shapeTreeFlattened) {
IShapeWorker worker = factory.build(shape);
if(worker != null)
worker.process(shape);
}
It still means that you have to write concrete implementations to work on new versions of 'shape' but because it is completely separated from the interface of shape, you can retrofit this solution without breaking the original interface and software that interacts with it. It acts as a sort of scaffolding around the implementations of IShape.
If you're using Java: Yes, it's called instanceof. People are overly scared to use it. Compared to the visitor pattern, it's generally faster, more straightforward, and not plagued by point #5.
If you have n IShapes and m operations that behave differently for each shape, then you require n*m individual functions. Putting these all in the same class seems like a terrible idea to me, giving you some sort of God object. So they should be grouped either by IShape, by putting m functions, one for each operation, in the IShape interface, or grouped by operation (by using the visitor pattern), by putting n functions, one for each IShape in each operation/visitor class.
You either have to update multiple classes when you add a new IShape or when you add a new operation, there is no way around it.
If you are looking for each operation to implement a default IShape function, then that would solve your problem, as in Daniel Martin's answer: https://stackoverflow.com/a/986034/1969638, although I would probably use overloading:
interface IVisitor
{
void visit(IShape shape);
void visit(Rectangle shape);
void visit(Circle shape);
}
interface IShape
{
//...
void accept(IVisitor visitor);
}
I have actually solved this problem using the following pattern. I do not know if it has a name or not!
public interface IShape
{
}
public interface ICircleShape : IShape
{
}
public interface ILineShape : IShape
{
}
public interface IShapeDrawer
{
void Draw(IShape shape);
/// <summary>
/// Returns the type of the shape this drawer is able to draw!
/// </summary>
Type SourceType { get; }
}
public sealed class LineShapeDrawer : IShapeDrawer
{
public Type SourceType => typeof(ILineShape);
public void Draw(IShape drawing)
{
if (drawing is ILineShape)
{
// Code to draw the line
}
}
}
public sealed class CircleShapeDrawer : IShapeDrawer
{
public Type SourceType => typeof(ICircleShape);
public void Draw(IShape drawing)
{
if (drawing is ICircleShape)
{
// Code to draw the circle
}
}
}
public sealed class ShapeDrawingClient
{
private readonly IDictionary<Type, IShapeDrawer> m_shapeDrawers =
new Dictionary<Type, IShapeDrawer>();
public void Add(IShapeDrawer shapeDrawer)
{
m_shapeDrawers[shapeDrawer.SourceType] = shapeDrawer;
}
public void Draw(IShape shape)
{
Type[] interfaces = shape.GetType().GetInterfaces();
foreach (Type #interface in interfaces)
{
if (m_shapeDrawers.TryGetValue(#interface, out IShapeDrawer drawer))
{
drawer.Draw(drawing);
return;
}
}
}
}
Usage:
LineShapeDrawer lineShapeDrawer = new LineShapeDrawer();
CircleShapeDrawer circleShapeDrawer = new CircleShapeDrawer();
ShapeDrawingClient client = new ShapeDrawingClient ();
client.Add(lineShapeDrawer);
client.Add(circleShapeDrawer);
foreach (IShape shape in shapes)
{
client.Draw(shape);
}
Now if someone as the user of my library defines IRectangleShape and wants to draw it, they can simply define IRectangleShapeDrawer and add it to ShapeDrawingClient's list of drawers!