I have a simple class hierarchy of two classes. Both classes call an init-method specific to that class. Therefor the init-method is overriden in the subclass:
class A
{
public A() { this->InitHandlers(); }
public virtual void InitHandlers() { // load some event handlers here }
}
class B: public A
{
public B() { this->InitHandlers(); }
public virtual void InitHandlers() {
// keep base class functionality
A::InitHandlers();
// load some other event handlers here
// ...
}
}
I know this is evil design:
The call of an overriden method from constructor is error-prone.
B::InitHandlers() would be called twice with this setup.
But semantically it makes sense to me: I want to extend the behaviour of class A in class B by loading more handlers but still keeping the handlers loaded by class A. Further this is a task that has to be done in construction. So how can this be solved with a more robust design?
You can do something like this:
class A
{
protected boolean init = false;
public A() { this->Init(); }
public virtual void Init() {
if (!this->init) {
this->init = true;
this->InitHandlers();
}
}
public virtual void InitHandlers() {
// load some event handlers here
}
}
class B: public A
{
public B() { this->Init(); }
public virtual void InitHandlers() {
// keep base class functionality
A::InitHandlers();
// load some other event handlers here
// ...
}
}
You can see it as a design pattern template method.
Related
If I pull a class A and class B onto a Code Map, VSE (Visual Studio Enterprise) will map the direct calls of class A calling methods in class B.
So,
public class A
{
public void DoSomething()
{
b.DoSomethingElse();
}
}
This will map. But if it's something like:
public class A
{
public void DoSomething()
{
d.DoManyThings();
}
}
public class D
{
public void DoManyThings()
{
c.DoThings();
}
}
public class C
{
public void DoThings()
{
b.DoSomethingElse();
}
}
public class B
{
public void DoSomethingElse()
{
// imagine code here
}
}
Then the Code Map won't map between class A and class B automatically. The only way I've found to show those dependencies is to go to each method and click "Show Methods This Calls".
Is there a way to get VSE make the Code Map all those dependencies initially without having to investigate every method?
I'm trying to create a class heirachy for a game, there is an Item class which is the base class for all items in the game. The problem is that some derived items (like potion) might not implement some of the abstract methods defined by the item.
Is it ok for derived classes to implement an abstract method with "do nothing"?
Example: https://dotnetfiddle.net/jJABN1
using System;
using System.Collections.Generic;
public abstract class Item
{
public abstract void Use();
}
public class Potion : Item
{
public override void Use()
{
// do nothing
return;
}
}
public class Sword : Item
{
public override void Use()
{
Console.WriteLine("Sword used!");
return;
}
}
public class Program
{
public static void Main()
{
List<Item> items = new List<Item>();
Item potion = new Potion();
Item sword = new Sword();
items.Add(potion);
items.Add(sword);
for (int i = 0; i < items.Count; i++)
{
Item item = items[i];
item.Use();
}
}
}
One of Robert Martin's SOLID Principles - Interface Segregation Principle addresses this situation. It basically says that a client should not be exposed to methods it doesn't need.
An example of violating the Interface Segregation Principle:
// Abstraction
public abstract class Printer
{
public abstract void Print();
public abstract void Scan();
}
// Implementations
public class SomeAllInOnePrinter : Printer
{
public override void Print()
{
Console.WriteLine("Printing...");
}
public override void Scan()
{
Console.WriteLine("Scanning...");
}
}
public class SomeBasicPrinter : Printer
{
public override void Print()
{
Console.WriteLine("Printing...");
}
public override void Scan()
{
// Basic printers can't scan
}
}
This is usually solved by separating an abstract class to multiple smaller abstract classes that can optionally inherit one other:
// Abstractions
public abstract class Printer
{
public abstract void Print();
}
public abstract class AllInOnePrinter : Printer
{
public abstract void Scan();
}
// Implementations
public class SomeAllInOnePrinter : AllInOnePrinter
{
public override void Print()
{
Console.WriteLine("Printing...");
}
public override void Scan()
{
Console.WriteLine("Scanning...");
}
}
public class SomeBasicPrinter : Printer
{
public override void Print()
{
Console.WriteLine("Printing...");
}
}
Technically, there could be an edge-case (should be uncommon!) where a deriving class doesn't need to implement all the methods, in such a case I'd rather it to override and throw an error to signal the user that this method should not be used.
That said, in the provided example there is only one method, so the question is: if a derived class doesn't need this method - why do you need to inherit the abstract class to begin with? if it's just in order to provide an example that's understandable - but better improve the example to include other methods that are used in the derived class.
Say, I have class A with method M:
private void M()
{
Do1();
Do2();
}
class B extends A.
Problem: I need Do2() to not to be executed when calling from an instance of B.
I have a couple of ideas but not sure which do not break OOP and SOLID rules.
Make Do2 virtual.
class A
{
protected virtual void Do2()
{
// Do something
}
}
class B
{
protected override void Do2()
{
// Do nothing
}
}
This solution looks weird to me because I override a method to "do nothing", when overriding is needed to "do something instead of something" or "do something in addition to something".
Create bool protected flag property
class A
{
protected virtual NeedCallDo2
{
get { return true; }
}
private void M()
{
Do1();
if (NeedCallDo2)
{
Do2();
}
}
}
class B
{
protected override NeedCallDo2
{
get { return false; }
}
}
This solution is also not perfect but I have a control of execution flow and can decide whether to call Do2 or not.
Pass constructor flag parameter
class A
{
private bool needCallDo2;
protected A(bool needCallDo2 = true)
{
this.needCallDo2 = needCallDo2;
}
private void M()
{
Do1();
if (this.needCallDo2)
{
Do2();
}
}
}
class B
{
public B()
: base(false)
{
}
}
This is a trick question! Given the solid-principles tag, there is no correct way to disable base class functionality, since that would violate liskov-substitution, which is the L in SOLID.
You could move the execution logic into an execution strategy class where class B uses a different strategy implementation than class A.
Imagine the following class hierarchy:
interface IRules
{
void NotifyPickup(object pickedUp);
void NotifyDeath();
void NotifyDamage();
}
class CaptureTheFlag : IRules
{
public void NotifyPickup(Pickup pickedUp)
{
if(pickedUp is Flag)
GameOver();
}
public void NotifyDeath()
{
}
public void NotifyDamage()
{
}
}
class DeathMatch : IRules
{
public void NotifyPickup(Pickup pickedUp)
{
points++;
}
public void NotifyDeath()
{
lives--;
}
public void NotifyDamage()
{
}
}
class GameWorld
{
IRules gameMode;
public Main(IRules gameMode)
{
this.gameMode = gameMode;
}
object[] worldObjects;
public void GameLoop()
{
foreach(object obj in worldObjects)
{
// This call may have a bunch of sideeffects, like getting a pickup
// Or a player dying
// Or damage being taken
// Different game modes are interested in different events / statistics.
obj.Update();
// Stuff happens...
gameMode.NotifyDamage();
// Stuff happens...
gameMode.NotifyDeath();
}
}
}
So here I've got an interface which contains Notify* functions. These are callbacks. Different game modes are interested in different events of the game. It's not really possible to access the concrete objects creating these events because they're buried in the worldObjects array. Imagine we are adding new game modes to our game. The IRules interface will get hugely bloated, containing all the possible things a game mode may be interested in, and most calls will be stubbed! How can I prevent this?
Edit 2: Concrete example
Seems like your Process logic sends out a lot of events. If you would give these events a name, you could subscribe your observers to them.
Then it would even be possible to create a 'filtering' observer that can forward the events to any other observer (a decorator pattern):
struct Event {
enum e { A, B, /*...*/ };
e name;
};
class IEventListener {
public:
virtual void event( Event e ) = 0;
};
// an event dispatcher implementation:
using namespace std;
class EventDispatcher {
public:
typedef std::shared_ptr<IEventListener> IEventListenerPtr;
map<Event::e,vector<IEventListenerPtr>> listeners;
void event(Event e){
const vector<IEventListenerPtr> e_listeners=listeners[e.name].second;
//foreach(begin(e_listeners)
// ,end(e_listeners)
// ,bind(IEventListener::event,_1,e));
for(vector<IEventListenerPtr>::const_iterator it=e_listeners.begin()
; it!=e_listeners.end()
; ++it)
{
(*it)->event(e);
}
}
};
You program could look like this:
Main main;
EventEventDispatcher f1;
f1.listeners[Event::A].push_back(listener1);
main.listener=f1;
Note: code untested - grab the idea.
If you really want to decouple the sender from the sink, you put an event system in between. The example given here is very dedicated and lightweight, but do sure take a look at various existing implementations: Signals and Slots implemented in Qt and in Boost, the delegates from C#, ...
Apologizes if I missed something but why not use event? Basically let IController expose void Callback() method, then Main would be able subscribe any callback to own event:
class Main
{
private event EventHandler SomeEvent;
public Main(IController controller)
{
// use weak events to avoid memory leaks or
// implement IDisposable here and unsubscribe explicitly
this.SomeEvent += controller.Callback;
}
public void ProcessStuff()
{
// invoke event here
SomeEvent();
}
}
EDIT:
This is what I would do: extract each rule action into the separate interface so you just implement what you need in concrete classes, for instance CaptureTheFlag class does only PickupFlag action for now so does not need Damage/Death methods, so just mark by IPickupable and that's it. Then just check whether concrete instance supports concrete actions and proceed with execute.
interface IPickupable
{
void NotifyPickup(object pickedUp);
}
interface IDeathable
{
void NotifyDeath();
}
interface IDamagable
{
void NotifyDamage();
}
class CaptureTheFlag : IPickupable
{
public void NotifyPickup(Pickup pickedUp)
{
if (pickedUp is Flag)
GameOver();
}
}
class DeathMatch : IPickupable, IDeathable
{
public void NotifyPickup(Pickup pickedUp)
{
points++;
}
public void NotifyDeath()
{
lives--;
}
}
class GameWorld
{
public void GameLoop()
{
foreach(object obj in worldObjects)
{
obj.Update();
IPickupable pickupable = gameMode as IPickupable;
IDeathable deathable = gameMode as IDeathable;
IDamagable damagable = gameMode as IDamagable;
if (pickupable != null)
{
pickupable.NotifyPickup();
}
if (deathable != null)
{
deathable.NotifyDeath();
}
if (damagable != null)
{
damagable.NotifyDamage();
}
}
}
}
My final solution was the C# equivalent of what xtofl posted. I created a class which stored a bunch of delegates in it. These delegates started off with default values (so they would never be null) and the different concrete IRules classes could choose to overwrite them or not. This worked better than abstract or stubbed methods because it doesn't clog the interface with unrelated methods.
class GameEvents
{
public Action<Player> PlayerKilled = p => {};
public Func<Entity, bool> EntityValid = e => true;
public Action ItemPickedUp = () => {};
public Action FlagPickedUp = () => {};
}
class IRules
{
GameEvents Events { get; }
}
class CaptureTheFlag : IRules
{
GameEvents events = new GameEvents();
public GameEvents Events
{
get { return events; }
}
public CaptureTheFlag()
{
events.FlagPickedUp = FlagPickedUp;
}
public void FlagPickedUp()
{
score++;
}
}
Each rule set can choose which events it wants to listen to. The game simply calls then by doing Rules.Events.ItemPickedUp();. It's guaranteed never to be null.
Thanks to xtofl for the idea!
Using interfaces won't work because I want a single implementation. Using this solution would end in a lot of redundant code because I plan on having quite a few sub classes (composition vs inheritance). I've decided that a problem-specific design solution is what I'm looking for, and I can't think of anything elegant.
Basically I want classes to have separate properties, and for those properties to be attached at design time to any sub class I choose. Say, I have class 'ninja'. I would like to be able to make arbitrary sub classes such as 'grayNinja' where a gray ninja will always have a sword and throwing stars. Then possibly 'redNinja' who will always have a sword and a cape. Obviously swords, stars, and capes will each have their own implementation - and this is where I can't use interfaces. The closest solution I could find was the decorator pattern, but I don't want that functionality at runtime. Is the best solution an offshoot of that? Where inside the Black Ninja class constructor, I pass it through the constructors of sword and throwingStar? (those being abstract classes)
haven't coded in a while and reading hasn't gotten me too far - forgive me if the answer is simple.
Edit: Answered my own question. I can't mark it as 'answer' until tomorrow. Please let me know if there's a problem with it that I didn't catch. All the reading this problem forced me to do has been awesome. Learned quite a bit.
You want classes to have separate properties. Have you considered coding exactly that?
For example, you want a RedNinja that is-a Ninja that has-a sword and cape. Okay, so define Ninja to have an inventory, make it accessible through Ninja, and pass in an inventory through RedNinja's constructor. You can do the same thing for behaviors.
I've done once a similar app. with a earlier "C++" compiler that supported only single inheritance and no interfaces, at all.
// base class for all ninjas
public class Ninja {
// default constructor
public Ninja() { ... }
// default destructor
public ~Ninja() { ... }
} // class
public class StarNinja: public Ninja {
// default constructor
public StarNinja() { ... }
// default destructor
public ~StarNinja() { ... }
public void throwStars() { ... }
} // class
public class KatannaNinja: public Ninja {
// default constructor
public KatannaNinja() { ... }
// default destructor
public ~KatannaNinja() { ... }
public void useKatanna() { ... }
} // class
public class InvisibleNinja: public Ninja {
// default constructor
public InvisibleNinja() { ... }
// default destructor
public ~InvisibleNinja() { ... }
public void becomeVisible() { ... }
public void becomeInvisible() { ... }
} // class
public class FlyNinja: public Ninja {
// default constructor
public FlyNinja() { ... }
// default destructor
public ~FlyNinja() { ... }
public void fly() { ... }
public void land() { ... }
} // class
public class InvincibleNinja: public Ninja {
// default constructor
public InvincibleNinja() { ... }
// default destructor
public ~InvincibleNinja() { ... }
public void turnToStone() { ... }
public void turnToHuman() { ... }
} // class
// --> this doesn't need to have the same superclass,
// --> but, it helps
public class SuperNinja: public Ninja {
StarNinja* LeftArm;
InvincibleNinja* RightArm;
FlyNinja* LeftLeg;
KatannaNinja* RightLeg;
InvisibleNinja* Body;
// default constructor
public SuperNinja() {
// -> there is no rule to call composed classes,
LeftArm = new StarNinja();
RightArm = new InvincibleNinja();
LeftLeg = new FlyNinja();
RightLeg = new KatannaNinja();
Body = new InvisibleNinja();
}
// default destructor
public ~SuperNinja() {
// -> there is no rule to call composed classes
delete LeftArm();
delete RightArm();
delete LeftLeg();
delete RightLeg();
delete Body();
}
// --> add all public methods from peers,
// --> to main class
public void throwStars() { LeftArm->throwStars(); }
public void useKatanna() { RightLeg->useKatanna(); }
public void becomeVisible() { Body->becomeVisible() }
public void becomeInvisible() { Body->becomeInvisible() }
public void fly() { LeftLeg->fly() }
public void land() { LeftLeg->land() }
public void turnToStone() { RightArm->turnToStone(); }
public void turnToHuman() { RightArm->turnToHuman(); }
} // class
Im afraid, that the most close example is the composition design pattern. In order, to become more similar to inheritance, I make a generic base class that all composite classes share, and I make a main class that will be the result of the multiple inheritance, that has a copy of all the public methods of the component classes.
If you want to use interfaces, to enforce that main class have all important methods,
then make an interface that matches each composing class, and implemented in the main class.
public interface INinja {
public void NinjaScream() { ... }
} // class
public interface IStarNinja {
void throwStars();
} // class
public interface IKatannaNinja {
void useKatanna();
} // class
public interface IInvisibleNinja {
void becomeVisible();
void becomeInvisible();
} // class
public interface CFlyNinja {
void fly();
void land();
} // class
public interface IInvincibleNinja {
void turnToStone() { ... }
void turnToHuman() { ... }
} // class
// base class for all ninjas
public class CNinja: public INinja {
// default constructor
public CNinja() { ... }
// default destructor
public ~CNinja() { ... }
public void NinjaScream() { ... }
} // class
public class CStarNinja: public CNinja, INinja {
// default constructor
public CStarNinja() { ... }
// default destructor
public ~CStarNinja() { ... }
public void NinjaScream() { ... }
public void throwStars() { ... }
} // class
public class CKatannaNinja: public CNinja, IKatannaNinja {
// default constructor
public CKatannaNinja() { ... }
// default destructor
public ~CKatannaNinja() { ... }
public void NinjaScream() { ... }
public void useKatanna() { ... }
} // class
public class CInvisibleNinja: public CNinja, IInvisibleNinja {
// default constructor
public CInvisibleNinja() { ... }
// default destructor
public ~CInvisibleNinja() { ... }
public void becomeVisible() { ... }
public void becomeInvisible() { ... }
} // class
public class CFlyNinja: public CNinja, IFlyNinja {
// default constructor
public CFlyNinja() { ... }
// default destructor
public ~CFlyNinja() { ... }
public void fly() { ... }
public void land() { ... }
} // class
public class CInvincibleNinja: public CNinja, IInvincibleNinja {
// default constructor
public CInvincibleNinja() { ... }
// default destructor
public ~CInvincibleNinja() { ... }
public void turnToStone() { ... }
public void turnToHuman() { ... }
} // class
// --> this doesn't need to have the same superclass,
// --> but, it helps
public class CSuperNinja: public CNinja,
IKatannaNinja,
IInvisibleNinja,
IFlyNinja,
IInvincibleNinja
{
CStarNinja* LeftArm;
CInvincibleNinja* RightArm;
CFlyNinja* LeftLeg;
CKatannaNinja* RightLeg;
CInvisibleNinja* Body;
// default constructor
public CSuperNinja() {
// -> there is no rule to call composed classes
LeftArm = new CStarNinja();
RightArm = new CInvincibleNinja();
LeftLeg = new CFlyNinja();
RightLeg = new CKatannaNinja();
Body = new CInvisibleNinja();
}
// default destructor
public ~SuperNinja() {
// -> there is no rule to call composed classes
delete LeftArm();
delete RightArm();
delete LeftLeg();
delete RightLeg();
delete Body();
}
// --> add all public methods from peers,
// --> to main class
public void throwStars() { LeftArm->throwStars(); }
public void useKatanna() { RightLeg->useKatanna(); }
public void becomeVisible() { Body->becomeVisible() }
public void becomeInvisible() { Body->becomeInvisible() }
public void fly() { LeftLeg->fly() }
public void land() { LeftLeg->land() }
public void turnToStone() { RightArm->turnToStone(); }
public void turnToHuman() { RightArm->turnToHuman(); }
} // class
I know this solution is complex, but, seems that there is not another way.
Cheers.
Alright so mix-ins through extension methods are going to be my preferred route. I couldn't figure out how to use dynamic proxies in vb.net (seemed to require libraries with lots of documentation that didn't cover specifically what I needed). Dynamic proxies also seems to be a bit dirtier of a solution than using extension methods. Composition would have been what I defaulted to if the previous two didn't work.
So one problem with extension methods, is that the code gets a little dirtier if you want to hold variables. Not much though. Another problem is that all the extension methods must be defined in modules, so the code might look a little goofy to a new eye. I will solve this by defining my interface and module with the corresponding extension method in the same file.
finally, here's some sample vb.net code if you don't want to see a full fledged example through the link.
Imports System.Runtime.CompilerServices 'for extension methods
Public Interface ISword
End Interface
Public Interface IThrowingStar
End Interface
Module ExtensionMethods
<Extension()>
Public Sub swingSword(ByVal hasASword As ISword)
Console.WriteLine("Sword has been swung")
End Sub
<Extension()>
Public Sub throwStar(ByVal hasAStar As IThrowingStar)
Console.WriteLine("Star has been thrown")
End Sub
End Module
Public Class RedNinja
Inherits Ninja
Implements IThrowingStar, ISword
Public Sub New()
End Sub
End Class
Public MustInherit Class Ninja
private curHealth as Integer
Public Sub New()
curHealth = 100
End Sub
Public Function getHP() As Integer
Return curHealth
End Function
End Class
Module Module1
Sub main()
Console.WriteLine("Type any character to continue.")
Console.ReadKey()
Dim a As New RedNinja
a.swingSword() 'prints "Sword has been swung"
a.throwStar() 'prints "Star has been thrown"
Console.WriteLine("End of program - Type any key to exit")
Console.ReadKey()
End Sub
End Module
Dirty solution, if you simply must have multiple inheritance, is using something like dynamic proxies in Java.
But I guess you're probably programming in C#, and this is language agnostic question, so here goes language agnostic answer: check out composite and factory design patterns, that should give you some ideas.
Also, it might not be needed to pass everything in constructor. Check out IoC pattern as well.