As title, is there any way to iterate or display Apache velocity template attributes?
for example, I have following code :
<code>
${ctx.messages.headerMessage}
</code>
And I want to know how many other attributes the variable ${ctx} has
It's only possible to discover and to loop on an object properties (that is, the ones with getters and/or setters) if you can add a new tool to your Velocity context. If you can't, you're rather stuck.
There are several ways to do this, I illustrate below how to do this with commons-beanutils.
First, add Apache commons-beanutils in your class path, and add it to your Velocity context from Java:
import org.apache.commons.beanutils.PropertyUtils;
...
context.put("beans", new PropertyUtils());
...
One remark: if you do not have access to the Java part, but if by chance commons-beanutils is already in the classpath, there is one hakish way of having access to it: #set($beans = $foo.class.forName('org.apache.commons.beanutils.PropertyUtils').newInstance()).
Then, let's say that I have the following object:
class Foo
{
public boolean isSomething() { return true; }
public String getName() { return "Nestor"; }
}
which is present in my context under $foo. Using your newly $beans properties introspector, you can do:
#set ($properties = $beans.getPropertyDescriptors($foo.class))
#foreach ($property in $properties)
$property.name ($property.propertyType) = $property.readMethod.invoke($foo)
#end
This will produce:
bar (boolean) = true
class (class java.lang.Class) = class Foo
name (class java.lang.String) = Robert
(you'll need to filter out the class property, of course)
One last remark, though. Templates are for coding the View layer of an MVC application, and doing such a generic introspection of objects in them is rather inadequate in the view layer. You're far better of moving all this introspection code on the Java side.
If I have a class
export class Person {
public name: string = "";
public foo: string = "";
}
ValidationRules
.ensure((p :Person) => p.name)
.required()
.withMessage("name is required")
.on(Person);
Is there any way to extend those rules on a per controller basis? For example in my App class I also want to ensure the foo property is set, however adding this as a rule here seems to override the name rule from the above code.
export class App {
public person: Person = new Person();
#observable
public message: string = "";
constructor(public vc: ValidationController, public vld: Validator) {
ValidationRules
.ensure((p: Person) => p.foo).required().withMessage("foo is required").on(this.person);
this.vc.addObject(this.person);
this.vc.validate();
}
}
Yes that's possible, but it requires a slightly different approach.
There are 2 important things to note here:
The fluent api initializer (static method ensure() on ValidationRules) always instantiates a new FluentEnsure object. It doesn't look for existing stuff - not even if you finalize on the same target. To extend a ruleset with more rules, you need to call .ensure() on the existing ruleset.
The finalizer (instance method on() on FluentEnsure) stores the rules in the .prototype.__rules__ (if it's a Function) or .__rules__ property of the target you pass in and will overwrite any existing one.
In other words when you finalize on an instance of Person, you're storing a brand new rules object on person.__rules__ which effectively hides Person.prototype.__rules__.
For the record, the static methods Rules.set(target, rules) and Rules.get(target) are wrappers around the .__rules__ property. You'll definitely want to call those and not try to access the property directly.
Now you might think something like Rules.get(Person).ensure(...).on(person) but that would also modify the original rules on Person.prototype.
So how to work with that?
Enter tags
It can get messy with fetching and combining rulesets, but here's the basic idea:
export class Person {}
ValidationRules.ensure(p => p.name).required().on(Person)
And elsewhere:
Rules.get(Person).ensure(p => p.foo).required().tag("foo");
And elsewhere:
Rules.get(Person).ensure(p => p.bar).required().tag("1234");
Then when it's time to validate:
const allRules = Rules.get(Person); // "1234" is included
const fooRules = ValidationRules.untaggedRules(allRules)
.concat(ValidationRules.taggedRules(allRules, "foo"); "1234" not included
vc.addObject(person);
vc.validate({ fooRules });
Or hand roll something
I've never actually used tags myself before and I've seen an issue or two regarding them. You could also do something similar yourself if you want more control/transparency:
export class Person {
public static scopedRules: { [scope: string]: any } = Object.create(null);
}
ValidationRules.ensure(p => p.name).required().on(Person)
And elsewhere:
Person.scopedRules.foo = Object.create(null);
ValidationRules.ensure(p => p.foo).required().on(Person.scopedRules.foo)
Then when it's time to validate:
const rules = Rules.get(Person).concat(Rules.get(Person.scopedRules.foo));
vc.addObject(person);
vc.validate({ rules });
Of course this is just a "simplest possible thing" example. In a real world scenario you'd probably have your rule storage/retrieval/merging etc tucked away somewhere.
I know how to create a class the will allow me to instantiate it and use across my project. What I want to be able to do is have functions without instantiating classes. For example, I know how to do this:
$core = new core();
$val = $core->convertToMyNotation($anotherval);
But what I want is to be able to do this ANYWHERE in any view, class whatever:
$val = convertToMyNotation($anotherval);
Where would I place these functions in order to be able to do that?
best way to do it, create a public function in components/Controller.php
public function globalFunction(){
// do something here.
}
and access it anywhere by
$this->globalFunction();
You can define a static method as an option.
class core{
public static function convertToMyNotation($value){
//do whatever here
return $value;
}
}
Then call it like so:
$val = core::convertToMyNotation($anotherval);
This requires no instantiation of the object to use. The only restriction is that you cannot use the $this property inside a static method.
Alternately, just define a file with your functions in it and include the file at some point early like, like within the boostrap script in your public_html/index.php file.
Edit: darkheir makes some good suggestions. Include such a class in your protected/components folder, and have it extend CComponent to gain some potentially useful enhancements.
By including the class in the protected/components folder, you gain the advantage of autoloading the class, by default.
There is no definitive question of your answer, it depends a lot on what the function will be doing!
If the function is performing some things specific to a model
(getting the last users, ...) this has to be in the User model as
Willem Renzema described:
class theModelClass {
public static function convertToMyNotation($value){
//do whatever here
return $value;
}
}
And you'll call it like
$val = theModelClass::convertToMyNotation($anotherval);
If the function is handling user inputs (sanitizing he inputs,
checking the values, ...) then it has to go to the controller and
you'll use Hemc solution:
Create a public function in components/Controller.php
public function globalFunction(){
// do something here.
}
and access it anywhere by
$this->globalFunction();
If the function is an Helper: performing some actions that do not
depend on models or user inoput then you can create a new class that
you'll put in your component directory:
class core extends CComponent{
public static function convertToMyNotation($value){
//do whatever here
return $value;
}
}
And
$val = core::convertToMyNotation($anotherval);
Actually, I think you're looking for this answer instead:
http://www.yiiframework.com/wiki/31/use-shortcut-functions-to-reduce-typing/
In essence, in your entry script, before you load up Yii, include a global functions file:
require('path/to/globals.php');
Then, any function defined in that file can be used as a shortcut. Be careful, but enjoy the power! :-)
Create something like
Class Core extends CApplicationComponent{
public function doSomething(){}
}
and in config main.php
'components'=>array(
'core'=>array(
'class' => 'Core'
),
),
and now you can call whenever you want
Yii::app()->core->doSomething();
Why should we make the constructor private in class? As we always need the constructor to be public.
Some reasons where you may need private constructor:
The constructor can only be accessed from static factory method inside the class itself. Singleton can also belong to this category.
A utility class, that only contains static methods.
By providing a private constructor you prevent class instances from being created in any place other than this very class. There are several use cases for providing such constructor.
A. Your class instances are created in a static method. The static method is then declared as public.
class MyClass()
{
private:
MyClass() { }
public:
static MyClass * CreateInstance() { return new MyClass(); }
};
B. Your class is a singleton. This means, not more than one instance of your class exists in the program.
class MyClass()
{
private:
MyClass() { }
public:
MyClass & Instance()
{
static MyClass * aGlobalInst = new MyClass();
return *aGlobalInst;
}
};
C. (Only applies to the upcoming C++0x standard) You have several constructors. Some of them are declared public, others private. For reducing code size, public constructors 'call' private constructors which in turn do all the work. Your public constructors are thus called delegating constructors:
class MyClass
{
public:
MyClass() : MyClass(2010, 1, 1) { }
private:
MyClass(int theYear, int theMonth, int theDay) { /* do real work */ }
};
D. You want to limit object copying (for example, because of using a shared resource):
class MyClass
{
SharedResource * myResource;
private:
MyClass(const MyClass & theOriginal) { }
};
E. Your class is a utility class. That means, it only contains static members. In this case, no object instance must ever be created in the program.
To leave a "back door" that allows another friend class/function to construct an object in a way forbidden to the user. An example that comes to mind would be a container constructing an iterator (C++):
Iterator Container::begin() { return Iterator(this->beginPtr_); }
// Iterator(pointer_type p) constructor is private,
// and Container is a friend of Iterator.
Everyone is stuck on the Singleton thing, wow.
Other things:
Stop people from creating your class on the stack; make private constructors and only hand back pointers via a factory method.
Preventing creating copys of the class (private copy constructor)
This can be very useful for a constructor that contains common code; private constructors can be called by other constructors, using the 'this(...);' notation. By making the common initialization code in a private (or protected) constructor, you are also making explicitly clear that it is called only during construction, which is not so if it were simply a method:
public class Point {
public Point() {
this(0,0); // call common constructor
}
private Point(int x,int y) {
m_x = x; m_y = y;
}
};
There are some instances where you might not want to use a public constructor; for example if you want a singleton class.
If you are writing an assembly used by 3rd parties there could be a number of internal classes that you only want created by your assembly and not to be instantiated by users of your assembly.
This ensures that you (the class with private constructor) control how the contructor is called.
An example : A static factory method on the class could return objects as the factory method choses to allocate them (like a singleton factory for example).
We can also have private constructor,
to enfore the object's creation by a specific class
only(For security reasons).
One way to do it is through having a friend class.
C++ example:
class ClientClass;
class SecureClass
{
private:
SecureClass(); // Constructor is private.
friend class ClientClass; // All methods in
//ClientClass have access to private
// & protected methods of SecureClass.
};
class ClientClass
{
public:
ClientClass();
SecureClass* CreateSecureClass()
{
return (new SecureClass()); // we can access
// constructor of
// SecureClass as
// ClientClass is friend
// of SecureClass.
}
};
Note: Note: Only ClientClass (since it is friend of SecureClass)
can call SecureClass's Constructor.
You shouldn't make the constructor private. Period. Make it protected, so you can extend the class if you need to.
Edit: I'm standing by that, no matter how many downvotes you throw at this.
You're cutting off the potential for future development on the code. If other users or programmers are really determined to extend the class, then they'll just change the constructor to protected in source or bytecode. You will have accomplished nothing besides to make their life a little harder. Include a warning in your constructor's comments, and leave it at that.
If it's a utility class, the simpler, more correct, and more elegant solution is to mark the whole class "static final" to prevent extension. It doesn't do any good to just mark the constructor private; a really determined user may always use reflection to obtain the constructor.
Valid uses:
One good use of a protected
constructor is to force use of static
factory methods, which allow you to
limit instantiation or pool & reuse
expensive resources (DB connections,
native resources).
Singletons (usually not good practice, but sometimes necessary)
when you do not want users to create instances of this class or create class that inherits this class, like the java.lang.math, all the function in this package is static, all the functions can be called without creating an instance of math, so the constructor is announce as static.
If it's private, then you can't call it ==> you can't instantiate the class. Useful in some cases, like a singleton.
There's a discussion and some more examples here.
I saw a question from you addressing the same issue.
Simply if you don't want to allow the others to create instances, then keep the constuctor within a limited scope. The practical application (An example) is the singleton pattern.
Constructor is private for some purpose like when you need to implement singleton or limit the number of object of a class.
For instance in singleton implementation we have to make the constructor private
#include<iostream>
using namespace std;
class singletonClass
{
static int i;
static singletonClass* instance;
public:
static singletonClass* createInstance()
{
if(i==0)
{
instance =new singletonClass;
i=1;
}
return instance;
}
void test()
{
cout<<"successfully created instance";
}
};
int singletonClass::i=0;
singletonClass* singletonClass::instance=NULL;
int main()
{
singletonClass *temp=singletonClass::createInstance();//////return instance!!!
temp->test();
}
Again if you want to limit the object creation upto 10 then use the following
#include<iostream>
using namespace std;
class singletonClass
{
static int i;
static singletonClass* instance;
public:
static singletonClass* createInstance()
{
if(i<10)
{
instance =new singletonClass;
i++;
cout<<"created";
}
return instance;
}
};
int singletonClass::i=0;
singletonClass* singletonClass::instance=NULL;
int main()
{
singletonClass *temp=singletonClass::createInstance();//return an instance
singletonClass *temp1=singletonClass::createInstance();///return another instance
}
Thanks
You can have more than one constructor. C++ provides a default constructor and a default copy constructor if you don't provide one explicitly. Suppose you have a class that can only be constructed using some parameterized constructor. Maybe it initialized variables. If a user then uses this class without that constructor, they can cause no end of problems. A good general rule: If the default implementation is not valid, make both the default and copy constructor private and don't provide an implementation:
class C
{
public:
C(int x);
private:
C();
C(const C &);
};
Use the compiler to prevent users from using the object with the default constructors that are not valid.
Quoting from Effective Java, you can have a class with private constructor to have a utility class that defines constants (as static final fields).
(EDIT: As per the comment this is something which might be applicable only with Java, I'm unaware if this construct is applicable/needed in other OO languages (say C++))
An example as below:
public class Constants {
private Contants():
public static final int ADDRESS_UNIT = 32;
...
}
EDIT_1:
Again, below explanation is applicable in Java : (and referring from the book, Effective Java)
An instantiation of utility class like the one below ,though not harmful, doesn't serve
any purpose since they are not designed to be instantiated.
For example, say there is no private Constructor for class Constants.
A code chunk like below is valid but doesn't better convey intention of
the user of Constants class
unit = (this.length)/new Constants().ADDRESS_UNIT;
in contrast with code like
unit = (this.length)/Constants.ADDRESS_UNIT;
Also I think a private constructor conveys the intention of the designer of the Constants
(say) class better.
Java provides a default parameterless public constructor if no constructor
is provided, and if your intention is to prevent instantiation then a private constructor is
needed.
One cannot mark a top level class static and even a final class can be instantiated.
Utility classes could have private constructors. Users of the classes should not be able to instantiate these classes:
public final class UtilityClass {
private UtilityClass() {}
public static utilityMethod1() {
...
}
}
You may want to prevent a class to be instantiated freely. See the singleton design pattern as an example. In order to guarantee the uniqueness, you can't let anyone create an instance of it :-)
One of the important use is in SingleTon class
class Person
{
private Person()
{
//Its private, Hense cannot be Instantiated
}
public static Person GetInstance()
{
//return new instance of Person
// In here I will be able to access private constructor
}
};
Its also suitable, If your class has only static methods. i.e nobody needs to instantiate your class
It's really one obvious reason: you want to build an object, but it's not practical to do it (in term of interface) within the constructor.
The Factory example is quite obvious, let me demonstrate the Named Constructor idiom.
Say I have a class Complex which can represent a complex number.
class Complex { public: Complex(double,double); .... };
The question is: does the constructor expects the real and imaginary parts, or does it expects the norm and angle (polar coordinates) ?
I can change the interface to make it easier:
class Complex
{
public:
static Complex Regular(double, double = 0.0f);
static Complex Polar(double, double = 0.0f);
private:
Complex(double, double);
}; // class Complex
This is called the Named Constructor idiom: the class can only be built from scratch by explicitly stating which constructor we wish to use.
It's a special case of many construction methods. The Design Patterns provide a good number of ways to build object: Builder, Factory, Abstract Factory, ... and a private constructor will ensure that the user is properly constrained.
In addition to the better-known uses…
To implement the Method Object pattern, which I’d summarize as:
“Private constructor, public static method”
“Object for implementation, function for interface”
If you want to implement a function using an object, and the object is not useful outside of doing a one-off computation (by a method call), then you have a Throwaway Object. You can encapsulate the object creation and method call in a static method, preventing this common anti-pattern:
z = new A(x,y).call();
…replacing it with a (namespaced) function call:
z = A.f(x,y);
The caller never needs to know or care that you’re using an object internally, yielding a cleaner interface, and preventing garbage from the object hanging around or incorrect use of the object.
For example, if you want to break up a computation across methods foo, bar, and zork, for example to share state without having to pass many values in and out of functions, you could implement it as follows:
class A {
public static Z f(x, y) {
A a = new A(x, y);
a.foo();
a.bar();
return a.zork();
}
private A(X x, Y y) { /* ... */ };
}
This Method Object pattern is given in Smalltalk Best Practice Patterns, Kent Beck, pages 34–37, where it is the last step of a refactoring pattern, ending:
Replace the original method with one that creates an instance of the new class, constructed with the parameters and receiver of the original method, and invokes “compute”.
This differs significantly from the other examples here: the class is instantiable (unlike a utility class), but the instances are private (unlike factory methods, including singletons etc.), and can live on the stack, since they never escape.
This pattern is very useful in bottoms-up OOP, where objects are used to simplify low-level implementation, but are not necessarily exposed externally, and contrasts with the top-down OOP that is often presented and begins with high-level interfaces.
Sometimes is useful if you want to control how and when (and how many) instances of an object are created.
Among others, used in patterns:
Singleton pattern
Builder pattern
On use of private constructors could also be to increase readability/maintainability in the face of domain-driven design.
From "Microsoft .NET - Architecing Applications for the Enterprise, 2nd Edition":
var request = new OrderRequest(1234);
Quote, "There are two problems here. First, when looking at the code, one can hardly guess what’s going
on. An instance of OrderRequest is being created, but why and using which data? What’s 1234? This
leads to the second problem: you are violating the ubiquitous language of the bounded context. The
language probably says something like this: a customer can issue an order request and is allowed to
specify a purchase ID. If that’s the case, here’s a better way to get a new OrderRequest instance:"
var request = OrderRequest.CreateForCustomer(1234);
where
private OrderRequest() { ... }
public OrderRequest CreateForCustomer (int customerId)
{
var request = new OrderRequest();
...
return request;
}
I'm not advocating this for every single class, but for the above DDD scenario I think it makes perfect sense to prevent a direct creation of a new object.
If you create a private constructor you need to create the object inside the class
enter code here#include<iostream>
//factory method
using namespace std;
class Test
{
private:
Test(){
cout<<"Object created"<<endl;
}
public:
static Test* m1(){
Test *t = new Test();
return t;
}
void m2(){
cout<<"m2-Test"<<endl;
}
};
int main(){
Test *t = Test::m1();
t->m2();
return 0;
}