A Way to implement Abstract Class In Flex

A Way to implement Abstract Class In Flex

It’s a fact that, until now(3.0) the ActionScript doesn’t implement the abstract class, it has Interface, but the abstract class is very useful when you have some logic that is fixed, and these class extends the class have some same methods(logic), and some other methods should be implemented in the subclasses, we should prevent the customer(of the super class) instantiating the super class. Compare to Interface, the abstract emphasize the “extending”, which means code reusing. But the Interface is more flexiable.

Okey, I found an article which introduce a way to implement the abstract class, but the way is not that perfect, it can implement the “Abstract” in runtime, it can’t check the error in complie time if the customer(of the super class) try to new the instance of the super class.

And, the runtime error is not recommended. There is no free lunch, to Force the AS3 to do something which is not native may cost a lot, In this case , you should do think carefully in your projects.

The following are copied from http://joshblog.net/2007/08/19/enforcing-abstract-classes-at-runtime-in-actionscript-3/, by JOSH TYNJALA.

As you probably know, abstract classes are not a language feature of ActionScript 3. If you are unfamiliar with the concept of an abstract class, let me explain. Think of an abstract class as a super-powered interface. It defines a series of functions that must be implemented, and it cannot be instantiated directly. It goes a step beyond an interface because it also defines some functions that are already fully implemented.

To use an analogy, all electronic devices generally have the same connector to plug into the wall. However, not all electronics have the same purpose. A coffee maker and a DVD player do very different things. We could make an interface IElectronicDevice to make sure they all have a plugIntoWall() function, but then every device will need to re-implement the common wall plug functionality that doesn’t differ very often. By making a class AbstractElectronicDevice, we can implement the wall plug functionality once, and all subclasses of AbstractElectronicDevice will be able to use it without re-implementing the same code.

The problem, of course, is that the coffee maker and DVD player have different controls for turning power on and off. The coffee maker might have a switch, while the DVD player has a button. We can’t implement the togglePower() function in AbstractElectronicDevice because many electronics will have different controls, so we need some way to force all subclasses of AbstractElectronicDevice to implement this function themselves. Using the methods developed to enforce Singletons in ActionScript 3 as a guide, I’ve found a way to enforce the abstractness of a class at runtime upon instantiation.

There are two main parts to enforcing an abstract class. First, we must stop a developer from instantiating the abstract class directly. To do this, the constructor needs a little special sauce. Since anyone can create an instance of a public class by simply using the new keyword, we need the constructor of our abstract class to require a parameter that only subclasses will be able to pass. Second, we must ensure that a developer implements functions that the abstract class has defined, but not implemented.

Stopping Direct Instantiation of an Abstract Class

The magic bean to stop a developer from instantiating a class directly is one keyword: this. You don’t have access to an instance of a class until after you call a constructor, so only an instance of a subclass will be able to pass a reference to itself to the super class. Let’s look at some code to help make things clearer.

The instance of MyAbstractType expects to receive a reference to itself as the first parameter in the constructor. If it does not, an error will be thrown.

package com.joshtynjala.abstract

{

    import flash.errors.IllegalOperationError;

 

    public class MyAbstractType

    {

       public function MyAbstractType(self:MyAbstractType)

       {

           if(self != this)

           {

              //only a subclass can pass a valid reference to self

              throw new IllegalOperationError("Abstract class did not receive reference to self. MyAbstractType cannot be instantiated directly.");

           }

       }

    }

}

Only MyConcreteType and other subclasses of MyAbstractType will be able to pass a reference to the instance to their super() constructors. Notice that users of MyConcreteType don’t need to know that it extends an abstract class. The signature of the MyConcreteType’s constructor can be completely different than MyAbstractType.

package com.joshtynjala.abstract

{

    public class MyConcreteType extends MyAbstractType

    {

       public function MyConcreteType()

       {

           //pass "this" to clear the abstract check

           super(this);

       }

    }

}

 

Forcing Implementation of Functions in Subclasses

Next, like with an interface, we need to force subclasses of our abstract class to implement specific functions. Ideally, we want this enforcement to happen immediately when the object is created (to make bugs visible as early as possible). After we check that the user isn’t trying to instantiate the abstract class directly, we should check to be sure the unimplemented methods are overridden. We can do by checking the results from theflash.utils.describeType() function against a list of unimplemented methods in the abstract class. Again, a little code should give us a clearer picture.

In MyAbstractType, after we check for a reference to the self parameter, we build a list of unimplemented functions in an Array. Next, we use describeType()to get a list of the methods declared in the instance. If a subclass of MyAbstractType overrides a method, the declaredBy attribute in the method XML will specify the name of the subclass rather than MyAbstractType.

package com.joshtynjala.abstract

{

    import flash.errors.IllegalOperationError;

    import flash.utils.describeType;

    import flash.utils.getQualifiedClassName;

 

    public class MyAbstractType

    {

       public function MyAbstractType(self:MyAbstractType)

       {

           if(self != this)

           {

              //only a subclass can pass a valid reference to self

              throw new IllegalOperationError("Abstract class did not receive reference to self. MyAbstractType cannot be instantiated directly.");

           }

 

           //these functions MUST be implemented in subclasses

           var unimplemented:Array = [mustBeOverridden];

 

           //get the fully-qualified name the abstract class

           var abstractTypeName:String = getQualifiedClassName(MyAbstractType);

 

           //get a list of all the methods declared by the abstract class

           //if a subclass overrides a function, declaredBy will contain the subclass name

           var selfDescription:XML = describeType(this);

           var methods:XMLList = selfDescription.method.(@declaredBy == abstractTypeName && unimplemented.indexOf(this[@name]) >= 0);

 

           if(methods.length() > 0)

           {

              //we'll only get here if the function is still unimplemented

              var concreteTypeName:String = getQualifiedClassName(this);

              throw new IllegalOperationError("Function " + methods[0].@name + " from abstract class " + abstractTypeName + " has not been implemented by subclass " + concreteTypeName);

           }

       }

 

       //implemented

       public function alreadyImplemented():void

       {

           trace("Don't forget to list me in the Array of valid functions.");

       }

 

       //unimplemented

       public function mustBeOverridden(param:String):void {};

    }

}

 

Now, in MyConcreteType, we can implement the mustBeOverridden() function. If we do not, an error will be thrown. To be certain, try commenting out the implementation of mustBeOverridden() in MyConcreteClass. If you instantiate it, you will receive a runtime error.

package com.joshtynjala.abstract

{

    public class MyConcreteType extends MyAbstractType

    {

       public function MyConcreteType()

       {

           //pass "this" to clear the abstract check

           super(this);

       }

 

       //implemented

       override public function mustBeOverridden(param:String):void

       {

           trace("param:", param);

       }

    }

}

Conclusions

As you probably noticed, the first part of implementing an abstract class is very simple. Making the abstract class receive a reference to itself enforces subclassing, and it can be done in only a few lines of code. The second part, making an abstract class work like an interface, requires a lot more code, and it may be more prone to errors. You, as a developer, must remember to add all the unimplemented methods to the Array in the constructor. Additionally, if the class gets large, I can imagine that calling describeType() often enough could lead to performance problems. In most cases, as long as you keep your classes clean, it should work well. I highly recommend the subclass enforcement, but I wouldn’t be heartbroken if you feel that checking for unimplemented methods is overkill.

Source code for this tutorial is available for download. It includes all the code above plus an extra implementation (for comparison) of the “electronic device” example I described at the beginning. It’s all under the terms of the MIT license.

 

Yes, it is really a good idea to implement the “Abstract Class”, but the checking is postponed to runtime, then some defects appear:

1. Cost

May be it costs too much, you have to call describeType, this is really costing, especially when you have to create a lot of instances of subclasses.