Classes

ActionScript 3.0 provides four special attributes that control access to properties defined inside a class: public, private, protected, and internal.

The public attribute makes a property visible anywhere in your script. For example, to make a method available to code outside its package, you must declare the method with the public attribute. This is true for any property, whether it is declared using the var, const, or function keywords.

The private attribute makes a property visible only to callers within the property’s defining class. This behavior differs from that of the private attribute in ActionScript 2.0, which allowed a subclass to access a private property in a superclass. Another significant change in behavior has to do with run-time access. In ActionScript 2.0, the private keyword prohibited access only at compile time and was easily circumvented at run time. In ActionScript 3.0, this is no longer true. Properties that are marked as private are unavailable at both compile time and run time.

For example, the following code creates a simple class named PrivateExample with one private variable, and then attempts to access the private variable from outside the class.

class PrivateExample 
{ 
    private var privVar:String = "private variable"; 
} 
 
var myExample:PrivateExample = new PrivateExample(); 
trace(myExample.privVar);// compile-time error in strict mode 
trace(myExample["privVar"]); // ActionScript 2.0 allows access, but in ActionScript 3.0, this is a run-time error. 

In ActionScript 3.0, an attempt to access a private property using the dot operator (myExample.privVar) results in a compile-time error if you are using strict mode. Otherwise, the error is reported at run time, just as it is when you use the property access operator (myExample["privVar"]).

The following table summarizes the results of attempting to access a private property that belongs to a sealed (not dynamic) class:

In classes declared with the dynamic attribute, attempts to access a private variable do not result in a run-time error. Instead, the variable is not visible, so the value undefined is returned. A compile-time error occurs, however, if you use the dot operator in strict mode. The following example is the same as the previous example, except that the PrivateExample class is declared as a dynamic class:

dynamic class PrivateExample 
{ 
    private var privVar:String = "private variable"; 
} 
 
var myExample:PrivateExample = new PrivateExample(); 
trace(myExample.privVar);// compile-time error in strict mode 
trace(myExample["privVar"]); // output: undefined

Dynamic classes generally return the value undefined instead of generating an error when code external to a class attempts to access a private property. The following table shows that an error is generated only when the dot operator is used to access a private property in strict mode:

The protected attribute, which is new for ActionScript 3.0, makes a property visible to callers within its own class or in a subclass. In other words, a protected property is available within its own class or to classes that lie anywhere below it in the inheritance hierarchy. This is true whether the subclass is in the same package or in a different package.

For those familiar with ActionScript 2.0, this functionality is similar to the private attribute in ActionScript 2.0. The ActionScript 3.0 protected attribute is also similar to the protected attribute in Java. It differs in that the Java version also permits access to callers within the same package. The protected attribute is useful when you have a variable or method that your subclasses need but that you want to hide from code that is outside the inheritance chain.

The internal attribute, which is new for ActionScript 3.0, makes a property visible to callers within its own package. This is the default attribute for code inside a package, and it applies to any property that does not have any of the following attributes:

• public

• private

• protected

• a user-defined namespace

The internal attribute is similar to the default access control in Java, although in Java there is no explicit name for this level of access, and it can be achieved only through the omission of any other access modifier. The internal attribute is available in ActionScript 3.0 to give you the option of explicitly signifying your intent to make a property visible only to callers within its own package.

The static attribute, which can be used with properties declared with the var, const, or function keywords, allows you to attach a property to the class rather than to instances of the class. Code external to the class must call static properties by using the class name instead of an instance name.

Static properties are not inherited by subclasses, but the properties are part of a subclass’s scope chain. This means that within the body of a subclass, a static variable or method can be used without referencing the class in which it was defined.

As an alternative to the predefined access control attributes, you can create a custom namespace for use as an attribute. Only one namespace attribute can be used per definition, and you cannot use a namespace attribute in combination with any of the access control attributes (public, private, protected, internal).

Constructor methods, sometimes called constructors, are functions that share the same name as the class in which they are defined. Any code that you include in a constructor method is executed whenever an instance of the class is created with the new keyword. For example, the following code defines a simple class named Example that contains a single property named status. The initial value of the status variable is set inside the constructor function.

class Example 
{ 
    public var status:String; 
    public function Example() 
    { 
        status = "initialized"; 
    } 
} 
 
var myExample:Example = new Example(); 
trace(myExample.status); // output: initialized

Constructor methods can only be public, but the use of the public attribute is optional. You cannot use any of the other access control specifiers, including private, protected, or internal, on a constructor. You also cannot use a user-defined namespace with a constructor method.

A constructor can make an explicit call to the constructor of its direct superclass by using the super() statement. If the superclass constructor is not explicitly called, the compiler automatically inserts a call before the first statement in the constructor body. You can also call methods of the superclass by using the super prefix as a reference to the superclass. If you decide to use both super() and super in the same constructor body, be sure to call super() first. Otherwise, the super reference does not behave as expected. The super() constructor should also be called before any throw or return statement.

The following example demonstrates what happens if you attempt to use the super reference before calling the super() constructor. A new class, ExampleEx, extends the Example class. The ExampleEx constructor attempts to access the status variable defined in its superclass, but does so before calling super(). The trace() statement inside the ExampleEx constructor produces the value null, because the status variable is not available until the super() constructor executes.

class ExampleEx extends Example 
{ 
    public function ExampleEx() 
    { 
        trace(super.status); 
        super(); 
    } 
} 
 
var mySample:ExampleEx = new ExampleEx(); // output: null

Although it is legal to use the return statement inside a constructor, it is not permissible to return a value. In other words, return statements must not have associated expressions or values. Accordingly, constructor methods are not allowed to return values, which means that no return type can be specified.

If you do not define a constructor method in your class, the compiler automatically creates an empty constructor for you. If your class extends another class, the compiler includes a super() call in the constructor it generates.

Static methods, also called class methods, are methods that are declared with the static keyword. Static methods, which are attached to a class rather than to an instance of a class, are useful for encapsulating functionality that affects something other than the state of an individual instance. Because static methods are attached to a class as a whole, static methods can be accessed only through a class and not through an instance of the class.

Static methods are useful for encapsulating functionality that is not limited to affecting the state of class instances. In other words, a method should be static if it provides functionality that does not directly affect the value of a class instance. For example, the Date class has a static method named parse(), which takes a string and converts it to a number. The method is static because it does not affect an individual instance of the class. Instead, the parse() method takes a string that represents a date value, parses the string, and returns a number in a format compatible with the internal representation of a Date object. This method is not an instance method, because it does not make sense to apply the method to an instance of the Date class.

Contrast the static parse() method with one of the instance methods of the Date class, such as getMonth(). The getMonth() method is an instance method, because it operates directly on the value of an instance by retrieving a specific component, the month, of a Date instance.

Because static methods are not bound to individual instances, you cannot use the keywords this or super within the body of a static method. Both the this reference and the super reference have meaning only within the context of an instance method.

In contrast with some other class-based programming languages, static methods in ActionScript 3.0 are not inherited.

Instance methods are methods that are declared without the static keyword. Instance methods, which are attached to instances of a class instead of the class as a whole, are useful for implementing functionality that affects individual instances of a class. For example, the Array class contains an instance method named sort(), which operates directly on Array instances.

Within the body of an instance method, both static and instance variables are in scope, which means that variables defined in the same class can be referenced using a simple identifier. For example, the following class, CustomArray, extends the Array class. The CustomArray class defines a static variable named arrayCountTotal to track the total number of class instances, an instance variable named arrayNumber that tracks the order in which the instances were created, and an instance method named getPosition() that returns the values of these variables.

public class CustomArray extends Array 
{ 
    public static var arrayCountTotal:int = 0; 
    public var arrayNumber:int; 
 
    public function CustomArray() 
    { 
        arrayNumber = ++arrayCountTotal; 
    } 
     
    public function getArrayPosition():String 
    { 
         return ("Array " + arrayNumber + " of " + arrayCountTotal); 
    } 
}

Although code external to the class must access the arrayCountTotal static variable through the class object using CustomArray.arrayCountTotal, code that resides inside the body of the getPosition() method can refer directly to the static arrayCountTotal variable. This is true even for static variables in superclasses. Though static properties are not inherited in ActionScript 3.0, static properties in superclasses are in scope. For example, the Array class has a few static variables, one of which is a constant named DESCENDING. Code that resides in an Array subclass can access the static constant DESCENDING using a simple identifier:

public class CustomArray extends Array 
{ 
    public function testStatic():void 
    { 
        trace(DESCENDING); // output: 2 
    } 
}

The value of the this reference within the body of an instance method is a reference to the instance to which the method is attached. The following code demonstrates that the this reference points to the instance that contains the method:

class ThisTest 
{ 
    function thisValue():ThisTest 
    { 
        return this; 
    } 
} 
 
var myTest:ThisTest = new ThisTest(); 
trace(myTest.thisValue() == myTest); // output: true

Inheritance of instance methods can be controlled with the keywords override and final. You can use the override attribute to redefine an inherited method, and the final attribute to prevent subclasses from overriding a method.

Get and set accessor functions, also called getters and setters, allow you to adhere to the programming principles of information hiding and encapsulation while providing an easy-to-use programming interface for the classes that you create. Get and set functions allow you to keep your class properties private to the class, but allow users of your class to access those properties as if they were accessing a class variable instead of calling a class method.

The advantage of this approach is that it allows you to avoid the traditional accessor functions with unwieldy names, such as getPropertyName() and setPropertyName(). Another advantage of getters and setters is that you can avoid having two public-facing functions for each property that allows both read and write access.

The following example class, named GetSet, includes get and set accessor functions named publicAccess() that provide access to the private variable named privateProperty:

class GetSet 
{ 
    private var privateProperty:String; 
     
    public function get publicAccess():String 
    { 
        return privateProperty; 
    } 
     
    public function set publicAccess(setValue:String):void 
    { 
        privateProperty = setValue; 
    } 
}

If you attempt to access the property privateProperty directly, an error occurs, as follows:

var myGetSet:GetSet = new GetSet(); 
trace(myGetSet.privateProperty); // error occurs

Instead, a user of the GetSet class uses something that appears to be a property named publicAccess, but that is really a pair of get and set accessor functions that operate on the private property named privateProperty. The following example instantiates the GetSet class, and then sets the value of the privateProperty using the public accessor named publicAccess:

var myGetSet:GetSet = new GetSet(); 
trace(myGetSet.publicAccess); // output: null 
myGetSet.publicAccess = "hello"; 
trace(myGetSet.publicAccess); // output: hello

Getter and setter functions also make it possible to override properties that are inherited from a superclass, something that is not possible when you use regular class member variables. Class member variables that are declared using the var keyword cannot be overridden in a subclass. Properties that are created using getter and setter functions, however, do not have this restriction. You can use the override attribute on getter and setter functions that are inherited from a superclass.

A bound method, sometimes called a method closure, is simply a method that is extracted from its instance. Examples of bound methods include methods that are passed as arguments to a function or returned as values from a function. New in ActionScript 3.0, a bound method is similar to a function closure in that it retains its lexical environment even when extracted from its instance. The key difference, however, between a bound method and a function closure is that the this reference for a bound method remains linked, or bound, to the instance that implements the method. In other words, the this reference in a bound method always points to the original object that implemented the method. For function closures, the this reference is generic, which means that it points to whatever object the function is associated with at the time it is called.

Understanding bound methods is important if you use the this keyword. Recall that the this keyword provides a reference to a method’s parent object. Most ActionScript programmers expect that the this keyword always represents the object or class that contains the definition of a method. Without method binding, however, this would not always be true. In previous versions of ActionScript, for example, the this reference did not always refer to the instance that implemented the method. When methods are extracted from an instance in ActionScript 2.0, not only is the this reference not bound to the original instance, but also the member variables and methods of the instance’s class are not available. This is not a problem in ActionScript 3.0, because bound methods are automatically created when you pass a method as a parameter. Bound methods ensure that the this keyword always references the object or class in which a method is defined.

The following code defines a class named ThisTest, which contains a method named foo() that defines the bound method, and a method named bar() that returns the bound method. Code external to the class creates an instance of the ThisTest class, calls the bar() method, and stores the return value in a variable named myFunc.

class ThisTest 
{ 
    private var num:Number = 3; 
    function foo():void // bound method defined 
    { 
        trace("foo's this: " + this); 
        trace("num: " + num); 
    } 
    function bar():Function 
    { 
        return foo; // bound method returned 
    } 
} 
 
var myTest:ThisTest = new ThisTest(); 
var myFunc:Function = myTest.bar(); 
trace(this); // output: [object global] 
myFunc(); 
/* output:  
foo's this: [object ThisTest] 
output: num: 3 */

The last two lines of code show that the this reference in the bound method foo() still points to an instance of ThisTest class, even though the this reference in the line just before it points to the global object. Moreover, the bound method stored in the myFunc variable still has access to the member variables of the ThisTest class. If this same code is run in ActionScript 2.0, the this references would match, and the num variable would be undefined.

One area where the addition of bound methods is most noticeable is with event handlers, because the addEventListener() method requires that you pass a function or method as an argument.

To embed an asset, first place the asset into a FLA file’s library. Next, use the asset’s linkage property to provide a name for the asset’s embedded asset class. If a class by that name cannot be found in the classpath, a class is automatically generated for you. You can then create an instance of the embedded asset class and use any properties and methods defined or inherited by that class. For example, the following code can be used to play an embedded sound that is linked to an embedded asset class named PianoMusic:

var piano:PianoMusic = new PianoMusic(); 
var sndChannel:SoundChannel = piano.play();

Alternatively, you can use the [Embed] metadata tag to embed assets in a Flash Professional project, described next. If you use the [Embed] metadata tag in your code, Flash Professional uses the Flex compiler to compile your project instead of the Flash Professional compiler.

If you are compiling your code using the Flex compiler, to embed an asset in ActionScript code use the [Embed] metadata tag. Place the asset in the main source folder or another folder that is in your project’s build path. When the Flex compiler encounters an Embed metadata tag, it creates the embedded asset class for you. You can access the class through a variable of data type Class that you declare immediately following the [Embed] metadata tag. For example, the following code embeds a sound named sound1.mp3 and uses a variable named soundCls to store a reference to the embedded asset class associated with that sound. The example then creates an instance of the embedded asset class and calls the play() method on that instance:

package 
{ 
    import flash.display.Sprite; 
    import flash.media.SoundChannel; 
    import mx.core.SoundAsset; 
 
    public class SoundAssetExample extends Sprite 
    { 
        [Embed(source="sound1.mp3")] 
        public var soundCls:Class; 
         
        public function SoundAssetExample() 
        { 
            var mySound:SoundAsset = new soundCls() as SoundAsset; 
            var sndChannel:SoundChannel = mySound.play(); 
        } 
    } 
}