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Flex 3 Cookbook by Joshua Noble, Todd Anderson

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Chapter 1. Flex and ActionScript Basics

A Flex application consists primarily of code written in two different languages: ActionScript and MXML. Now in its 3.0 incarnation, ActionScript has gone from a prototype-based scripting language to a fully object-oriented, strictly typed, ECMAScript language. MXML is a markup language that will feel comfortable immediately to anyone who has spent time working with Hypertext Markup Language (HTML), Extensible Markup Language (XML), or a host of newer markup-based languages.

How do MXML and ActionScript relate to one another? The compiler, after parsing through the different idioms, translates them into the same objects, so that

<mx:Button id="btn" label="My Button" height="100"/>

and

var btn:Button = new Button();
btn.label = "My Button";
btn.height = 100;

produce the same object. The major difference is that whereas creating that object in ActionScript (second example) creates the button and nothing else, creating the object in MXML adds the button to whatever component contains the MXML code. The Flex Framework handles calling the constructor of the object described in MXML and either adding it to the parent or setting it as a property of a parent.

MXML files can include ActionScript within an <mx:Script> tag, but ActionScript files cannot include MXML. Although it’s tempting to think of MXML as the description of the appearance and components that make up your application, and of ActionScript as the description of the event handlers and custom logic your application requires, this is not always true. A far better way to think of their relationship is to understand that both languages ultimately describe the same objects via different syntax. Certain aspects of the Flash platform cannot be accessed without using ActionScript for loops, function declarations, and conditional statements, among many others. This is why the use of ActionScript and the integration between MXML and ActionScript is necessary for all but the very simplest applications.

This chapter discusses many aspects of integrating MXML and ActionScript: creating components in MXML, creating classes in ActionScript, adding event listeners, creating code-behind files by using ActionScript and MXML, and creating function declarations. Although it doesn’t contain all the answers, it will get you started with the basics of ActionScript and MXML.

1.1. Create a Flex Project in Flex Builder

Problem

You want to create a project in Flex Builder.

Solution

Use the Create New Project wizard.

Discussion

Flex Builder is built on top of Eclipse, the venerable and well-respected integrated development environment (IDE) most strongly associated with Java development. Although Flex Builder certainly is not necessary for Flex development, it is the premier tool for creating Flex applications and as such provides a wealth of features to help you design and develop applications more effectively. You can use Flex Builder either as a stand-alone application or as a plug-in to an existing installation of Eclipse.

The first thing you need to do as a Flex developer is to create a Flex project. A Flex project is different from the other types of projects in Flex Builder because it includes theSWC (Flex library) Flex library SWC (unlike an ActionScript project) and is compiled to a SWF file that can be viewed in the Flash Player (unlike a Flex Library project). To create a project, right-click or Control-click (Mac) in Flex Builder’s project navigator to display the contextual menu (Figure 1-1), or use the File menu at the top of the application. From either, choose New→Flex Project. A dialog box appears to guide you through creating a project.

Creating a new Flex project

Figure 1-1. Creating a new Flex project

When prompted to specify how the project will get its data, choose Basic, which brings you to the New Flex Project dialog box (Figure 1-2).

Creating a new project in Flex Builder

Figure 1-2. Creating a new project in Flex Builder

Enter an application name and below, a location where the files will be stored on your system. The default location is C:/Documents and Settings/Username/Documents/workspace/Projectname on a Windows machine, and Users/Username/Documents/workspace/Projectname on a Mac. You can of course, uncheck Use Default Location and store your files wherever you like. The name of the project must be unique. The Application Type section lets you select whether you are making an Adobe Integrated Runtime (AIR) application or an application that will run in a browser via the Flash Player plug-in. Finally, the Server Technology settings let you indicate whether the application will be connecting to a server, and if so, what server type and separate configuration type are needed.

If you have nothing more to add, click Finish. To change the location where the compiled SWF file will be placed, click Next to reach the screen shown in Figure 1-3.

Setting the location where the compiled SWF will be placed

Figure 1-3. Setting the location where the compiled SWF will be placed

After the location of the generated SWF has been set, you can either finish or add source folders or SWC files to the project. To add another folder or set of folders, click the Source Path tab (Figure 1-4). To add SWC files to the project, click the Library Path tab (Figure 1-5). On this screen, you can also change the main MXML application file, which is by default the same name as the project.

Setting the source folder and main application file

Figure 1-4. Setting the source folder and main application file

Setting any additional libraries for a Flex project

Figure 1-5. Setting any additional libraries for a Flex project

With all paths and names specified, click Finish. Your project is now configured, and you are ready to begin development.

1.2. Create a Flex Library Project in Flex Builder

Problem

You need to create a Flex Library project.

Solution

From the Flex Navigator, choose New→Flex Library Project to access the Create New Project wizard.

Discussion

A Flex Library project does not have a main MXML file that it is compiled into a SWF. Instead the files are compiled into a SWC file that can be used in other applications or as the source for a runtime shared library (usually referred to as an RSL). The classes within the library are used to create a group of assets that can be reused in multiple projects at either compile time or runtime. To create a Flex Library project, right-click or Control-click (Mac) in the Flex Builder’s project navigator to open the contextual menu (Figure 1-6) or use the File menu. In either case, then choose New→Flex Library Project.

Creating a Flex Library project

Figure 1-6. Creating a Flex Library project

In the resulting dialog box (Figure 1-7), specify a name for your project as well as its location.

Setting the project location and SDK for the compiler

Figure 1-7. Setting the project location and SDK for the compiler

If you have nothing more to add, click Finish now. If you need to include files, assets, or other SWC files, including the Adobe AIR libraries, click Next and select them from the resulting screen. To set classes that can be selected and added into the recipe, first browse to a source path you would like to include and then set either classes or graphical assets that will be compiled into the library. Click Finish to create the project.

1.3. Create an ActionScript Project

Problem

You want to create an ActionScript project that does not use the Flex 3 libraries.

Solution

Use the Create New Project wizard and select ActionScript Project.

Discussion

An ActionScript project is different from a Flex project in that it does not include the Flex Framework at all. ActionScript projects rely on the core ActionScript classes within the Flash code base and do not have access to any of the components in the Flex Framework. To create an ActionScript project, choose File→New→ActionScript Project (Figure 1-8).

Creating an ActionScript project

Figure 1-8. Creating an ActionScript project

In the resulting dialog box, specify a name for the project and a location where the files and compiled SWFs will reside. Click Finish to finalize the project with default settings, or click Next to add libraries or other source folders to the project, change the main file of the application, add SWC files that your code can access, or change the location of the output SWF. By default, the name of the main ActionScript file for the application will be set as the name of the project. The default output location of the SWF file will be the bin-debug folder in the project.

1.4. Set Compiler Options for the MXML Compiler in Flex Builder

Problem

You need to set specific compiler options for the MXML compiler.

Solution

Set the options for the compiler arguments in the Flex Compiler screen of the Project Properties dialog box.

Discussion

The MXML compiler, also called the mxmlc, is the application that compiles ActionScript and MXML files into a SWF file that can be viewed in the Flash Player. When you run or debug a Flex application in Flex Builder, the MXML compiler is invoked and the files are passed to the compiler as an argument to the application. When you debug the player, an argument to create a debug SWF is passed to the MXML compiler. Flex Builder lets you pass other arguments to the MXML compiler, as well; for example, you can pass arguments to specify the location of an external library path, allow the SWF to access local files, or set the color of the background.

To change the compiler settings for a project, right-click or Control-click (Mac) on the project and select Properties from the contextual menu (Figure 1-9), or choose Project→Properties from the menu bar.

Changing the properties of a project

Figure 1-9. Changing the properties of a project

In the resulting Project Properties dialog box (Figure 1-10), select Flex Compiler. Here you have several options to control how the SWF file is compiled. In the input field labeled Additional Compiler Arguments, you can add multiple options; simply type a hyphen (-) in front of each option and separate the options with spaces.

Setting compiler options

Figure 1-10. Setting compiler options

Some of the most commonly used options are as follows:

verbose-stacktraces

Specifies whether the SWF will include line numbers and filenames when a runtime error occurs. This makes the generated SWF larger, and a SWF with verbose-stacktraces is different than a debug SWF.

source-path path-element

Adds any directories or files to the source path to have any MXML or ActionScript files be included. You can use wildcards to include all files and subdirectories of a directory. Also you can use += to append the new argument to the default options or any options set in a configuration file, for example:

-source-path+=/Users/base/Project
include-libraries

Specifies a SWC file to be compiled into the application and links all the classes and assets in the library into the SWF. This option is useful if the application will load in other modules that may need access to the classes in a SWC that the SWF will not be using.

library-path

Similar to the include-libraries option but includes only classes and assets that are used in the SWF. This lets you keep the size of the SWF file manageable.

locale

Specifies a locale to be associated with a SWF file. For example, use -locale=es_ES to specify that the SWF is localized for Spanish.

use-network

Indicates whether the SWF will have access to the local file system and is intended for use on a local machine, or whether the standard Flash Player security will apply. For example, use -use-network=false to specify that the SWF will have local file system access but will not be able to use any network services. The default value is true.

frames.frame

Enables you to add asset factories that stream in after the application and then publish their interfaces with the ModuleManager class. The advantage of doing this is that the application starts faster than it would have if the assets had been included in the code, but does not require moving the assets to an external SWF file. One of the more difficult and more useful parameters.

keep-all-type-selectors

Ensures that all style information, even if it is not used in the application, is compiled into the SWF. This is important if the application will be loading other components that require style information. The default value is false, which means that style information not used in the application is not compiled into the SWF.

After setting the options for the compiler, click the Apply button to save the options for that project.

1.5. Compile a Flex Project Outside of Flex Builder

Problem

You are not using Flex Builder for your Flex project and need to compile your project.

Solution

Use a terminal or command prompt to invoke the MXML compiler.

Discussion

Although Flex Builder is a powerful tool for Flex development, it is certainly not necessary for creating Flex applications. The MXML compiler (mxmlc) is free to anyone and can be downloaded from Adobe. To compile a Flex application outside of Flex Builder, open a command prompt (Windows) or a terminal (Mac OS X), invoke the MXML compiler, and pass the file containing the application as an argument, using a command such as the following:

home:base$ . /Users/base/Flex SDK 3/bin/mxmlc ~/Documents/FlexTest/FlexTest.mxml

This will compile the MXML file into a SWF that by default compiles into the folder where the MXML file is located. Any warnings or errors from the compiler will be displayed in the terminal or command-prompt window. To add further options to the MXML compiler, you append arguments to the call to the compiler. For example:

home:base$ ./mxmlc ~/Documents/FlexTest/FlexTest.mxml  -output=/Users/base/test/genera
ted/Index.swf -library-path+=/Users/lib/MyLib.swc

generates a SWF file named Index.swf, places it in the directory at /Users/base/test/generated/, and includes the SWC library /Users/lib/MyLib.swc.

To invoke the MXML compiler directly from the command line without providing the full path to your SDK installation (which in this example is C:\flex_sdk_3), you will need to add the /bin directory the compiler resides in to the Path systems variable.

On Windows:
  1. Open System from the Control Panel.

  2. Select the Advanced tab.

  3. Click Environment Variables.

  4. Within the System variables grid, navigate to and double-click Path.

  5. In the Variable Value field, if the last character is not set to a semicolon (;), enter a semicolon and then the path to the /bin folder within your Flex SDK installation directory (Figure 1-11).

    Setting the Flex SDK 3 Path variable

    Figure 1-11. Setting the Flex SDK 3 Path variable

  6. With the path to the MXML compiler directory set, open a command prompt, navigate to your project directory, and enter the following command:

    C:\Documents\FlexTest> mxmlc FlexTest.mxml

    This generates the FlexTest.swf file within C:\Documents\FlexTest, just as the first command presented in this section does. Setting the path to the /bin directory of the Flex 3 SDK installation lets you invoke the compiler from any directory including, in this example, your current project directory.

  7. If step 6 results in the following error message

    Error: could not find JVM

    you must manually enter the path to the directory in which the Java Runtime Environment (JRE) is installed on your machine. To manually enter the path, navigate to the /bin directory of your Flex 3 SDK installation. Open the jvm.config file in a text editor, and append the path to your JRE installation directory to the variable java.home. Assuming the Java installation is on the root of your drive, you enter the following:

    java.home=C:/Java/jre
On Mac OS X or Linux:
  1. Open your .bash_profile file (if you are using Bash) and edit the path variable by adding the location of the MXML compiler. Your .bash_profile file should look something like this:

    PATH="${PATH}:~/flex3SDK/bin"
    export PATH

    The .bash_profile will be located in your user home directory (which you can always access via a command line by typing cd ~). If you are using tsch, the path to the MXML compiler should be added to the .profile file.

  2. If the Java runtime is not set properly, set the following path variable in your terminal shell:

    PATH="${PATH}:~/flex3SDK/bin"
    export PATH

1.6. Add an Event Listener in MXML

Problem

You need to add an event listener in MXML that will listen for any events dispatched by children within the MXML file.

Solution

Pass a method name to the event tag of the component either with or without an event object sent.

Discussion

Flex components dispatch events whenever an action occurs, such as a user clicking a button, the selected item in a combo box changing, or data loading. To listen to these events being broadcast, simply add a reference to a function that will handle the event. For example:

<mx:Canvas xmlns:mx="http://www.adobe.com/2006/mxml" width="400" height="300">

    <mx:Script>
        <![CDATA[

            private function buttonClick():void
            {
                trace(" Button has been clicked ");
            }

        ]]>
    </mx:Script>

    <mx:Button click="buttonClick()" label="Click Me"/>
</mx:Canvas>

Adding click="buttonClick()" invokes the function buttonClick whenever the button dispatches a click event.

You can also pass the event object itself to the function. Every time a component dispatches an event, the component sends an object of type Event that any object listening to the event can receive. For example:

<mx:HBox xmlns:mx="http://www.adobe.com/2006/mxml" width="400" height="300">

    <mx:Script>
        <![CDATA[

            private function buttonClick(event:Event):void
            {
                trace(event.target.id);
                if(event.target.id == "buttonOne")
                {
                    trace(" button one was clicked")
}
                else
                {
                    trace(" button two was clicked")
}
            }

        ]]>
    </mx:Script>

    <mx:Button click="buttonClick(event)"
label="Click Me One" id="buttonOne"/>
    <mx:Button click="buttonClick(event)" label="Click Me Two" id="buttonTwo"/>
</mx:HBox>

By telling the event listener to listen for an object of type Event, you can send the event to the event listener and then respond to that event in different ways depending on specified criteria. In this example, the response depends on where the event originated.

The event object and the event dispatching system in Flex are some of the most important things to understand. All events contain a type that is used when the event is being listened for; if an event is of type click, then the event-listening method will be added to the click event of the child:

<mx:Button click="trace('I was clicked')"/>

Notification for user interactions, messages sent to an application from a server, or timers are sent via events. The event object defines several properties that you can access in any listening function. They are as follows:

bubbles

Indicates whether an event is a bubbling event, that is, whether it will be redispatched from the object that has received the event to any listeners further up the event chain.

cancelable

Indicates whether the behavior associated with the event can be prevented.

currentTarget

The object that is actively processing the event object with an event listener.

eventPhase

The current phase in the event flow.

Target

The event target, which is the object that has dispatched the event.

Type

The type of event.

You can also write event handlers in the MXML itself by using the binding tags {} to indicate that the code inside of the braces should be executed when the event is fired. For example:

    <mx:Button click="{textComponent.text = 'You clicked the button}" label="Click Me"/
>
    <mx:Text id="textComponent"/>

When it compiles this code, the Flex compiler creates a function and then sets textComponent.text = 'You clicked the button' as the body of that function. It may look different from the previous method, but the end result of this function is the same: It listens for the event and executes its code. There’s nothing inherently wrong with this approach, but for anything more complex than setting a single property, use a defined function to make your code easier to read and understand.

1.7. Set Properties of a Child Defined in MXML in ActionScript

Problem

You need to set the properties of a child defined in MXML in a script tag as a part of a method.

Solution

Refer to the child component by its id property, and set properties or call methods by using the id property.

Discussion

It’s easy to think of the script part of a component as being somehow separate from the MXML part, but they are not separate at all. Consider an example:

<mx:HBox xmlns:mx="http://www.adobe.com/2006/mxml" width="400" height="300">
    <mx:Script>
        <![CDATA[

            private function changeAppearance():void
            {
                this.width = Number(widthInputField.text);
                this.height = Number(heightInputField.text);
            }

        ]]>
    </mx:Script>
    <mx:Image id="imageDisplay"/>
    <mx:Text text="Enter a width"/>
    <mx:TextInput id="widthInputField"/>
    <mx:Text text="Enter an height"/>
    <mx:TextInput id="heightInputField"/>
    <mx:Button click="changeAppearance()" label="Change Size"/>
</mx:HBox>

As you can see in the changeAppearance method, the use of this refers to the component itself, the HBox that contains the child components; that reference changes the width and height of the component. References to the two TextFields, widthInputField and heightInputField, are also being used to grab the text data from those TextInputs. Each TextInput is being referred to by its id, in much that same way that you would refer to the id of an element in Document Object Model (DOM) scripting. The id must be a unique name that can be used throughout the application to refer to the child components Within one component there is a single-level hierarchy, so no matter where a child is nested within the component (for example, within another child), the id still refers to that child. Look at a similar example of actively setting the property of a child:

<mx:VBox xmlns:mx="http://www.adobe.com/2006/mxml" width="520" height="650">
    <mx:Script>
        <![CDATA[

            private var fileName:String = "";

            private function saveResume():void
            {
                //....a service call to send the data and set the filename
                fileNameDisplay.text = "Your resume has been saved as "+fileName;
            }

        ]]>
    </mx:Script>
    <mx:Text id="fileNameDisplay" text="" width="500"/>
    <mx:RichTextEditor id="richTextControl" width="500" height="400"/>
    <mx:Button id="labelButton" label="Submit Resume" click="saveResume()"/>
</mx:VBox>

In the preceding example, the child component is referenced by its id property and can have its properties set by using that id. All components added in MXML are by default visible to any component that has access to the parent component.

1.8. Define Arrays and Objects

Problem

You need to define an Array object or a hash table—style object to store values or other objects.

Solution

Use the ActionScript syntax for creating a new object or array with a constructor call, or define them in MXML.

Discussion

As with many things in Flex, arrays and objects, the two most commonly used types for containing data, can be defined in ActionScript or in MXML. To define an array in MXML, use the <mx:Array> tag to wrap all the items inside the array:

<mx:Array>
        <mx:String>Flex</mx:String>
        <mx:String>Flash</mx:String>
        <mx:String>Flash Media Server</mx:String>
        <mx:String>Flash Lite</mx:String>
        <mx:String>AIR</mx:String>
    </mx:Array>

All the items in the array are accessible through their zero-based index. You can create multiple nested arrays in MXML by adding multiple Array tags within an array:

    <mx:Array>
        <mx:Array>
            <mx:String>Flex</mx:String>
            <mx:String>Flash</mx:String>
        <mx:Array>
    </mx:Array>

To create an object in MXML, use the <mx:Object> tag and add all the properties of the object and their values as attributes in the tag. For example:

    <mx:Object id="person" firstName="John" lastName="Smith" age="50" 
socialSecurity="123-45-6789"/>

The limitation of creating an object in MXML is that you cannot create multiple nested objects. By creating your objects within a script tag, however, you can create objects containing multiple complex objects within them. You can create an object by creating a variable of type Object and calling the constructor on it and then attaching properties later:

    var object:Object = new Object();
    var otherObject:Object = new Object();
    object.other = otherObject;

You can also create an object by simply using curly brackets to delimit the properties within the object. For example:

    var person:Object = {name:"John Smith", age:22, position:{department:"Accounting",
salary:50000, title:"Junior Accountant"}, id:303};

Note how for the Person object the position properties point to another object that contains its own distinct properties. Notice how you don’t even need to declare a variable name for the object that the position property will point to.

To create an array in ActionScript, create a variable and call the Array constructor:

    var arr:Array = new Array("red", "blue", "white", "black", "green", "yellow");

You can also create an array without using the constructor by using square brackets and supplying any objects that will populate the array, as shown:

    var noConstructorArray:Array = [2, 4, 6, 8, 10, 12, 14, 16];

This is the same as calling the Array constructor and then passing the objects in the constructor.

1.9. Set the Scope of Variables in ActionScript

Problem

You need to make certain variables publicly accessible but protect others from external access.

Solution

Use the ActionScript scope modifiers for variables and methods.

Discussion

Within an ActionScript file or MXML file, there are different scopes of variables. Private variables and functions are visible only within the component itself; no other component can access them. This definition is useful for variables or methods that are used by a component only, that contain data only a single component should modify, or that are not modified except within the class. When you design complex classes, a good practice is to mark all properties that are not explicitly needed by outside components as private. Public variables are visible to any object that has a reference to the object of the class defining the property. Carefully considering which properties will be needed by outside classes and limiting access to those properties creates a far better class structure and frequently helps the programmer clarify the needs of a certain part of the application. Properties marked as private will not be visible to inheriting classes either, only the class or component in which they are defined. Finally, protected variables are accessible only to any object of a class that extends the class in which the property is defined, but not to any external objects.

Variables and functions marked private are visible only within the class or component in which they are defined and any inheriting class. For example, here is a class with protected and private properties:

package oreilly.cookbook
{
    public class Transport
    {
        protected var info:Object;
        private var speed:Object;
    }
}

By using the extends keyword, you can share the properties and methods of one class with another. In the following example, marking the Car class with the keyword extends the Transport class so that Car inherits all of its nonprivate properties and methods. Car does not have access to the private properties of the Transport class, however, and attempting to access them results in an error stating that the properties cannot be found.

package oreilly.cookbook
{
    public class Car extends Transport
    {
        public function set data(value:Object):void
        {
            info = value;
            speed = value.speed; /* this will throw an error because the speed 
variable is private and access to it is not allowed to any other class */
        }
    }
}

The protected properties of the Transport class are available to the Car class, but they are not available to any class that has an instance of either of these classes as a property.

Any public properties of both classes, however, are available to any class that instantiates one of these objects. The static keyword indicates that any object that references the class can access the specified property without needing to instantiate an object of that class. In other words, the static keyword defines a variable for all instances of a class. Adding the following line to the Car class

public static const NUMBER_OF_WHEELS:int = 4;

means you can now access the NUMBER_OF_WHEELS property with a reference to the class without needing to instantiate an object of that class, such as

    import oreilly.cookbook.Car;
    public class CompositionTest
    {
        private var car:Car;

        public function CompositionTest()
        {
            trace(" a Car object has "+Car.NUMBER_OF_WHEELS+" wheels");
        }
    }

So far, this recipe has examined the scope of variables that are affiliated with classes. Variables, however, need not be properties of a class. They can be created and destroyed within a function, being meaningful only within a function body. For example:

private function processSpeedData():void
{
    var speed:int;
    var measurement:String = "kilometers";
}

Outside the body of this function, the variables speed and measurement are meaningless. ActionScript uses something called variable hoisting, which means that variables defined anywhere within a function are in scope throughout the function. Unlike languages that support block-level scoping, the following code traces the newCar object as being valid:

        private function makeCars():void
        {
            for(var i:int = 0; i<10; i++)
            {
                var newCar:Car = new Car();
                carArray.push(newCar);
            }
            trace(newCar);
        }

The reference to newCar goes out of scope only when the function returns.

ActionScript also defines a final modifier that indicates to the compiler that the method cannot be altered by any classes that extend that class. This modifier enables you to define methods that are not private but that will not be altered or overridden. For example, defining this method in the Transport class

public final function drive(speed:Number):void{ /* final and cannot be overriden in 
any other class*/ }

indicates that any class that extends Transport will possess this method. Its public scoping means any properties will be accessible to any object with access to that Transport object but that subclasses cannot redefine the method.

1.10. Create a Component in ActionScript

Problem

You want to create a component in ActionScript without using any MXML.

Solution

Create an ActionScript file and extend one of the Flex library components.

Discussion

In addition to creating components in MXML, you can create them in ActionScript without using any MXML at all. You just need to do a few things differently. The first is to ensure that your class is properly packed in relation to the main application file. In the following example, the component is from the application-level folder; specifically, it’s within oreilly/cookbook/, which is reflected in the package name:

package oreilly.cookbook
{

The next difference is that any classes to be included or referenced in the component must be imported by using their full package name. This includes any class that the component will extend, in this case mx.containers.Canvas:

    import mx.containers.Canvas;
    import mx.controls.Text;
    import mx.controls.Image;
    import oreilly.cookbook.Person;

    public class PersonRenderer extends Canvas
    {

Any constants and variables for the components are listed just below the class declaration generally. In the following example, all the private properties of this class are listed. These are properties that the component can access but that no other component can access or alter. To access these properties, get and set methods will be provided so that if any processing is necessary when these properties are accessed, it can be done when the property is accessed. Getter and setter methods are common ways to provide access to private variables through functions.

        private var _data:Object;
        private var nameText:Text;
        private var ageText:Text;
        private var positionText:Text;
        private var image:Image;

In ActionScript, the constructor function always must be public, not return a value, and have the same name as the class itself. For example:

        public function PersonRenderer ()
        {
            super();

Any components that will be added to the component need to have their constructors called and then passed as a parameter to the addChild method in order to be added to the display list and so that their properties can be altered by the component.

            nameText = new Text();
            addChild(nameText);
            ageText = new Text();
            addChild(ageText);

In the following example, the ageText Text component is manually positioned, a necessity because its component, PersonRenderer, is a Canvas and does not have any layout management, unlike a VBox or HBox component.

            ageText.y = 20;
            positionText = new Text();
            addChild(positionText);
            positionText.y = 40;
            image = new Image();
            addChild(image);
            image.y = 60;
        }

If a component already defines a method to set data, such as mx.containers.Canvas here, you must override the method if you want the component to perform any custom actions. To do so, use the override keyword to indicate to the compiler that you intend to override the method of the superclass. For example:

        override public function set data(value:Object):void
        {
            _data = value;
            nameText.text = value.name;
            ageText.text = String(value.age);
            positionText.text = value.position;
            image.source = value.image;
        }

        override public function get data():Object
        {
            return _data;
        }

A final method finishes the component; this one originates in the class and is publicly scoped:

        public function retrievePerson():Person
        {
            /* do some special employee processing */
            return null;
        }
    }
}

To add this class to any other component, you can use ActionScript:

    var renderer:PersonRenderer  = new PersonRenderer();
    addChild(renderer);

Or you can use MXML:

    <renderers:PersonRenderer id="renderer"/>

In ActionScript, the constructor is explicitly called, and in the MXML version it is called when the constructor for the component that the PersonRenderer object is nested inside of gets called.

1.11. Use Event Bubbling

Problem

You want to listen for events passed up from children components to parent components without adding a long chain of event listeners.

Solution

Use the event-bubbling mechanism in the Flash Player to listen for events passed up from children.

Discussion

Understanding bubbled events requires looking at several classes. Several types of events can be bubbled up: mouse-down events, click events, and keyboard events. The term bubbling up refers to the event working its way up through the display list to the application container, like a bubble rising to the surface through the water. When the user clicks on any component, that event is passed up through the hierarchy. This means that the parent of a component can listen on a component for a click event, and if one is dispatched anywhere within the child component, the parent will be notified. To have the parent listen for all events of a certain type within the child, the parent simply needs to add an event listener to that child to receive all bubbled-up events.

Consider this class defined in BubblingComponent.mxml:

<mx:HBox xmlns:mx="http://www.adobe.com/2006/mxml" width="400" height="200">
    <mx:Script>
        <![CDATA[
            private function sendClick():void
            {
                trace(" BubblingComponent:: click ");
            }
        ]]>
    </mx:Script>
    <mx:Button click="sendClick()"/>
</mx:HBox>

This component contains a button that will dispatch a click event up the display list to any component that contains an instance of the BubblingComponent. To listen to this event, use the click handler in a component that contains BubblingComponent:

<cookbook:BubblingComponent click="handleClick()" id="bubbler"/>

A BubblingHolder that contains a BubblingComponent could be defined as shown in the following code snippet:

<mx:Canvas xmlns:mx="http://www.adobe.com/2006/mxml" width="400" height="300" 
xmlns:cookbook="oreilly.cookbook.*"
 creationComplete="complete()">
    <mx:Script>
        <![CDATA[
            private function handleClick():void
            {
                trace(" BubblingComponentHolder:: click ");
            }
        ]]>
    </mx:Script>
    <cookbook:BubblingComponent click="handleClick()" id="bubbler"/>
</mx:Canvas>

This component will dispatch an event up to any component listening, even to the application level. When we add the BubblingHolder to the main application file

<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="vertical" 
xmlns:cookbook="oreilly.cookbook.*">
    <mx:Script>
        <![CDATA[
            public function createName():void
            {
                name = "Flex Cookbook";
            }
        ]]>
    </mx:Script>
    <cookbook:BubblingComponentHolder click="handleClick()"/>
</mx:Application>

the click event from BubblingComponent.mxml will be broadcast all the way up to the application level.

The sequence of events in a MouseEvent sends the information about the event, such as a click and the location of the click, up the display list through all the children, to the child that should receive the event, and then back down the display list to the Stage.

The Stage detects the MouseEvent and passes it down the display list until it finds the target of the event, that is, the last component that the user’s mouse is interacting with. This is called the capturing phase. Next, the event handlers within the target of the event are triggered. This is called the targeting phase, when the event is given an actual target. Finally, the bubbling phase occurs, sending the event back up the display list to any interested listeners, all the way back to the Stage.

1.12. Use a Code-Behind Model to Separate MXML and ActionScript

Problem

You want to use a code-behind model to keep ActionScript and MXML code separate.

Solution

Create a component in ActionScript that extends the Flex library class that provides any needed functionality and then add any properties and methods there. Then create an MXML file that extends that class you’ve created.

Discussion

If you’re familiar with ASP.NET development, you’ve doubtless heard the term code-behind—likewise, if you’re familiar with the notion of separating the controller and the view in any type of application that mixes markup and another language (Ruby on Rails, JavaServer Pages (JSP) developments, PHP, and so forth). Separating the actual layout elements from the code that controls them is a good strategy for keeping the different aspects of a view cleanly separated for readability and clarity. There are times when the sheer number of files required to use this approach throughout an application makes navigating a project difficult, because two files are created for each component. Moreover, separating business logic and view logic is frequently difficult and can lead to difficult-to-follow separations of code within a component. Many developers, however, prefer this approach, and sometimes it helps clarify an application and its workings.

To begin, first look at the behind part of the code-behind: a component that extends the class (mx.containers.Canvas in this example) and contains methods to listen for the component to be added to the stage, as well as a method that can handle any event but is intended to handle button clicks specifically.

package oreilly.cookbook
{
    import mx.containers.Canvas;
    import flash.events.Event;

    public class CodeBehindComponent extends Canvas
    {
        public function CodeBehindComponent()
        {
            super();
            addEventListener(Event.ADDED_TO_STAGE, addedToStageListener);
        }

        protected function addedToStageListener(event:Event):void
        {
            trace(" Added to Stage from Code Behind ");
        }

        protected function clickHandler(event:Event):void
        {
            trace(" Click handled from component "+event.target);
        }

    }
}

In this example, methods that would normally be marked as private are scoped as protected. This is because the code portion of the code-behind, the MXML, will inherit from the CodeBehindComponent class and to function, requires access to these methods. Here you have the MXML component of the component:

<cookbook:CodeBehindComponent xmlns:mx="http://www.adobe.com/2006/mxml" width="200" 
height="400" xmlns:cookbook="oreilly.cookbook.*">
    <mx:Button click="clickHandler(event)"/>
</cookbook:CodeBehindComponent>

1.13. Make Properties of a Component Bindable

Problem

You are creating a component and would like to allow properties on that component to be bindable, for other components to bind to.

Solution

Create getter and setter methods, and mark those methods with a Bindable metadata tag that contains the name of the event that the methods will dispatch when the property is set.

Discussion

Any object can define bindable properties by dispatching an event when the property’s changed and using the Bindable metadata tag above the property. The best practice is to use get and set functions to define these bindable properties. When the property is set, an event is dispatched using the same name indicated in the Bindable tag. For example:

package oreilly.cookbook
{
    import flash.events.EventDispatcher;
    import flash.events.Event;

    public class Person extends EventDispatcher
    {
        public static var NAME_CHANGE:String = "nameChange";
        private var _name:String;

        [Bindable(event=NAME_CHANGE)]
        public function get name():String
        {
            return _name;
        }

        public function set name(value:String):void
        {
            dispatchEvent(new Event(NAME_CHANGE));
            _name = value;
        }
    }
}

The Bindable tag requires the name of the event that will be dispatched when the name property is set. This ensures that any component binding a property to the name property of the Person object is notified when the value changes.

Now that the bindable property has been set on the Person object, you can use all instances of Person in binding expressions:

<mx:Canvas xmlns:mx="http://www.adobe.com/2006/mxml" width="400" height="300">
    <mx:Script>
        <![CDATA[

        [Bindable]
        private var _person:Person;

        public function set person(person:Person:void
        {
            _person = person;
        }

        ]]>
    </mx:Script>
    <mx:Label text="{_person.name}"/>
</mx:Canvas>

1.14. Use Custom Events and Dispatch Data with Events

Problem

You want to dispatch data with an event by using a custom event class.

Solution

Create a class that extends the flash.events.Event class and create a property for the data that you would like to be available from the event.

Discussion

At times you may need to dispatch data objects with events so that listeners can access that data without accessing the object that dispatched the event. Renderers or deeply nested objects that are dispatching events up through multiple components to listeners will frequently want to send data without requiring the listening component to find the object and access a property. As a solution, create an event type and add any data types that you want to include to the constructor of the event. Remember to call the super method of the Event class so that the Event object is properly instantiated. For example:

package oreilly.cookbook
{
    import flash.events.Event;

    public class CustomPersonEvent extends Event
    {

        public var person:Person;
        public var timeChanged:String;

        public function CustomPersonEvent(type:String, bubbles:Boolean=false, 
cancelable:Boolean=false, personValue:Person=null, timeValue:String="")
        {
            super(type, bubbles, cancelable);
            person = personValue;
            timeChanged = timeValue;
        }

        override public function clone():Event
        {
            return new CustomPersonEvent(type, bubbles, cancelable, personValue, 
timeValue);
        }
    }
}

In this custom Event class, the inherited Event.clone method is overridden as well in order for the CustomPersonEvent to be able to duplicate itself. If an event listener attempts to redispatch this custom event, as shown here:

private function customPersonHandler(event:CustomPersonEvent):void {
    dispatchEvent(event);
}

the event that is dispatched is not the event that is received; instead, it is a copy of the CustomPersonEvent created using the clone method. This is done inside the flash.events.EventDispatcher class. If the clone method is not overridden to ensure that all properties of the CustomPersonEvent are carried into a clone of itself, then the event returned from the clone will be of type flash.events.Event and will not have any properties of the CustomPersonEvent.

1.15. Listen for a Keyboard Event

Problem

You need to listen for the user to press a key and determine which key was pressed and handle the event accordingly.

Solution

Add an event listener for the keyDown event either on the component or on the stage of the application and read the KeyboardEvents keyCode property.

Discussion

To listen for a KeyboardEvent, use the keyDown event handler, which all classes that extend UIComponent possess. The KeyboardEvent class defines a keyCode property which contains the code for the key that the user pressed. For example:

<mx:HBox xmlns:mx="http://www.adobe.com/2006/mxml" width="400" height="300" 
keyDown="keyHandler(event)" backgroundColor="#0000ff">
    <mx:Script>
        <![CDATA[

            import flash.events.KeyboardEvent;

            private function keyHandler(event:KeyboardEvent):void
            {
                switch(event.keyCode)
                {
                    case 13:
                        trace(" Enter pressed ");
                    break;
                    case 32:
                        trace(" Space Bar pressed ");
                    break;
                    case 16:
                        trace(" Shift Key pressed ");
                    break;
                    case 112:
                        trace(" F1 pressed ");
                    break;
                    case 8:
                        trace(" Delete pressed ");
                    break;
                }
            }

        ]]>
    </mx:Script>
    <mx:Button label="One"/>
</mx:HBox>

A note about this component: It will listen only for events that occur while the button has focus. If you remove the button from this component, there is nothing left that can have focus, and the keyHandler function will never be called. To catch all KeyEvents that occur in the application, whether or not the component has focus, add the following to the opening tag of the component:

     addedToStage="stage.addEventListener(KeyboardEvent.KEY_DOWN, keyHandler)"

This ensures that the keyHandler method will handle all KeyEvents that the stage catches, which would be all of them.

1.16. Define Optional Parameters for Methods

Problem

You want to define methods for a parameter that have default values or null values so that the method does not always need those values passed.

Solution

Use default values or null values in the method declaration by setting the parameter equal to a default value or equal to null.

Discussion

To define an optional method or multiple optional methods for a method, simply set the default value of an object to null in the signature of the event. The ActionScript primitives String, Number, int, and Boolean cannot be null values, however; you must supply a default value. For example:

        public function optionalArgumentFunction(value:Object, string:String, 
count:int = 0, otherValue:Object = null):void
        {
            if(count != 0)
            {
               /*if the count is not the default value handle the value the call 
passes in*/
            }
            if(otherValue != null)
            {
               /* if the otherValue is not null handle the value the call passes in */
            }
        }

Another strategy for providing not only optional parameters to the method but also an indeterminate number of arguments is to use the ... marker in front of a variable name. This is referred to officially as the ...(rest) parameter. This variable will contain an array of arguments that can be looped over and processed.

        public function unlimitedArgumentsFunction(...arguments):void
        {
            for each(var arg:Object in arguments)
            {
                /* process each argument */
            }
        }

1.17. Determine the Type of an Object

Problem

You need to determine the type of an object that has been passed to a method.

Solution

Use the is operator to determine the object’s type or the superclass of the object’s type.

Discussion

To determine the type of an object, ActionScript provides the is operator, which tests the type of an object and returns true or false. The is operator returns true if the object is of the type tested against or if the object extends the type indicated. For example, because the Canvas object extends UIComponent, the is operator returns true if the Canvas object is tested as being of type UIComponent. A UIComponent however, will not return true if tested as being of type Canvas, because UIComponent does not inherit from Canvas. Consider this code:

        public function TypeTest()
        {
            var uiComponent:UIComponent = new UIComponent();
            var canvas:Canvas = new Canvas();
            trace(" uiComponent is UIComponent "+(uiComponent is UIComponent));
            trace(" uiComponent is Canvas "+(uiComponent is Canvas));
            trace(" canvas is UIComponent "+(canvas is UIComponent));
        }

which produces the following output:

 uiComponent is UIComponent true
 uiComponent is Canvas false
 canvas is UIComponent true

A common use of type testing is to determine the component that has thrown an event. This lets you use a single method for simple event handling and test the type of the object to determine appropriate actions.

        private function eventListener(mouseEvent:MouseEvent):void
        {
            if(mouseEvent.target is Button)
            {
                /* handle button specific actions */
            }
            else if(mouseEvent.target is ComboBox)
            {
                /* handle combobox specific things */
            }
            else
            {
                /* handle all other cases */
            }
        }

1.18. Define and Implement an Interface

Problem

You need to create an interface and then create a component that implements that interface.

Solution

Create an ActionScript file, declare that file as an interface, and define any methods you would like the interface to require. To implement the interface, use the implements keyword in the class declaration of the component that will use the interface.

Discussion

Interfaces are powerful tools that let you describe a contract that an object must fulfill: the interface must contain a specified set of methods with a certain scope, name, parameters, and return type; components using the object, in turn, will expect that this set of methods is present. This lets you create lightweight descriptions of a class without actually creating a new class that would clutter your inheritance trees. Classes that implement an interface are considered to be of that interface type, and this can be used to set the types for parameters of methods or to set the return types of methods as shown here:

public function pay(payment:IPaymentType):IReceipt

This method can accept any object that implements IPaymentType and will return an object that implements the IReceipt interface.

The interface cannot define the method body nor can it define any variable. In the following code snippet, IDataInterface is declared and defines five methods that any object that implements the interface must also possess and define:

package oreilly.cookbook
{
    public interface IDataInterface
    {
        function set dataType(value:Object):void;

        function get dataType():Object;

        function update():Boolean;

        function write():Boolean;

        function readData():Object;
    }
}

To implement the interface, declare the class and add the implements marker to the class declaration. All methods defined in an interface must be implemented by the class. In the following code snippet, all the methods of the preceding interface are included and are given function bodies:

package oreilly.cookbook
{
    import flash.events.EventDispatcher;
    import flash.events.IEventDispatcher;

    public class ClientData extends EventDispatcher implements IDataInterface
    {

        private var _dataType:Object;

        public function ClientData(target:IEventDispatcher=null)
        {
            super(target);
        }

        public function set dataType(value:Object):void
        {
            _dataType = value;
        }

        public function get dataType():Object
        {
            return _dataType;
        }

        public function update():Boolean
        {
            //do the actual updating
            var updateSuccessful:Boolean;
            if(updateSuccessful)
            {
                return true;
            }
            else
            {
                return false;
            }
        }

        public function write():Boolean
        {
            var writeSuccess:Boolean;
            if(writeSuccess)
            {
                return true;
            }
            else
            {
                return false;
            }
        }

        public function readData():Object
        {
            var data:Object;
            //get all the data we need
            return data;
        }
    }
}

To implement an interface in MXML, use implements. in the top-level tag for the component. For example:

<mx:HBox xmlns:mx="http://www.adobe.com/2006/mxml" width="400" height="300" 
implements= "IDataInterface">

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