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A Sample Migration: Chapter 18 - ActionScript 3.0 Quick Reference Guide

by David Stiller, Rich Shupe, Darren Richardson, Jen deHaan

This excerpt is from The ActionScript 3.0 Quick Reference Guide. If you're ready to give your Flash projects a considerable performance boost, learning ActionScript 3.0 is a must. This Quick Answer Guide is designed specifically to help Flash designers and developers make the leap from ActionScript 2.0 to the new object-oriented ActionScript 3.0 quickly and painlessly. You'll learn key differences between the two language versions, allowing you to more easily leverage ActionScript 3.0 using Flash CS4 and other Adobe technologies like Flex and AIR.

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Table of Contents

A Simple Particle System
ActionScript 2.0
Main Timeline
Particles Class
ActionScript 3.0
Main Timeline
Particles Class
Migration Sample Summary

You’ll typically find it fairly straightforward to choose which version of ActionScript to use when you start a new project. You usually decide based on which version of the Flash Player you’re trying to target, the need for enhanced performance, or a specific feature you wish to use. Deciding what to do with existing projects, however, is another matter. Often you need to determine whether or not it’s worth the time and effort to migrate to ActionScript 3.0 from a prior version of the language, or just to start over.

You can’t easily determine what to do; each project’s characteristics probably significantly affect your decision. The clarity of your existing code, the extent to which you use particular features, project size, and your comfort level, among other factors, help you decide.

This chapter walks you through a small-scale, manufactured example of a migration from ActionScript 2.0 to ActionScript 3.0. Read the text linearly, as code isn’t always explained in detail twice. It is very important to understand that this example isn’t a demonstration of real-world best practices at work. This example is significantly constrained, and attempts to insert as many migration issues as possible into its tiny footprint. Throughout the example, what may appear to be odd choices, poorly optimized code, or even mistakes, have been intentionally injected into the code to either set up a migration task or bring the two versions into a parallel structure.

For example, objects are added out of order intentionally to demonstrate depth management, different methods of providing or checking default values have been used, objects are created in a variety of ways, both component and custom buttons are used, function/method placement isn’t consolidated, and so on. The project demonstrates both timeline- and class-based coding, but is by no means an example of good object oriented programming practices. The class and timeline are coupled too tightly, to mention one example.

If you remember that this scenario is artificial, designed to illustrate migration issues, it may help you form a plan or checklist of sorts to help when it comes time to update your next legacy project.

A Simple Particle System

This representative example creates a particle system that performs a few basic tasks. At startup, it plays an ambient audio loop. It then attempts to draw a blue square particle upon each enter frame event, based on a simple conditional. If successful, the particle moves away from the center of the stage, rotating and fading to transparency. Each time a particle is created, its name is added to a text field.

Finally, two buttons add functionality to the system. The first opens a web page. The second selects the previously created particle, replaces its content with a larger red square, and plays a new random sound. The altered particle affects its associated sound, panning based on the particle’s position, and fading based on the particle’s alpha.

Both examples require a custom button with a linkage of "BtnLink" and a Button component. The ActionScript 2.0 FLA file also requires an empty movie clip called "Particles." These assets are provided in the downloadable source files (see Preface).

ActionScript 2.0

To start, the complete ActionScript 2.0 code will be presented, with numbered lines. The project features a single class used by a brief frame script in the main timeline. This structure helps demonstrate migration issues related to the use of classes, as well as general syntax. Discussion of each script follows, with a general explanation following the ActionScript 2.0 code, and migration comments following the ActionScript 3.0 code.

Main Timeline

The main timeline builds the user interface, and creates an instance of the Particles class by adding a movie clip to which the class is linked. The class does all the work with the particles, and will be explained in a moment.

Production: The lines of code in this chapter contain turnover characters and soft returns to indicate that code lines are continued on the next line. The continued line should not have a line number. Robyn

1  import mx.controls.Button;
2
3  if (!imgURL) {
4      var imgURL:String = "bg.jpg";
5  }
6
7  var txtFrmt:TextFormat = new TextFormat();
8  txtFrmt.align = "right";
9  var txtFld:TextField = this.createTextField("particleInfo", 1, 380, ¬
     10, 100, 380);
10 txtFld.setNewTextFormat(txtFrmt);
11
12 bg = this.createEmptyMovieClip("bckgrnd", 2);
13 bg.loadMovie(imgURL);
14 bg.swapDepths(txtFld);
15
16 var snd:Sound = new Sound();
17 snd.loadSound("../audio/bass_back.mp3");
18 snd.onLoad = function(success:Boolean):Void  {
19     if (success) {
20         snd.setVolume(10);
21         snd.start(0, 100);
22     }
23 };
24
25 var particles:MovieClip = attachMovie("Particles", "particles", 3);
26
27 var controls:MovieClip = this.createEmptyMovieClip("btns", 4);
28 controls._y = 360;
29
30
31 var linkBtn:MovieClip = controls.attachMovie("BtnLink","link", 5);
32 linkBtn._x = 20;
33 linkBtn.siteLink = "http://www.learningactionscript3.com/";
34 linkBtn.onRelease = function():Void  {
35     getURL(this.siteLink, "_blank");
36 };
37
38 var changePBtn:Button = controls.createClassObject(mx.controls.Button, ¬
     "chng", 6, {label:"Change"});
39 changePBtn.move(changePBtn.x + 120, 0);
40 function changeParticle():Void {
41     var p:MovieClip = particles["particle" + (particles.count - 1)];
42     p.clear();
43     p.beginFill(0xFF0000);
44     makeRect(p, -20, -20, 40, 40);
45     p.endFill();
46     particles.particleSound(p);
47 }
48 changePBtn.addEventListener("click", changeParticle);
49
50 function makeRect(mc:MovieClip, xp:Number, yp:Number, w:Number, ¬
     h:Number):Void {
51     mc.moveTo(xp, yp);
52     mc.lineTo(xp + w, yp);
53     mc.lineTo(xp + w, yp + h);
54     mc.lineTo(xp, yp + h);
55     mc.lineTo(xp, yp);
56 }

The following includes discussions of ActionScript 2.0 syntax, but also a basic explanation of the project—much of which applies to both ActionScript 2.0 and 3.0 versions.

Import

Line 1 imports the Button component class to make it accessible to the compiler.

FlashVars

Lines 3 through 5 check for the presence of a variable called imgURL, and then initialize it to the URL of a background image in case no value for the variable is found. This step lets you pass a path to a background image into the project through the HTML host file. If this feature isn’t used, the hard-coded background image isn’t displayed.

TextFormat

Lines 7 and 8 create a simple TextFormat instance to right-justify text in a field.

TextField

Lines 9 and 10 dynamically initialize a TextField. Line 9 creates the field and gives it an instance name of particleInfo, places the field at a depth of level 1, places the field at point (380, 10), and sizes the field to a width of 100 pixels and height of 380 pixels. Line 10 applies the previously created TextFormat to the field.

Depth Management

ActionScript 2.0 requires that you set a level for every asset added to your project. Asset levels are hard-coded in lines 9, 12, 25, 27, 31, and 38. This step requires either careful preplanning or arbitrary level assignment (buttons between 100 and 200, movie clips between 300 and 400, and so on) and a good memory.

You can determine the next available level for symbol instances like movie clips, using the getNextHighestDepth() method. However, significant problems arise when adding components (which occurs later in the script) because the getNextHighestDepth() method can return errant levels beginning with 1048576. Not only does this wreak havoc with your level management, it’s outside the valid level range, making it impossible to remove assets dynamically.

To get around this problem, you can adopt the significantly more confusing approach of using the DepthManager class, created for managing depths of Version 2 Components, generally making things more difficult.

Line 14 swaps the depths of the text field and background image, making the background image the bottom-most asset. The text field and background were added in reverse order to demonstrate this feature.

Variable Declaration

ActionScript 2.0 allows sloppy variable use such as not declaring or typing the variable bg in line 12.

Image Loading

Lines 12 and 13 display a background image. Line 12 creates an empty movie clip, and sets its depth to level 2. Line 13 loads the image.

Sound

Lines 16 through 23 play a background sound. Line 16 creates an instance of the Sound class. Line 17 loads the sound.

Handling Events

Lines 18 through 23 create an event handler to process, in this case, a load complete event. Upon load completion, it sets the volume of the sound to 10 percent, so you can clearly hear other sounds atop this one, and plays the sound from the beginning, looping 100 times for longer play time. Another event handler appears in lines 34 through 36.

Void

Line 18 uses Void to tell the compiler that no data’s returned from the function. This action occurs again on lines 34 and 40.

Percent Values Scale

Line 20 manipulates a percent scale, and uses values between 0 and 100.

Dynamic Creation of Movie Clip and Instantiation of Custom Class

Line 25 creates an instance of a Library-based movie clip with a linkage name of Particles. It gives the clip an instance name of particles, and sets its depth to level 3. Note here that the Particles class responsible for particle control can only easily be integrated into the project by associating the class with a movie clip. This quality means that you have to have at least an empty movie clip in your Library, already set up with the appropriate class name.

Alternatives to this approach include manipulating the movie clip prototype, which is a bit messy and very difficult to bring forward into ActionScript 3.0, or switching to object-oriented techniques and using composition. Both approaches are significantly more involved than relying on a Library symbol.

Lines 27 and 31 also dynamically create movie clip instances. Line 27 creates an empty container (positioned in line 28) to hold buttons, and line 31 creates the first of two buttons, the functionally of which is discussed next. Both lines hard code depths, to 4 and 5, respectively.

Finally, line 38 also dynamically creates a movie clip equivalent, in this case a component. This step is relevant because, in the space of a dozen or so lines of code, you see three separate ways, each with unique characteristics, to place visible content on the stage.

Property Underscores

Line 28 demonstrates that the movie clip’s _y property, like most properties in ActionScript 2.0, is preceded by an underscore.

Custom Button Instantiation

Lines 31 through 36 add a Library-based custom button with a linkage name of BtnLink to the project. Line 31 places it into the controls movie clip, gives it an instance name of link, and sets its depth to level 5.

Line 32 positions the x location of the button. The y location remains 0, and appears to be 360 because the button is in the controls movie clip—which, itself, appears at a y location of 360.

Lines 34 through 36 add a mouse release event handler to the button.

Dynamic versus Sealed Classes

Line 33 dynamically creates a property called siteLink, and then populates it with a string. This step’s really nothing more than a variable, but here it’s an example of dynamic versus sealed classes. In ActionScript 2.0, you could add properties to instances of most classes, even though this practice wasn’t recommended. In ActionScript 3.0, however, most classes are sealed, meaning you can’t dynamically alter them in this way. You can use this approach only with select dynamic classes in ActionScript 3.0.

The siteLink property is being added to the button, which you can’t do in ActionScript 3.0. The the section called “ActionScript 3.0” discusses why this example was included, and also discusses another more directly analogous example.

Opening a URL

Line 35 opens a URL in a new window, getting the URL from the siteLink property of the button to which the event handler is attached.

Dynamically Instantiating a Component

Lines 38 through 48 add and empower a Button component. (As with a custom asset, a Button component must be in your Library.) Two different kinds of user-clickable objects demonstrate both the use of movie clips and the use of components.

Line 38 adds the button to the controls container movie clip, gives it an instance name of chng, sets it depth to level 6, and gives it a label of “Change.” Line 39 uses the button’s move() method to set it to an x position of 120, leaving the y position at 0, to be affected by the position of the parent container. The extraneous use of the x property (rather than setting the value to 120 directly) is to show that ActionScript 2.0 v2 components use properties without underscores, contrary to other properties.

Lines 40 through 48 apply an event listener to the button. Lines 40 through 47 define the function triggered by the button’s click event, set in line 48.

Lines 41 through 45 are discussed in the following paragraphs, and line 46 triggers the particleSound() method of the selected particle in the Particles class.

Accessing Objects by Instance Name

Line 41 creates a reference to the previously created particle by accessing the movie clips within the particles movie clip and finding the particle by instance name. The instance name is built with the string "particle" and the number of the current particle minus 1.

Using the Drawing API

Lines 42 through 45 clear the contents of the particle, create a red fill, call a function that draws a centered, 40 × 40 pixel rectangle in the selected particle, and close the fill.

Lines 50 through 56 use the lineTo() and moveTo() methods to draw a rectangle, as the drawRect() method doesn’t exist in ActionScript 2.0.

Particles Class

The Particles class creates each particle, establishes its behavior, and ultimately removes it from the project.

1 import mx.utils.Delegate;
2
3 class Particles extends MovieClip {
4
5         private var _count:Number;
6         private var _soundNum:Number = 0;
7         private var _tempSound:Sound;
8
9         public function Particles() {
10              _x = Stage.width / 2;
11              _y = Stage.height / 2;
12
13              _tempSound = new Sound();
14              _tempSound.loadSound("../audio/note0.mp3");
15              _tempSound.onLoad = Delegate.create(this, ¬
     onSoundPreloaded);
16          }
17
18          private function onSoundPreloaded(success:Boolean):Void {
19              if (success) {
20                  if (_soundNum < 7) {
21                      _soundNum++;
22                      _tempSound.loadSound("../audio/note" + _¬
     soundNum + ".mp3");
23                  }
24              }
25          }
26
27          private function onEnterFrame():Void {
28              makeParticle(0x0066CC, Math.random() * 10 + 10);
29          }
30
31          private function makeParticle(col:Number, ¬
     size:Number):Void {
32              if (!col){ col = 0x003366; }
33              if (!size){ size = 20; }
34
35              if (Math.random() * 10 <= 2) {
36                  if (_count == undefined) { _count = 0; }
37
38                  var p:MovieClip = this.createEmptyMovieClip("particle" + _¬
                     count, this.getNextHighestDepth());
39                  _count++;
40                  p.beginFill(col);
41                  _parent.makeRect(p, -size/2, -size/2, size, size);
42                  p.endFill();
43
44                  p.xVel = Math.random() * 10 - 5;
45                  p.yVel = Math.random() * 2 - 1;
46                  p.onEnterFrame = onRunParticle;
47
48                  _parent.txtFld.text += p._name + "\n"
49                  _parent.txtFld.scroll = _parent.txtFld.maxscroll;
50              }
51          }
52
53          public function particleSound(p:MovieClip):Void {
54              if (!p.snd && _soundNum > 0){
55                  var num:Number = int(Math.random() * _soundNum);
56                  p.snd = new Sound();
57                  p.snd.loadSound("../audio/note" + num + ".mp3");
58                  p.snd.onLoad = function(success:Boolean):Void {
59                      p.snd.start();
60                  }
61              }
62          }
63
64          private function onRunParticle():Void {
65              var p:MovieClip = this;
66              p._x += p.xVel;
67              p._y += p.yVel;
68              p._rotation += 5;
69              p._alpha -= 2;
70
71              if (p.snd) {
72                  p.snd.setVolume(p._alpha / 10);
73                  p.snd.setPan(p._x / this._parent._x * 200)
74              }
75
76              if (p._alpha <= 0) {
77                  p.removeMovieClip();
78              }
79          }
80
81          public function get count():Number {
82              return _count;
83          }
84  }

The following features a brief explanation of particle behavior. Where appropriate, comments have been added to address ActionScript 2.0-specific concepts and, occasionally, to explain decisions made to demonstrate migration issues. Basic syntax issues discussed in the main timeline aren’t mentioned again.

Import

Line 1 imports the Delegate class to make it accessible to the compiler.

Class Structure

Lines 3 and 9 make up the key elements of the ActionScript 2.0 class structure. You’ll see later that ActionScript 3.0 begins a class with the package identifier. Line 3 shows that this class extends MovieClip. (It’s linked to a Library movie clip.) The class constructor beginning on line 9 centers the particle system on the stage, and creates and preloads the first particle-specific sound.

Class Properties

Lines 5 through 7 create class properties but only initialize one. This step factors later into the use of default values.

Number Data Types

Lines 5 and 6 use the Number data type for integer counters because no other number data types are available. You can also see this property in lines 31 and 55. The value assigned in line 55, as a good example, is always an integer, but must still be typed as Number.

Access to the Stage

Lines 10 and 11 show that Stage is a global object.

Method Closure

Line 15 assigns the onLoad() event handler for the sound created and loaded in lines 13 and 14, respectively. Delegate is used here, however, to demonstrate that ActionScript 2.0 does not have method closures. Due to this setup, the Delegate class must pass the relevant scope to the event handler for it to access the needed properties.

Preloading Sounds

To prevent delays when triggering sounds later, lines 18 through 25 load seven additional sounds, but don’t play them. The name of the audio file contains indices 0 through 7. The first sound, note0.mp3, was loaded in the class constructor, so the soundNum property is incremented before the load.

Class Enter Frame Method

Lines 27 through 29 demonstrate that, because this class extends MovieClip, ActionScript 2.0 lets you create a method for a movie clip event handler, without any further assignment. That is, because the method has the same name as a MovieClip event handler, it’s executed upon every movie clip enter frame event. This method attempts to create a blue particle that’s between 10 and 20 pixels square.

Particle Creation

Lines 31 through 51 create each particle. Individual aspects of this function will be discussed in separate headers, but the basic creation process is as follows. Line 35 checks to see if a newly created random number between 0 and 10 is less than or equal to 2. If so, it creates the particle. This action both prevents a particle from being created on every enter frame, and adds a nice feeling of randomness to the process.

Each particle is drawn into an empty movie clip, with its depth set to the next highest available level. It’s given an instance name of particleN, where N is an integer from the _count variable, incremented each time a particle’s created. A fill of the color passed into the method is created, a rectangle is drawn using the makeRect() function discussed in the main timeline (using the size passed into the function), and the fill’s closed.

Next, random x and y velocities are chosen for each particle, providing movement between 5 and −5 for x and between 1 and −1 for y. The onRunParticle() method is then assigned as the enter frame event handler for each particle. (This step’s in contrast to the enter frame event handler assigned at the class level that creates the particles.)

Finally, the name of the particle, and a subsequent new line, are added to the text field in the main timeline (the particle’s parent). This step is discussed in detail in the upcoming note, “Manipulating Text Fields.”

Default Values

Lines 32 and 33 validate the argument values of col and size, assigning values if none are found. ActionScript 2.0 has no built-in mechanism for assigning default values in functions. As such, you must assign them manually.

Further, line 36 checks for a value of undefined in _count and, if found, initializes the property to 0. This demonstrates a change in the way ActionScript 3.0 handles default values, and this is also why _count wasn’t initialized in line 5.

Accessing Objects in the Parent

Lines 41, 48, and 49 all access objects in the parent, in this case the main timeline.

Manipulating Text Fields

The name of the particle, and a subsequent new line, are added to the text field in the main timeline (the particle’s parent). The text field is then scrolled to the bottom line so you can always see the newly added name.

Particle Sound

Lines 53 through 62 create and play particle-specific sound. A validation first tests to be sure a sound for this particle doesn’t already exist, and that at least one sound has preloaded (via the incremented counter in line 21). If so, a random number is chosen from the current number of preloaded sounds (line 57) and a new sound is created and stored in the snd property within the current particle. The random sound is then loaded and played upon load completion.

(Many people consider it a best practice to group private and public methods (and properties). However, this method’s optional, and has been neglected in order to arrange this example in a slightly more linear fashion for easier explanation.

Particle Behavior

Lines 64 through 79 establish the independent behavior for each particle. For each enter frame event, lines 66 and 67 add the x and y velocity values to the particle’s location, line 68 rotates the particle 5 degrees, and line 69 reduces the alpha by 2 percent.

Lines 71 through 74 controls particle-specific audio. The volume and pan of the sound are set according to the particle’s alpha and x coordinate, respectively.

Finally, lines 76 through 78 remove the particle when its alpha is less than or equal to 0.

Getter

The last three lines of the class create a getter that returns the value of the _count property when requested, as seen in line 41 of the main timeline frame script.

ActionScript 3.0

The following are the ActionScript 3.0 versions of the two previous scripts. Only language version-specific comments are included here so, if you’re unclear on overall functionality, see the equivalent ActionScript 2.0 section.

Main Timeline

The following is the main timeline frame script.

1  import fl.controls.Button;
2
3  var imgURL:String = "bg.jpg";
4  if (root.loaderInfo.parameters.imgURL) {
5      imgURL = root.loaderInfo.parameters.imgURL;
6  }
7  var txtFrmt:TextFormat = new TextFormat();
8  txtFrmt.align = TextFormatAlign.RIGHT;
9  var txtFld:TextField = new TextField();
10 txtFld.x = 380;
11 txtFld.y = 10;
12 txtFld.width = 100;
13 txtFld.height = 380;
14 txtFld.defaultTextFormat = txtFrmt;
15 addChild(txtFld);
16
17 var bg:Loader = new Loader();
18 addChildAt(bg, 0);
19 bg.load(new URLRequest(imgURL));
20
21 var snd:Sound = new Sound();
22 snd.load(new URLRequest("../audio/bass_back.mp3"));
23 snd.addEventListener(Event.COMPLETE, onSoundLoaded, false, 0, ¬
    true);
24 function onSoundLoaded(evt:Event):void {
25     var sndChannel:SoundChannel = new SoundChannel();
26     sndChannel = evt.target.play(0, 100);
27     var sndTransform:SoundTransform = sndChannel.soundTransform;
28     sndTransform.volume = .1;
29     sndChannel.soundTransform = sndTransform;
30     evt.target.removeEventListener(Event.COMPLETE, onSoundLoaded);
31 }
32
33 var particles:Particles = new Particles();
34 addChild(particles);
35
36 var controls:MovieClip = new MovieClip();
37 controls.y = 360;
38 addChild(controls);
39
40 var siteLink:String = "http://www.learningactionscript3.com/";
41 var linkBtn:SimpleButton = new BtnLink();
42 linkBtn.x = 20;
43 controls.addChild(linkBtn);
44
45 linkBtn.addEventListener(MouseEvent.CLICK, onShowLink, false, 0, ¬
     true);
46 function onShowLink(evt:MouseEvent):void {
47     navigateToURL(new URLRequest(siteLink), "_blank");
48 }
49
50 var changePBtn:Button = new Button();
51 changePBtn.x = 120;
52 changePBtn.label = "Change";
53 controls.addChild(changePBtn);
54
55 changePBtn.addEventListener(MouseEvent.CLICK, onChangeParticle,¬
    false, 0, true);
56 function onChangeParticle(evt:MouseEvent):void {
57     var p:MovieClip = MovieClip(particles.getChildByName(¬
     "particle" + (particles.count - 1)));
58     if (p != null) {
59         p.graphics.clear();
60         p.graphics.beginFill(0xFF0000);
61         p.graphics.drawRect(-20, -20, 40, 40);
62         p.graphics.endFill();
63         particles.particleSound(p);
64     }
65 }

The following is an explanation of ActionScript 3.0-specific issues that appear in the main timeline frame script.

Import

Notice in Line 1 that the class path has changed from mx to fl.

FlashVars

FlashVars are no longer stored as global variables in the root timeline. Instead, they’re stored in the parameters object of the LoaderInfo instance of the root, as seen in line 4.

TextFormat

Although you can use appropriate string values, it is a best practice to use relevant constants for many property values in ActionScript 3.0. In this case, the format’s align property is populated with the RIGHT constant of the TextFormatAlign class, used in line 8.

The application of the TextFormat instance in line 14 has been changed from setNewTextFormat() to defaultTextFormat().

TextField

All display objects are created with a simple consistent new <class name>() structure. The ActionScript 2.0 TextField creation method is replaced with the ActionScript 3.0 instantiation (line 9) and followed by the assignment of property values (lines 10 through 14).

Depth Management

The ActionScript 3.0 display list automatically handles depth management so you don’t have to manually assign levels or worry about methods like getNextHighestDepth() or the DepthManager. As such, you don’t see any level assignments in any of the object instantiation routines.

However, you can still control depths. For example, you still have a swapDepths() method for moving the background image below the text field, as seen in line 14 of the ActionScript 2.0 main timeline frame script code. However, you have an easier way to handle this when objects are added to the display list. In ActionScript 2.0, existing objects are replaced when a new object is added to the same level. ActionScript 3.0, however, moves all objects above the target level one level higher, and then inserts the addition where specified.

Therefore, you can easily place the background image behind the text field when the image is added, as seen in line 18. The addChildAt() method is used, specifying level 0. The background image appears in level 0, and the text field is moved to level 1.

Variable Declaration

While you can in some cases omit typing a variable, all variables must be declared with the var identifier.

Image Loading

Rather than creating an empty movie clip, a Loader display object is used lines (17 through 19). Instead of using the image path as a string for the load() method, ActionScript 3.0 requires a consistent use of the URLRequest class for processing the URL prior to use.

Sound

Although the creation of the Sound instance is the same (line 21), sound management diverges significantly from that point on. Loading is similar, with a change of method name to the more consistent load() and the ever-needed URLRequest instance instead of a string. The event handling is significantly different in ActionScript 3.0 (and is explained in a moment), but the idea behind it, as it pertains to sound, is the same: wait until the sound is loaded, and then proceed.

However, three new classes play a big part of sound management. First, each sound is typically played into its own discrete sound channel, an instance of the SoundChannel class (lines 25 and 26). This step is a requirement if you wish to perform sound transformations. Where the volume and pan properties existed in the Sound class in ActionScript 2.0, they’re now accessible through the soundTransform property of the SoundChannel class.

To effect such a change, an instance of the second new class, SoundTransform is derived from the SoundChannel soundTransform property (line 27), the desired property is changed (volume, line 28) and the new instance is reapplied to the SoundChannel soundTransform property once again (line 29).

The third new class, not used in this example, is the SoundMixer class. This class lets you manipulate all the sounds at once. The isolation of sounds into their own discrete channels lets you control each sound separately and with greater precision.

Handling Events

Event handling is very different in ActionScript 3.0. For detailed information, see Chapter 14, How Do I Work with Events?. From a migration standpoint, event handlers are no longer attached to the target of the event. Instead, event listeners are created, specifying an event to listen for, and a function to trigger upon an occurrence of that event (lines 23 through 31).

A mandatory parameter is used to receive information from the event that can be used inside the function. For example, the target of the event in the mentioned listener is the snd object. That is referenced by evt.target in lines 26 and 30.

The events are specified as constants, as discussed previously with the TextFormat align property, and optional parameters allow more granular control over when the event is processed (capture or target/bubbling phases and priority) as well as whether weak references are used for a little backup help in the memory management department.

Finally, you should remove the listener when you no longer need it, for optimal memory management (line 30). You also find event listeners in lines 45 through 48, and 55 through 65.

Void

Void is now lower case (lines 24, 46, 56).

Percent Values Scale

Percent value scales are now from 0 to 1 (instead of 0 to 100).

Dynamic Creation of Movie Clip and Instantiation of Custom Class

ActionScript 3.0 lets you much more easily use custom classes as display objects. You don’t need to rely on a Library-based symbol, or more convoluted methods, to instantiate the class. Instead, provided the class extends MovieClip, Sprite, Shape, or another applicable display object, you just need to instantiate it the way you would any other display object: new <classname>(); (line 33). You must then add the instance to the display list for the user to see it.

Another movie clip is dynamically generated in line 36. Note the simplicity of creating an empty movie clip container (to hold buttons). Rather than using one of many methods, such as createEmptyMovieClip(), the consistent new MovieClip() approach is all you need, coupled with the addChild() method on line 38.

Property Underscores

Line 37 is one example of the fact that ActionScript 3.0 properties are not preceded by an underscore.

Custom Button Instantiation

The same custom button used in the ActionScript 2.0 version can be instantiated here as a proper button (SimpleButton, line 41) rather than using a MovieClip method and typing the instance as a MovieClip or Object. It’s then positioned, and added to the display list (lines 42 and 43, respectively).

Dynamic versus Sealed Classes

In this case, however, the SimpleButton class is a sealed class, so you can’t add the site URL used by the button as a property. In this case, it’s stored in a standard variable.

You could have brought these two examples into a more parallel structure by using movie clips for buttons in both cases, because MovieClip is a dynamic class and would allow the addition of a property. However, the purpose of this chapter is not to change the way you want to work, but to understand how best to migrate a legacy project to the new syntax of ActionScript 3.0. Taking advantage of the new SimpleButton class is desirable, and even lets you create a button entirely from code (no Library assets) if preferred.

To see an exact parallel, you can add an example property to the text field instance in both versions of the project. In the ActionScript 2.0 version, adding

txtFld.inUse = true;

after line 10 works. However, adding the same line in the ActionScript 3.0 version after line 15 generates an error because the TextField class is sealed in ActionScript 3.0.

Opening a URL

Line 47 shows the new syntax for accessing a URL, using the navigateToURL() method and URLRequest instance.

Dynamically Instantiating a Component

Adding a component to your project on the fly is really no different from adding a movie clip or other display object. Just use the Button class as you would another display object class, as seen in line 50. (As with a custom asset, a Button component must be in your Library.)

Accessing Objects by Instance Name

You can’t access a dynamically created object directly by instance name. That is, setting the name property in ActionScript 3.0 doesn’t make it possible to reference the object using the dot syntax object model. Instead, you must use the getChildByName() method, as seen in line 57.

Using the Graphics Class (formerly the Drawing API)

Although the clear(), beginFill(), and endFill() methods are the same in ActionScript 3.0, they’re methods of the Graphics class, accessed through the graphics property instance of each relevant display object. Further, you don’t need a custom function to draw a rectangle, as the new drawRect() method does that for you.

Particles Class

The Particles class functionality is the same in ActionScript 3.0. It creates each particle, establishes its behavior, and ultimately removes it from the project.

1 package {
2
3     import flash.display.*;
4     import flash.events.*;
5     import flash.media.*;
6     import flash.net.*;
7
8     public class Particles extends Sprite {
9
10         private var _count:int;
11         private var _soundNum:int = 0;
12         private var _tempSound:Sound;
13
14         public function Particles() {
15             addEventListener(Event.ADDED_TO_STAGE, onAdded, ¬
                 false, 0, true);
16             addEventListener(Event.ENTER_FRAME, onLoop, false, ¬
                 0, true);
17             _tempSound = new Sound(new URLRequest( ¬
                  "../audio/note0.mp3"));
18             _tempSound.addEventListener(Event.COMPLETE, ¬
                  onSoundPreloaded, false, 0, true);
19         }
20
21         private function onAdded(evt:Event):void {
22             x = this.stage.stageWidth / 2;
23             y = this.stage.stageHeight / 2;
24             removeEventListener(Event.ADDED_TO_STAGE, onAdded);
25         }
26
27         private function onSoundPreloaded(evt:Event=null):void {
28             _tempSound.removeEventListener(Event.COMPLETE, ¬
                  onSoundPreloaded);
29             if (_soundNum < 7) {
30                 _soundNum++;
31                 _tempSound = new Sound(new URLRequest(¬
                      "../audio/note" + _soundNum + ".mp3"));
32                 _tempSound.addEventListener(Event.COMPLETE, ¬
                      onSoundPreloaded, false, 0, true);
33             }
34         }
35
36         private function onLoop(evt:Event):void {
37             makeParticle(0x0066CC, Math.random() * 10 + 10);
38         }
39
40         private function makeParticle(col:uint=0x003366, ¬
             size:Number=20):void {
41             if (Math.random() * 10 <= 2) {
42                 var p:MovieClip = new MovieClip();
43
44                 if (isNaN(_count)) { _count = 0; }
45                 p.name = "particle" + _count;
46                 _count++;
47
48                 p.graphics.beginFill(col);
49                 p.graphics.drawRect(-size/2, -size/2, size, size);
50                 p.graphics.endFill();
51
52                 p.xVel =  Math.random() * 10 - 5;
53                 p.yVel = Math.random() * 2 - 1;
54
55                 p.addEventListener(Event.ENTER_FRAME, ¬
                     onRunParticle, false, 0, true);
56                 addChild(p);
57
58                 MovieClip(parent).txtFld.appendText(p.name + "\n");
59                 MovieClip(parent).txtFld.scrollV = ¬
                     MovieClip(parent).txtFld.maxScrollV;
60             }
61         }
62
63         public function particleSound(p:MovieClip):void {
64             if (!p.snd && _soundNum > 0) {
65                 var num:int = int(Math.random()*_soundNum);
66                 p.snd = new Sound(new URLRequest(¬
                     "../audio/note" + num + ".mp3"));
67                 p.channel = new SoundChannel();
68                 p.channel = p.snd.play();
69             }
70         }
71
72         private function onRunParticle(evt:Event):void {
73             evt.target.x += evt.target.xVel;
74             evt.target.y += evt.target.yVel;
75             evt.target.rotation += 5;
76             evt.target.alpha -= .02;
77
78             if (evt.target.snd) {
79                 var trans = evt.target.channel.soundTransform;
80                 trans.volume = evt.target.alpha / 10;
81                 trans.pan = (evt.target.x / this.x)  * 2;
82                 evt.target.channel.soundTransform = trans;
83             }
84
85             if (evt.target.alpha <= 0) {
86                 if (evt.target.snd) {
87                     evt.target.channel.stop();
88                 }
89                 evt.target.removeEventListener(Event.ENTER_FRAME, ¬
                     onRunParticle);
90                 removeChild(MovieClip(evt.target));
91             }
92         }
93
94         public function get count():int {
95             return _count;
96         }
97     }
98 }

As with the ActionScript 2.0 section, basic syntax issues discussed in the main timeline won’t be mentioned again.

Class Structure

Line 1 shows that all ActionScript 3.0 classes must be enclosed in a package statement. This line would also be where you would include a path to the class, if desired. Lines 8 and 14 remain consistent with ActionScript 2.0.

Import

Lines 3 through 6 import all the classes to make them accessible to the compiler. Unlike ActionScript 2.0, even classes in Flash Player must be imported.

Class Properties

Lines 10 through 12 are consistent with ActionScript 2.0.

Number Data Types

Lines 10 and 11 use the int data type because you don’t need float values. You can also see this characteristic in lines 40 and 65. 40 is a good example, as the uint data type is used, because a color value can’t be negative.

Much has been made of the performance of the uint data type and, to a lesser degree, the int data type, so you can decide whether or not to use them. This is just an example.

Access to Stage

Unlike ActionScript 2.0, the stage isn’t a global object. Instead, you must access the stage through a display object. The Particles class both extends MovieClip, and is added to the display list in the main timeline frame script, so you can access the stage without passing a reference to it through the constructor.

However, you can access the stage only after the display object has been added to the display list. Therefore, this class can’t access the stage within its constructor, as the class hasn’t yet been fully initialized. Instead, a new event listener is added to listen for the Event.ADDED_TO_STAGE event (line 15). Once this event fires, the display object is part of the display list, and the stage reference doesn’t return null.

Lines 21 through 25 contain the function used for this purpose and, because the listener is no longer necessary, it’s removed upon execution of this function. (The this keyword is not strictly needed because the relevant scope is the class itself, but it’s been added to emphasize that you’re accessing the stage through a display object.)

Class Enter Frame Event

The use of the enter frame event for the class is the same; however, note that, because event handlers no longer exist, you can’t just name a function onEnterFrame() and expect it to work. You must convert that structure to an event listener design, seen in lines 16 and 36 through 38.

Method Closures

You no longer need the Delegate class, as ActionScript 3.0 supports method closures.

Preloading Sounds

The sound preloading routine hasn’t changed, and doesn’t include any ActionScript 3.0 syntax issues that haven’t already been discussed, with one small exception. If you pass a valid URLRequest instance to the Sound class constructor, as in line 31, the load() method is automatically called.

It’s also a good idea to look this method over with regard to removing listeners. It’s important to remove the load complete listener from _tempSound after each sound has been loaded (or, if you prefer, after the last sound has loaded) to prevent the listener from remaining on the last sound.

Particle Creation

ActionScript 3.0 has nothing unique in the makeParticle() method that hasn’t been, or won’t be, discussed elsewhere. However, be sure to read about changes to default values, accessing objects in the parent, and using the Drawing API (now commonly referred to as the Graphics class).

Default Values

ActionScript 3.0 allows the assignment of default values to method arguments, as seen in line 40. This action makes the associated arguments optional, but all optional arguments must appear at the end of the method signature.

Further, default values for data types have changed in ActionScript 3.0. For example, line 44 can no longer test for undefined, as the default value for number data types is NaN (not a number). As such, you must use the isNan() method to validate its value.

As is true with many intentionally injected migration issues in this exercise, this could have been handled a different way. This property could have been initialized in line 10, for example, but was not so this issue could be discussed.

Accessing Objects in the Parent

In Lines 58 and 59, the particle must cast the type of its parent before it can access the parent’s methods or properties. Without this step, the compiler knows only that the parent’s a display object container, but not what kind. The compiler, therefore, doesn’t recognize the txtFld property of the parent.

When cast to a MovieClip, however, the compiler knows that MovieClip is a dynamic class and can, therefore, have custom properties. It then looks for txtFld in the parent. and finds the text field you created.

Manipulating Text Fields

When adding text to the text field (line 58), the appendText() method was used, as it’s much faster than the compound operator +=. Furthermore, the property scrollV must be updated to the value of maxScrollV (line 59).

Particle Sound

Nothing about the particleSound() method that is unique to ActionScript 3.0 hasn’t already been discussed. Line 64 checks to make sure the particle’s Sound instance hasn’t already been created, and that _soundNum has been incremented to be sure the sounds have preloaded. Line 65 creates a random number within the count of available sounds, line 66 creates an instance of the Sound class and loads the sound, and lines 67 and 68 create a SoundChannel instance and play the sound.

Particle Behavior

The behavior of the ActionScript 3.0 particles isn’t unique, but a few very important concepts should be discussed. To begin, the first number of the product used for the sound transformation pan value is calculated using the particle’s x divided by the Particles class’ x (line 81). This step’s in contrast to the ActionScript 2.0 calculation, which divides the first number by the class’s parent’s x value (line 73 of the ActionScript 2.0 class code). ActionScript 2.0 requires the Library movie clip to instantiate the class this way, so the movie clip must be referenced in the calculation. ActionScript 3.0 lets you add this class to the display list directly, so only the class needs to be referenced.

Next, you must stop the sound, and remove the event listener, before removing the particle. Otherwise, the particle and its attendant objects, (such as listeners) won’t be collected by the garbage collector and purged from memory.

The compiler must be told that the object is a MovieClip to prevent an error from occurring, because the compiler sees only the target of the method as an Object that may or may not be removable. However, this issue has already been addressed in the “Accessing Objects in the Parent (Type Casting)” section of this discussion.

Getter

Nothing unique about the count() getter method is unique to ActionScript 3.0 hasn’t already been discussed

Migration Sample Summary

This is a small example of one possible migration path used to update a legacy project to ActionScript 3.0. Although awkward coding choices were made to show the largest number of migration issues practical in this size example, the exercise is still relatively close to a real-world scenario.

Having read this chapter, you may want to see if you can migrate this example on your own. Once you try the process a few times, you’ll have a pretty good idea of what you need, and you can evaluate the effectiveness of migration on a case-by-case basis. Depending on the extent of the changes, you may wish to use the old project as a kind of template, and then code the new version from scratch.

This excerpt is from The ActionScript 3.0 Quick Reference Guide. If you're ready to give your Flash projects a considerable performance boost, learning ActionScript 3.0 is a must. This Quick Answer Guide is designed specifically to help Flash designers and developers make the leap from ActionScript 2.0 to the new object-oriented ActionScript 3.0 quickly and painlessly. You'll learn key differences between the two language versions, allowing you to more easily leverage ActionScript 3.0 using Flash CS4 and other Adobe technologies like Flex and AIR.

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