Using Classes in ActionScript 3

An application is not one giant chunk of functionality. Under the hood, applications are made up of many moving parts. One of the basic parts that applications have are classes:

If you have some familiarity with programming in ActionScript, the chances are very high that you have used classes such as String, Number, MovieClip, etc. Despite how common classes are, writing your own class is not a walk in the park.

In this article, you will learn what classes are, how classes can be used with Flash, and how to create your own class.

What are Classes?
Fundamental to object oriented programming is the concept of objects. These objects have to be defined somewhere, and that "somewhere" is where classes come in. Let's take a look at the example where you have an application that is made up of ten blue circles:

Each blue circle is an object, and each object is created from some sort of a template. This template specifies the type of your object and any characteristics this object exhibits, and these templates are known as....classes! For example, all of your circle objects could be based on a BlueCircle class that defines the various characteristics of the circle itself:

In the example I have provided, each of your circle objects was built from a class that contained information about your object's characteristics. Digging one level deeper, the following is an example of what a class defined in code vaguely looks like:

package {

import flash.display.*; 

 

public class BlueCircle {

 

private var someVariable:uint;

 

public function Circle() {

//initial tasks

}

 

public function SetColor(newColor:uint) {

 //code for setting the color

}

 

public function SetSize(newSize:uint):

{

 //code for setting the size

}

}

}

Don't worry if the above code doesn't make sense. By the end of this tutorial, the brief overview you just saw above will make more sense as more details and layers are revealed to you.

Creating your Own Class
To go beyond simply looking at illustrated examples, let's create a project and get our hands dirty in creating a class. In Flash CS3 or Flash CS4, create a new Flash File whose language is ActionScript 3:

Once you have created your Flash file, go to File | Save and save this file to a location. Feel free to name your file whatever you want, but I'm going to call my application AS3Classes.

Now that you created your FLA file, it's time to create a class. Go to File | New, and from the New Document window, select the entry for ActionScript file:

Once you have created your new ActionScript file, go to File | Save. Give this file the name Name, and save this file in the same location as your FLA. At the end of this file creation and saving set of tasks, your folder containing your FLA should look as the following:

Great. You have just created a Flash file and an ActionScript class file. The next couple of steps may not make some sense right now, but just follow along. I promise to explain in great detail what happens shortly.

Let's define our class. Switch back to Flash and make sure your Name ActionScript file is open for editing.

Because the Name file is an ActionScript file, all of your familiar panels and palettes will be grayed out. All you have is a code editor that resembles Notepad. That's OK. Copy and paste the following code into your code editor:

package

{

public class Name

{

private var firstName:String;

private var lastName:String;

 

function Name()

{

 

}

 

public function SetFirstName(val:String):void

{

firstName = val;

}

 

public function SetLastName(val:String):void

{

lastName = val;

}

 

public function GetFullName():String

{

return firstName + " " + lastName;

}

}

}

After you have copied and pasted the code, save this file. Your Name class has been created. The next task is to actually use this class.

Using the class requires adding some code to your Flash file. So, jump back to your Flash file - which I called AS3Classes. You will see your traditional Flash interface with the stage and timeline displayed, and from here, let's add some code to take advantage of the class file you just fiddled with a few seconds ago.

Right click on an empty keyframe in your timeline and select Actions. The Actions window will appear:

Inside the Actions window, copy and paste the following code:

var author:Name = new Name();

author.SetFirstName("Charles");

author.SetLastName("Dickens");

 

trace(author.GetFullName());

After you have copied and pasted the above code, press Ctrl + Enter to test your application. You should see the text Charles Dickens displayed in your Output window:

All right! You have just created a simple application that uses a class that you created to display some information. Let's look into greater detail at what you just did and learn more about classes.

The Structure of a Class in AS3
While a class can contain a variety of data, all classes share a certain structure that allows the Flash Compiler to know what to do when you build your application. Let's look at the contents of our Name file to learn more:

package

{

public class Name

{

private var firstName:String;

private var lastName:String;

 

function Name()

{

 

}

 

public function SetFirstName(val:String):void

{

firstName = val;

}

 

public function SetLastName(val:String):void

{

lastName = val;

}

 

public function GetFullName():String

{

return firstName + " " + lastName;

}

}

}

The first thing your class needs is a package declaration:

package

{

public class Name

{

private var firstName:String;

private var lastName:String;

 

function Name()

{

 

}

 

public function SetFirstName(val:String):void

{

firstName = val;

}

 

public function SetLastName(val:String):void

{

lastName = val;

}

 

public function GetFullName():String

{

return firstName + " " + lastName;

}

}

}

For the sake of simplicity, think of a package as a giant folder that links various classes together. Because we only have one class in our application, the name of the package does not particularly matter. Despite this not mattering, you still need the package keyword in order to build your application.

Next up is the class definition:

package

{

public class Name

{

private var firstName:String;

private var lastName:String;

 

function Name()

{

 

}

 

public function SetFirstName(val:String):void

{

firstName = val;

}

 

public function SetLastName(val:String):void

{

lastName = val;

}

 

public function GetFullName():String

{

return firstName + " " + lastName;

}

}

}

The class definition tells Flash what the name of your class will be and what type of access restrictions it will have. The main thing to note is the class keyword that precedes the name of your Name class.

Preceding the class keyword, you see the word public displayed. The public keyword is something that is known as an access modifier, and it describes how accessible this particular class is. I won't describe access modifiers in great detail in this article, so just note that this value is commonly public (as in this example) or private.

Next up are two things that are probably familiar to you, and they are variables:

package

{

public class Name

{

private var firstName:String;

private var lastName:String;

 

function Name()

{

 

}

 

public function SetFirstName(val:String):void

{

firstName = val;

}

 

public function SetLastName(val:String):void

{

lastName = val;

}

 

public function GetFullName():String

{

return firstName + " " + lastName;

}

}

}

Variables allow you to store data inside them, and the names of our variables are firstName and lastName. All variables in ActionScript are declared using the var keyword.

What you can also do is specify the type of content your variable will be storing by following the variable name with a colon and the name of the type. In this case, both our variables will be storing data of type String, so I specify that after the variable name.

Finally, you will see both of our variables starting off with the private keyword. Just like the public keyword you saw earlier as part of our class definition, the private keyword is another access modifier.

Next up is a function whose name is the same as that of our class:

package

{

public class Name

{

private var firstName:String;

private var lastName:String;

 

function Name()

{

 

}

 

public function SetFirstName(val:String):void

{

firstName = val;

}

 

public function SetLastName(val:String):void

{

lastName = val;

}

 

public function GetFullName():String

{

return firstName + " " + lastName;

}

}

}

This function is known as a constructor. I will explain what a constructor does in a little bit, but to give you a sneak preview, a constructor is responsible for setting the spark that creates the object this class is the template for.

The last things we are going to look at are functions (also known as methods):

package

{

public class Name

{

private var firstName:String;

private var lastName:String;

 

function Name()

{

 

}

 

public function SetFirstName(val:String):void

{

firstName = val;

}

 

public function SetLastName(val:String):void

{

lastName = val;

}

 

public function GetFullName():String

{

return firstName + " " + lastName;

}

}

}

In a nutshell, a function is a block of code that does something when asked to. It may just do something and stop, or it may do something and return some data back to whatever asked it.

In our class, we have three functions called SetFirstName, SetLastName, and GetFullName. Based on the names of these functions, what they do should be pretty straightforward, and the code is very simple as well.

Instead, let's focus on the structure of a typical function by looking at SetFirstName:

public function SetFirstName(val:String):void

{

firstName = val;

}

Let's read the first line from left-to-right and stop at the interesting keywords. Just like variables and class definitions, functions too have access modifiers. This function's access level is public.

A function isn't quite a function without it being defined as one, so the next keyword to look at is the word function itself. This keyword defines this whole block of code as a function.

After that, you have the function name and any variables it takes in as an argument. Our function is called SetFirstName, and it takes just one variable as its argument - a variable called val whose type is String.

The final thing to note is that the entire function declaration is followed by a return type. This means that, if your function is going to be returning any data, the data's type (class) needs to be specified. Because the SetFirstName function isn't returning anything, its return type is set to void.

An example of a function that does return some data is GetFullName:

public function GetFullName():String

{

return firstName + " " + lastName;

}

Notice that in the function declaration, the return type is set to String. When you specify a return type, Flash will expect this function to return data whose type matches the return type specified.

In this case, we are returning data in the form of a string by combining the data from the firstName and lastName variables, and that is good because that is what our function declaration indicates as well.

Creating Objects from a Class
Creating a class and populating it with properties and functions is just one part of writing an application. An equally important part is to create objects out of those classes. You've already done that, and the code for doing that can be found in your FLA file where you pasted the following lines of code on a keyframe:

var author:Name = new Name();

author.SetFirstName("Charles");

author.SetLastName("Dickens");

 

trace(author.GetFullName());

Let's look at the above lines of code in greater detail:

var author:Name = new Name();

In the first line, we declare a variable whose name is author. Beyond just giving it a name, we specify its type to be Name as wel. In case you are wondering, the type Name is exactly the same Name class we looked at a few moments ago.

Creating a variable is only the first half of what the above line does. The second-half deals with creating your Name object. On the right-hand side of the equal sign, you set the variable to an actual Name object using the new keyword followed by the Name constructor that you saw earlier inside the name class.

This is where a constructor comes in handy. When you call a constructor, you are essentially constructing a new object, and this is commonly known also as instantiating an object. At the end of this first line, your author variable is now an instance - a real manifestation of the class Name.

When you have an instance of a type, which is what we have with our author variable, you have the ability to access the functions and any variables from your type easily such as what I do in the following two lines:

author.SetFirstName("Charles");

author.SetLastName("Dickens");

SetFirstName and SetLastName are both functions defined in your Name class, but because the author variable is an instance of the name class, you can call it directly from author itself.

Let's look at the final line of our code:

trace(author.GetFullName());

In this line, I am tracing the value returned by the GetFullName function from our author object. As you recall, the GetFullName function returns a value of type string, and this means that you can manipulate its output like you would any other string-based data.

I am keeping things simple by just tracing the values that you see. Next, let's look at this in a bit more complicated light by creating many objects and visualizing how they probably look.

Creating Many Objects
To help in explaining what is going, the code you currently have only creates a single object. Let's make things a bit more interesting by creating many objects.

In the code you pasted in your Flash file's keyframe, replace your current code with the following:

//Book Author

var author:Name = new Name();

author.SetFirstName("Charles");

author.SetLastName("Dickens");

 

trace(author.GetFullName());

 

//Video Game Character

var game:Name = new Name();

game.SetFirstName("Marcus");

game.SetLastName("Fenix");

 

trace(game.GetFullName());

 

//TV Personality

var tv:Name = new Name();

tv.SetFirstName("Mike");

tv.SetLastName("Rowe");

 

trace(tv.GetFullName());

 

//Normal by day, Superhero by night

var superHero:Name = new Name();

superHero.SetFirstName("Bruce");

superHero.SetLastName("Wayne");

 

trace(superHero.GetFullName());

When you run your application now by pressing Ctrl + Enter, the following is what you will see displayed in your Output window:

In the first page of this article, I showed you the following diagram describing the relationship between classes and objects:

Let's reuse the idea from the above diagram and fill in some missing details by visualizing the Name class and the objects that it contains:

You have the four objects that we created based on the Name class, and notice that each object stores the values that are unique to it. In the above diagram, I have populated the firstName and lastName values.

Conclusion
Hopefully this article helped you to get a better understanding of classes and their place in modern applications. If you have done some minor ActionScript 3 programming in Flash already, you probably have used classes already. Any variable that you declare has a type associated with it, and that type is basically a class. For example, a common class you saw in this tutorial is String, and there are dozens more built-in types that the ActionScript language provides.

Fortunately, you are not limited to just using the built-in types. As you saw in this article, creating your own class is pretty straightforward. The Name class is probably on the simpler side of the classes that you can create, but even more complicated classes will follow a similar pattern.

A great follow-up tutorial to read is Classes and MovieClips where you learn how to associate movie clips on your stage to classes, but otherwise, that's it for this article!

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