AS3 - Object Oriented Programming - Part1

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AS3 – Object Oriented Programming – Part1

April 16th, 2009

We talked a lot about classes and objects in the introductory part, now let’s look how we can create a class.

In AS3, a class must be created in a separate .as file, with the name of the class being the name of the file.
Also, a class must be contained in a package. We’ll discuss in the next part more about packages.

To define a class in actionscript we have the keyword class.

To write our first class, named Apple, create a file named Apple.as, open it in a text editor and paste in the next code (make sure you edit with a text editor and not a word processor like MS Word):

package {
public class Apple {
}
}

There it our first class, however is quite empty; it has no functionality.
Notice that i’ve used the keyword package. That way I defined the package for the Apple class. But more on packages in the next part of this tutorial.
Also notice another keyword introduced: public. This is an access specifier.
An access specifier sets the visibility for a class, method, property or namespace.

A method is a function defined inside a class.
A property or member is a variable defined inside a class.
A namespace is like a package, controls the visibility of definitions, but it’s not directly related to classes, so it won’t be discussed in this tutorial.

In our case, the public specifier sets the class to be visible and accessible anywhere. Without specifying public, the class would be visible only inside the package, because the default access is internal.
A class can only be defined public or internal, however there are 2 other access specifiers: private and protected.
Private means that the property or method is only visible and accesible inside the class. About protected we’ll talk a bit later.

With this said, let’s add a method and a property to our class:

package {
public class Apple {
private var stones:uint = 4;

public function Apple() {
}
}
}

Now our Apple class has a property of type uint, named stones, which is declared private (this means that only methods of the class Apple can access this property) and has a default value of 4.
It also has a method, declared public, named Apple. Notice the name of this method is exactly the name of the class. A method with the same name as its class is named constructor.
In actionscript 3.0 a constructor can only be declared public. Also a constructor cannot have a return type (cannot return a value).
It is named “constructor” because when an object is created this method will be called. So it is a good place to put here any initializing code.

With these being said we can construct our first object of the class Apple, but first some words about the import statement.
Before we can use our class, we must import it where we want to use it. For that, we have the “import” statement, so we can use it like this:

import Apple;

Ok. Now we’re ready.
Open the Flash IDE, create a new Flash Actionscript 3.0 File, save it to a name of your choice and copy the Apple.as file in the same directory as your .fla file.
On the timeline, select the first keyframe and open the Actions window (Windows->Actions or press F9), and write the next lines:

import Apple;
var myApple : Apple = new Apple();

That’s it! Your first object is created out of your first class! You can test your file now, but really there isn’t much to see…
Notice the new keyword: it is used to construct new objects.

So let’s add something so that we can see what’s going on.
Add a trace action inside our constructor.
Our class should look like this:

package {
public class Apple {
private var stones:uint = 4;

public function Apple() {
trace("A new apple is constructed!");
}
}
}

Now run again your movie and you should see “A new apple is constructed!” in the Output window. That’s because when we construct our object using the new statement, the constructor of our class is called, thus the trace method is called.
If you would construct more objects, you will see that the message is outputted once for every object.

Next, let’s say we would like to know how many stones has our apple object. But the stones property is private, which means we can’t access it from outside the class. So, to access it let’s add a new method, named “getStones”, that will return the number of stones, and make it public.

package {
public class Apple {
private var stones:uint = 4;

public function Apple() {
trace("A new apple is constructed!");
}

public function getStones() : uint {
return this.stones;
}
}
}

Make note of this keyword. “this” refers to the “current object”.

Now we can call the new method on our object like this:

myApple.getStones()

In Flash, change the code with this one: