FlashDevelop: AS3 FlashDevelop Default AS3 Project

AS3 FlashDevelop Default AS3 Project

Time to explain what all that automatically created code in Main.as means. This tutorial assumes that you already have FlashDevelop up and running, and able to compile and run ActionScript 3. If you aren't at this point, check out my previous post.

This tutorial is really beginner programming info, If you already understand the code in Main.as, you can just scan, or skip to the next tutorial.

This is how Main.as looks initially:

package
{
 import flash.display.Sprite;
 import flash.events.Event;

 /*
  *
  * ...
  * @author your name
  */
 public class Main extends Sprite
 {

  public function Main():void
  {
   if (stage) init();
   else addEventListener(Event.ADDED_TO_STAGE, init);
  }

  private function init(e:Event = null):void
  {
   removeEventListener(Event.ADDED_TO_STAGE, init);
   // entry point
  }

 }

}

• Basic syntax info:
  Blocks of code are opened and closed using { and }. The general format is "Statement { code; }". In AS3 blocks without a statement essentially do nothing because there is no block level scope for variables. If you don't what that means don't worry about it yet.

Breaking it down line by line:

• Package:
  

  package

  You'll see this in all classes, it can be used for organizing your code. Unless you are planning on releasing your code to the public, or are creating an organized library for yourself, you can generally leave the package as is. One other point, your package must reflect your file system, so for example flash.display.Sprite would have to be in the path "..\src\flash\display\" and have the classname "Sprite".



• Import:
  

   import flash.display.Sprite;
   import flash.events.Event;

  These statements make the Sprite class, and Event class accessible to your class. In flash you must import all of the classes you intend to use. You shouldn't have to worry about import too much because code completion in FlashDevelop automatically adds necessary import statements for you when you use it.



• Documentation Comments:

   /*
    *
    * ...
    * @author your name
    */

  Commented lines. This is a specific type of comment meant to be used by a documentation generator. It is opened with /** and closed with */. If you type "@" on one of the comment lines, autocomplete will pop up and give you a bunch of tag options. These tags will be displayed in specific ways if you generate documentation. Even if you don't feel like setting up documentation for your class/project, it's still a good idea to put in a description of what the class is supposed to do where the "..." is to help you understand your code if you come back to it.



• Declaring the class:
  

   public class Main extends Sprite

  This declares the class you are making. "public" means this class is accessible to any other class that wants to use it. "class" is a keyword labels the contents of the related code block as a class. Main is the name of your class. Quick side note about names, in AS3 public classes must be created inside files that match the name of the class, this is why our class "Main" is declared in the file "Main.as". If you screw this convention up, really it's ok because FlashDevelop gives you a nice descriptive error. "extends Sprite" declares this class as a subclass of Sprite. What this means is that our class will essentially be a Sprite, with whatever we code added to it. If you don't know about subclasses and inheritance yet, it might be wise to google "class inheritance" and "polymorphism".



• Declaring a constructor:
  

    public function Main():void

This is the function that will be called when the Main class is used to create an object. Constructors will generally be declared public, unless you are making an abstract or static class, which isn't in the scope of this tutorial. function is a keyword that labels the related code block as a function. Main is the name of the function. In this case, the name must match the name of the class, which is what makes it a constructor in AS3. ( and ) are a space for parameters, but there are none to this constructor. Finally :void tells AS3 that this function does not return a value when called.



• Class creation code:
  

  if (stage) init();
  else addEventListener(Event.ADDED_TO_STAGE, init);

This is an if-else statement. It first checks if this object's stage has been defined. This is somewhat like the _root of AS2, it is basically the main drawing area of a movie. If the stage exists in this object, then your code begins running the init() function. If not, addEventListener makes this class wait until it has been added to the stage, at which point it will call init. addEventListener arguments are the event time, and then the name of the method the event should call when it occurs.



• Declaring an initialization function:
  

  private function init(e:Event = null):void

This function will be called either by the Main function, or the EventListener that the main function creates. It is private because it should only be called from within the Main class. The parameter e:Event = null declares a parameter named e, that is of the type Event. It is assigned to null which means that if you call init using init(); with no parameters, e will automatically be null. e is required to be there because functions called by event listeners will use the event parameter to report which event caused the function to be called. The :void means the function has no return value.



• Coding an initialization function:
  

  removeEventListener(Event.ADDED_TO_STAGE, init);

This simply removes the event listener from this object if it was added in the constructor. This stops any potential problems of init being called by the handler again accidentally.


• Code comments:
  

  // entry point

  Code comments start with a // and are terminated by a line end. They are just another way to document what your code does. Comments inside of code will not show up in documentation like the previously shown comments do.
  

Final Comments:
This default Main file works ideally for components that involve drawing. Using the init function and event listeners ensures that you don't attempt to do any drawing before your object is added to the stage. If your class doesn't require the stage you can just put your init code in the constructor and delete the init function.

Next Tutorial, Hello World >>