ActionScript

• An object oriented programming language used to create interactive content on web pages.
• Somewhat similar in syntax to JavaScript, Java, C++.
• Loosely typed, object oriented.
• It requires a Flash player plug-in to run.
• Created by Macromedia that is now part of Adobe.
• Popular for multimedia content on the web.
• The programs are called apps or movies, and they are heavily event-driven.

More about ActionScript

• The ActionScript code resides in action panels.
• Classes can be stored in external files (.as).
• The action panels are attached to Frames, Buttons, or Movie Clips. The code in a panel attached to an entity or clip controls its behavior.
• Files related to a program:
  .fla - flash source code (project) and resources
  .swf - compiled flash file, can be played by the Flash player, but the source code cannot be retrieved from it (some special programs exists that can decompile it).

Variables - Types

• Variables do not need types but you can specify them. Flash can help you find appropriate methods if you do.
• Example:
  var name:String = "John";
• Variables are local to the scope in which they are declared. If it's inside a function body, they are local to the function.
• Variables declared in an action panel are global but only within the code in that action panel.

Control Structures

• Curly braces are not mandatory to delimit blocks of a single instruction, but they help the syntax checking.
  
• Conditional: the usual, else is optional.
  

  if (x == "food") 
  { 
      Eat(x); 
  } 
  else 
  {
      dontEat(x);
  }

• Loops
• For loops and while loops with the C++ structure.
  

  for (i=0; i<7; i++) 
  {
      if (found == false) 
      {
          gameover = false; 
      }
  }
  while (!gameover) 
  {
      play();
  }
  

Functions

• The parameters don't need types, but you can specify them the same way as for variables.
  

  function displayArea (width, height) 
  { 
      trace(width * height);
      return width*height; 
  } 

• Types can be specified for the arguments. The type of the returned value can also be specified. Both are optional:
  function displayArea (width :int, height :int) :int {
• This declares a function named displayArea, with two arguments width and height.
• trace is a debugging tool that displays the arguments in the Output window.
• Functions are called as usual:
  displayArea(10, 20);

Events

• Most ActionScript programs need to be react to events. Example of code in the action panel of a button:
  

  on (release) 
  {
      if (words._visible == true) 
      {
          setProperty(words, _visible, false);
      } 
      else 
      {
          setProperty(words, _visible, true);
      }
  }

• words is a reference to an object (image) to be displayed on the web page. It has a property that allows it to be visible or not.
• release is an event generated by the release of the mouse button.
• on (event) is a special syntax that defines behavior related to an event.
• Most events in ActionScript 3.0 are attached to an object with a function call to addEventListener:
  object.addEventListener(event, function);

ActionScript 3.0 Classes

• ActionScript 3.0 has a library of classes and also allows you to define your own as external files.
• Some examples of useful classes.
  

  var skySprite:Sprite = new Sprite(); 
  var tile:BitmapData=new SkyBg(550, 700);
  skySprite.graphics.drawRect(0, 0, tile.width, tile.height);
  var scrollTimer:Timer=new Timer(5);
  scrollTimer.addEventListener(TimerEvent.TIMER, scrollLayers);
  scrollTimer.start();

User-Defined Classes

• A class must be in a file by the same name and extension .as.
• In the file, the class must be in a package named after the subdirectory the class is in.
• Attributes are variables declared inside the class.
• Class methods are functions defined inside the class.
• They can both be declared "public". By default they are internal, which works like private for classes not defined inside a named package.
• A class should have one constructor which is a function with the same name as the class. Only one is allowed, but the parameters can have optional values (par = val).

Class Properties

• Any class attribute, method, or constant.
• Qualifier for any property:
• internal: accessible within the package.
• public, protected, private: the same meaning as in C++.
• static: a property of the class and not of an instance. Does not need an object of the class to me accessed. Math.PI, a static attribute of the class Math.