As we saw earlier, ActionScript leniently allows methods to assign new properties to objects. In fact, ActionScript is even more flexible than that—it allows any code to create a new property or method on a single object instance (rather than on all instances of a class). For example, the following code creates an instance, obj, of the generic Object class and immediately assigns it the new properties x (set to 10) and y (set to 20):
var obj = new Object( ); obj.x = 10; obj.y = 20;
Assigning properties and methods to individual objects is, of course, bad OOP practice. But in ActionScript, it is also a completely legitimate way to store a group of named values (much like a hash or associative array). In fact, many of ActionScript's built-in methods take generic objects as parameters. For example, MovieClip.globalToLocal( ) takes a point object parameter that must supply an x property and a y property. The x and y properties represent a global position that will be converted to a movie clip's local coordinate system.
pt = new Object( ); // Create generic object to hold our point
pt.x = 0; // Left border of main Stage
pt.y = 0; // Top border of main Stage
this.globalToLocal(pt); // Convert pt to local coordinates
trace("From the current clip, the top-left corner of the main Stage is at ");
trace("x: " + pt.x + "y: " + pt.y);
Similarly, MovieClip.attachMovie( ) takes an initObj parameter used to transfer an object's properties to a new movie clip instance, car_mc, as follows:
// Create an object
var initObj = new Object( );
// Create properties on the object
initObj.speed = 3;
initObj.onEnterFrame = function ( ) {
this._x += this.speed;
};
// Transfer the object's properties to car_mc
this.attachMovie("CarSymbol", "car_mc", 1, initObj);
Notice that this code defines onEnterFrame( ) as a method on initObj, which, given the task at hand, is perfectly legal (and convenient!).
To create a generic object of the Object class directly in our code without using the Object constructor, we can use an object literal, just as we might use a string literal or an array literal. An object literal is a series of comma-separated property name/value pairs, enclosed in curly braces. Here's the general syntax:
{ property1: value1, property2: value2, property3: value3 }
The names of properties in an object literal must be legal identifiers, as described in Chapter 14. The values can be any valid expression. For example:
// An object literal with two numeric properties
myObject = { x: 30+10, y: 20*2 };
Object literals are used for the sake of terseness. For example, in the following code we transfer the speed and onEnterFrame properties to car_mc more succinctly (though perhaps less legibly) than we did earlier:
this.attachMovie("CarSymbol", "car_mc", 1, {speed:3, onEnterFrame: function ( ) {
this._x += this.speed;}});