Actionscript Cookbook

12.3. Adding Properties to an Object Instance

Problem

You want to add custom properties (variables) to an object (an instance of a class).

Solution

Attach the new property to the object using an assignment statement and the dot operator.

Discussion

Objects derived from the built-in ActionScript classes have standard properties. For example, arrays have length properties, movie clips have _x and _y properties, and Sound objects have position and duration properties. But aside from these standard properties, you can add custom properties to any object in ActionScript. You can add new properties to an object (or change an existing property’s value) by simply assigning the property a value. If the property does not yet exist, ActionScript creates it automatically.

myMovieClip.myProperty = "some value";

Custom properties of movie clip objects are sometimes referred to as timeline variables, as discussed in Recipe 1.12. However, you can apply the same principle not only to movie clip objects, but to any kind of object. For example, the following code adds a custom property to an array named myArray:

myArray.myProperty = "some value";

Attaching properties to objects is invaluable. For example, custom properties can store a value that is used internally within an event handler method (or any other method) of the object. You can initialize the property value outside any of the methods and then access it within a method. This technique creates a variable whose value persists between function invocations ...