Actionscript Cookbook

12.7. Defining Read-Only Properties

Problem

You want to create a read-only property for a class.

Solution

Use the addProperty( ) method in the constructor and pass it a null value for the setter method.

Discussion

There are at least two good reasons to have read-only properties:

• You want to create constants for your class.

• You want to have a dependent property whose value depends on other properties/factors.

The addProperty( ) method creates getter/setter properties for a class. Passing addProperty( ) a null value for the setter method parameter effectively creates a read-only property.

See Recipe 12.6 for the initial implementation of the Square class. Here is a modified version:

_global.Square = function (side) {
  this.side = side;

  // Create a read-only property, version (effectively, a constant). The getter
  // method is getVersion(  ). The setter method is null.
  this.addProperty("version", this.getVersion, null);

  // Create a dependent property, area (its value changes, so it is not a constant,
  // but the value depends on other property values and can't be set directly). The
  // getter method is getArea(  ). The setter method is null. this.addProperty("area", this.getArea, null); }; Square.prototype.getArea = function ( ) { return Math.pow(this.side, 2); }; Square.prototype.getVersion = function ( ) { // Always returns the string "1.0". Therefore, it is a constant. return "1.0"; }; // Create a square with a side length of 5. sq = new Square(5); trace(sq.area); // Displays: 25 // Try to set ...