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Object.watch() 677

The Object.watch() method behaves similarly to the Object.watch() function in JavaScript

1.2 and later. The primary difference is the

userData parameter, which is a Flash addition to

Object.watch() that Netscape Navigator does not support. You can pass the userData

parameter to the event handler and use it in the event handler.

The

Object.watch() method cannot watch getter/setter properties. Getter/setter properties

operate through lazy evaluation— the value of the property is not determined until the property is

actually queried. Lazy evaluation is often efficient because the property is not constantly updated;

it is, rather, evaluated when needed. However,

Object.watch() needs to evaluate a property to

determine whether to invoke the

callback function. To work with a getter/setter property,

Object.watch() needs to evaluate the property constantly, which is inefficient.

Generally, predefined ActionScript properties, such as

_x, _y, _width, and _height, are getter/

setter properties and cannot be watched with

Object.watch().

Example

The following example uses watch() to check whether the speed property exceeds the speed

limit:

// Create a new object

var myObject:Object = new Object();

// Add a property that tracks speed

myObject.speed = 0;

// Write the callback function to be executed if the speed property changes

var speedWatcher:Function = function(prop, oldVal, newVal, speedLimit) {

// Check whether speed is above the limit

if (newVal > speedLimit) {

trace ("You are speeding.");

}

else {

trace ("You are not speeding.");

}

// Return the value of newVal.

return newVal;

}

// Use watch() to register the event handler, passing as parameters:

// - the name of the property to watch: "speed"

// - a reference to the callback function speedWatcher

// - the speedLimit of 55 as the userData parameter

myObject.watch("speed", speedWatcher, 55);

// set the speed property to 54, then to 57

myObject.speed = 54; // output: You are not speeding

myObject.speed = 57; // output: You are speeding

// unwatch the object

myObject.unwatch("speed");

myObject.speed = 54; // there should be no output