N-R > Object.watch

 

Object.watch

Availability

Flash Player 6.

Usage

myObject.watch( prop, callback [, userData] )

Parameters

prop A string indicating the name of the object property to watch.

callback The function to invoke when the watched property changes. This parameter is a function object, not a function name as a string. The form of callback is callback(prop, oldval, newval, userData).

userData An arbitrary piece of ActionScript data that is passed to the callback method. If the userData parameter is omitted, undefined is passed to the callback method. This parameter is optional.

Returns

A value of true if the watchpoint is created successfully, otherwise, returns a false value.

Description

Method; registers a callback function to be invoked when a specified property of an ActionScript object changes. When the property changes, the callback function is invoked with myObject as the containing object.

A watchpoint can filter (or nullify) the value assignment, by returning a modified newval (or oldval). If you delete a property for which a watchpoint has been set, that watchpoint does not disappear. If you later recreate the property, the watchpoint is still in effect. To remove a watchpoint, use the Object.unwatch method.

Only a single watchpoint may be registered on a property. Subsequent calls to Object.watch on the same property replace the original watchpoint.

The Object.watch method behaves similarly to the Object.watch function in Netscape 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 callback function and use it in the callback function.

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 in order to fire watchpoints on it. To work with a getter/setter property, Object.watch needs to evaluate the property constantly, which is inefficient.

Generally, ActionScript predefined properties, such as _x, _y, _width and _height, are getter/setter properties, and thus cannot be watched with Object.watch.

Example

This example shows a CheckBox component with methods that set the label or value of each check box instance:

myCheckBox1.setValue(true);
myCheckBox1.setLabel("new label");
...

It's convenient to think of the value and label of a check box as properties. It's possible to use Object.watch to make accessing the value and label look like property access rather than method invocation, as in the following:

// Define constructor for (and thus define) CheckBox class
function CheckBox() {
	...
	this.watch ('value', function (id, oldval, newval)) {
		...
	}
	this.watch ('label', function (id, oldval, newval)) {
		...
	}
}

When the value or label property is modified, the function specified by the component is invoked to perform any tasks needed to update the appearance and state of the component to reflect its new parameters. Therefore, the following assignment statement uses an Object.watch handler to notify the component that the variable has changed and causes it to update its graphical representation.

myCheckBox1.value = false;

This syntax is more concise than the former syntax:

myCheckBox1.setValue(false);

See also

Object.addProperty, Object.unwatch