Object.watch() 581
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 an event handler to be invoked when a specified property of an ActionScript
object changes. When the property changes, the event handler is invoked with
myObject as the
containing object. You must return the new value from the
Object.watch() method, or the
watched object property is assigned a value of
undefined.
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 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 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().
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