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
![]() ![]() ![]() |