Warning: Generally you should avoid using watch()
and unwatch()
when possible. These two methods are implemented only in Gecko, and they're intended primarily for debugging use. In addition, using watchpoints has a serious negative impact on performance, which is especially true when used on global objects, such as window
. You can usually use setters and getters or proxies instead. See Browser compatibility for details. Also, do not confuse Object.watch
with Object.observe
.
The watch()
method watches for a property to be assigned a value and runs a function when that occurs.
語法
obj.watch(prop, handler)
參數
prop
- 所需要監聽其值是否改變的物件屬性
handler
- 當監聽的變數其數值變換時所執行的function
回傳值
Description
Watches for assignment to a property named prop
in this object, 呼叫 handler(prop, oldval, newval)
whenever prop
is set and storing the return value in that property. A watchpoint can filter (or nullify) the value assignment, by returning a modified newval
(or by returning oldval
).
當你刪掉所監聽的物件屬性,並不會結束針對該物件屬性的監聽。當你重新產生該屬性時,監聽依舊維持作用。
要停止該次監聽, 須使用 unwatch()
函式. By default, the watch
method is inherited by every object descended from Object
.
The JavaScript debugger has functionality similar to that provided by this method, as well as other debugging options. For information on the debugger, see Venkman.
In Firefox, handler
is only called from assignments in script, not from native code. For example, window.watch('location', myHandler)
will not call myHandler
if the user clicks a link to an anchor within the current document. However, window.location += '#myAnchor'
will call myHandler
.
Note: Calling watch()
on an object for a specific property overrides any previous handler attached for that property.
範例
使用watch 和 unwatch
var o = { p: 1 };
o.watch('p', function (id, oldval, newval) {
console.log('o.' + id + ' changed from ' + oldval + ' to ' + newval);
return newval;
});
o.p = 2;
o.p = 3;
delete o.p;
o.p = 4;
o.unwatch('p');
o.p = 5;
上述程式執行結果:
o.p changed from 1 to 2 o.p changed from 2 to 3 o.p changed from undefined to 4
使用 watch
驗證物件的屬性
You can use watch
to test any assignment to an object's properties. This example ensures that every Person always has a valid name and an age between 0 and 200.
Person = function(name, age) {
this.watch('age', Person.prototype._isValidAssignment);
this.watch('name', Person.prototype._isValidAssignment);
this.name = name;
this.age = age;
};
Person.prototype.toString = function() {
return this.name + ', ' + this.age;
};
Person.prototype._isValidAssignment = function(id, oldval, newval) {
if (id === 'name' && (!newval || newval.length > 30)) {
throw new RangeError('invalid name for ' + this);
}
if (id === 'age' && (newval < 0 || newval > 200)) {
throw new RangeError('invalid age for ' + this);
}
return newval;
}
will = new Person('Will', 29);
console.log(will); // Will, 29
try {
will.name = '';
} catch (e) {
console.log(e);
}
try {
will.age = -4;
} catch (e) {
console.log(e);
}
上述程式執行結果:
Will, 29 RangeError: invalid name for Will, 29 RangeError: invalid age for Will, 29
規格
Not part of any specifications. Implemented in JavaScript 1.2.
瀏覽器相容性
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | No support | (Yes) | No support | No support | No support |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | No support | No support | (Yes) | No support | No support | No support |
Compatibility notes
- This Polyfill offers
watch
to all ES5 compatible browsers. - Using a
Proxy
enables you do even deeper changes to how property assignments work. - Calling
watch()
on theDocument
object throws aTypeError
since Firefox 23 (bug 903332). This regression has been fixed with Firefox 27.