Object.prototype.watch()

概述

监视一个对象的某个属性是否发生变化,在该属性变化时立即触发指定的回调函数.

语法

object.watch(prop, handler)

参数

prop

想要监视值是否发生变化的指定对象的某个属性的属性名称

handler

当指定的属性发生变化时执行的回调函数

描述

Watches for assignment to a property named prop in this object, calling 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).

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 unwatch() method. 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.

例子

例子: 使用 watch 和 unwatch

var o = {p:1};
o.watch("p",
  function (id, oldval, newval) {
    console.log("o." + id + "由" + oldval + " 变为 " + newval);
    return newval;
  });

o.p = 2;
o.p = 3;
delete o.p;
o.p = 4;

o.unwatch('p');
o.p = 5;

上面的代码显示结果如下:

o.p 由 1 变为 2
o.p 由 2 变为 3
o.p 由 undefined 变为 4

例子: 使用 watch 来验证一个对象的属性

你可以使用 watch 来检测一个对象的属性赋值是否是合法的.下例演示了如何确保每个人始终具有一个合法的名字和0 到 200之间的年龄.

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("不合法的名字 " + this);
  }
  if (id === "age"  && (newval < 0 || newval > 200)) {
    throw new RangeError("不合法的年龄 " + this);
  }
  return newval;
}

will = new Person("Will", 29);
print(will);   // Will, 29

try {
  will.name = "";
} catch (e) {
  //print(e);
  console.log(e);
}

try {
  will.age = -4;
} catch (e) {
  //print(e);
  console.log(e);
}

上面的代码显示结果如下:

Will, 29
RangeError: 不合法的名字 Will, 29
RangeError: 不合法的年龄 Will, 29

兼容性

• Polyfill 在所有的支持ES5的浏览器上提供了watch方法.
• Using a Proxy enables you do that even deeper changes to how property assignments work

相关链接

unwatch()