		# @feng3d/watcher
		用于监听对象属性的变化以及同步两个对象的属性值。

		### 监听对象属性的变化
		```
		const { watcher } = require('@feng3d/watcher');

		const o = { a: 1 };
		let out = '';
		const f = (_h, _p, _o) => { out += 'f'; };
		const f1 = (_h, _p, _o) => { out += 'f1'; };
		watcher.watch(o, 'a', f);
		watcher.watch(o, 'a', f1);
		o.a = 2;
		watcher.unwatch(o, 'a', f);
		o.a = 3;
		console.assert(out === 'ff1f1', out);
		```

		### 绑定两个对象的指定属性，保存两个属性值同步。
		```
		const { watcher } = require('@feng3d/watcher');

		const o1 = { a: 1 };
		const o2 = { a: 1 };

		watcher.bind(o1, 'a', o2, 'a');

		o1.a = 2;
		console.assert(o1.a == o2.a && o2.a === 2);

		o2.a = 5;
		console.assert(o1.a == o2.a && o1.a === 5);
		```

		### 监听对象属性链值变化
		```
		const { watcher } = require('@feng3d/watcher');

		const o = { a: { b: { c: 1 } } };
		let out = '';
		const f = (_h, _p, _o) => { out += 'f'; };
		const f1 = (_h, _p, _o) => { out += 'f1'; };
		watcher.watchchain(o, 'a.b.c', f);
		watcher.watchchain(o, 'a.b.c', f1);
		o.a.b.c = 2;
		watcher.unwatchchain(o, 'a.b.c', f);
		o.a.b.c = 3;
		console.assert(out === 'ff1f1', out);
		//
		out = '';
		watcher.unwatchchain(o, 'a.b.c', f1);
		o.a.b.c = 4;
		console.assert(out === '', out);
		//
		out = '';
		watcher.watchchain(o, 'a.b.c', f);
		o.a.b.c = 4;
		o.a.b.c = 5;
		console.assert(out === 'f', out);
		//
		out = '';
		o.a = { b: { c: 1 } };
		o.a.b.c = 3;
		console.assert(out === 'ff', `out:${out}`);
		//
		out = '';
		watcher.unwatchchain(o, 'a.b.c', f);
		o.a.b.c = 4;
		console.assert(out === '', `out:${out}`);
		//
		out = '';
		watcher.watchchain(o, 'a.b.c', f);
		o.a = null;
		o.a = { b: { c: 1 } };
		o.a.b.c = 5;
		console.assert(out === 'fff', out);
		```

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