$watch $apply and $evalAsync vs $timeout

$watch

$scope对象上的$watch方法会给Angular事件循环内的每个$digest调用装配一个脏值检查。如果在表达式上检测到变化， Angular总是会返回$digest循环。

$watch函数本身接受两个必要参数和一个可选的参数：

1.  watchExpression
   watchExpression可以是一个作用域对象的属性，或者是一个函数。在$digest循环中的每个$digest调用都会涉及它。如果watchExpression是一个字符串， Angular会在$scope上下文中对它求值。如果它是一个函数，那么Angular会认为它会返回应该被监控的值。
2.  listener/callback
   作为回调的监听器函数，它只会在watchExpression的当前值与先前值不相等（除了首次运行初始化期间）时调用。
3.  objectEquality（可选）
   objectEquality是一个进行比较的布尔值，用来告诉Angular是否检查严格相等。$watch函数会给监听器返回一个注销函数，我们可以调用这个注销函数来取消Angular对当前值的监控。

$evalAsync vs $timeout

http://stackoverflow.com/questions/17301572/angularjs-evalasync-vs-timeout
To summarize:
if code is queued using $evalAsync from a directive, it should run after the DOM has been manipulated by Angular, but before the browser renders
if code is queued using $evalAsync from a controller, it should run before the DOM has been manipulated by Angular (and before the browser renders) -- rarely do you want this
if code is queued using $timeout, it should run after the DOM has been manipulated by Angular, and after the browser renders (which may cause flicker in some cases)

$apply

http://blog.csdn.net/dm_vincent/article/details/38705099
如果你在AngularJS上下文之外的任何地方修改了model，那么你就需要通过手动调用$apply()来通知AngularJS。这就像告诉AngularJS，你修改了一些models，希望AngularJS帮你触发watchers来做出正确的响应。