AngularJS 指令绑定 & 简介

指令中独立scope 的 & 官方说明：

1. 绑定表达式

2. 经常用来绑定回调函数

诡异的地方在于，这个 & 某次听人说在子组件中是不能传值给callback的，好奇查了一下官方文档，更加诡异了：

We saw earlier how to use =attr in the scope option, but in the above example, we're using &attr instead. The & binding allows a directive to trigger evaluation of an expression in the context of the original scope, at a specific time. Any legal expression is allowed, including an expression which contains a function call. Because of this, & bindings are ideal for binding callback functions to directive behaviors.

When the user clicks the x in the dialog, the directive's close function is called, thanks to ng-click. This call to close on the isolated scope actually evaluates the expression hideDialog(message) in the context of the original scope, thus running Controller's hideDialog function.

Often it's desirable to pass data from the isolate scope via an expression to the parent scope, this can be done by passing a map of local variable names and values into the expression wrapper function. For example, the hideDialog function takes a message to display when the dialog is hidden. This is specified in the directive by calling close({message: 'closing for now'}). Then the local variable message will be available within the on-close expression.