Javascript 何时在 Angularjs 中使用 $watch 或 ng-change
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When to use $watch or ng-change in Angularjs
提问by levi
When should I use angular $watchfunctions and when use ng-changeangularjs directive? To me, they both can do the same.
什么时候应该使用 angular$watch函数，什么时候应该使用ng-changeangularjs 指令？对我来说，他们都可以做同样的事情。
Are there any differences or usage patterns between them?
它们之间有什么区别或使用模式吗？
回答by New Dev
They are not the same, clearly. One is used solely in the controller; the other is a directive on an input element.
他们不一样，很明显。一个单独用于控制器；另一个是输入元素上的指令。
But even in their application they differ.
但即使在它们的应用中，它们也不同。
When you use $watchthe watched expression will be evaluated on every digest cycle, and if there is a change, the handler is invoked.
当您使用$watch被监视的表达式时，将在每个摘要周期上评估，如果有更改，则调用处理程序。
With ng-change, the handler is invoked explicitly in response to an event.
使用ng-change，处理程序被显式调用以响应事件。
With $watch, change can come from anywhere: user action, controller function, service - all will trigger the handler.
使用$watch，更改可以来自任何地方：用户操作、控制器功能、服务——所有这些都将触发处理程序。
With ng-change, the change is restricted to a user action on a particular input element.
使用ng-change，更改仅限于用户对特定输入元素的操作。
It is worth to note also that ng-changeworks onlyin combination with ng-model- in other words, the ng-changeexpression is evaluated only when ngModel.$viewValue(refer to ngModelControllerdocumentation for more info) is changed, which typically happens in response to a user-initiated event.
还值得指出的是ng-change作品只有与组合ng-model-换句话说，该ng-change表达式只有当评估ngModel.$viewValue（参考ngModelController文档获取更多信息）被改变，这通常发生在响应用户启动的事件。
