为什么scope。$ watch无法正常工作？

Question

I have angularjs directive with template which consists of two tags - input and the empty list. What I want is to watch input value of the first input tag. The $watch() method is called once when directive is initialised but that's it. Any change of input value is silent. What do I miss?

here is my plunk

and here is the code:

.directive('drpdwn', function(){
   var ref = new Firebase("https://sagavera.firebaseio.com/countries");

function link(scope, element, attrs){

  function watched(){
    element[0].firstChild.value = 'bumba'; //this gets called once
    return element[0].firstChild.value;
  }

  scope.$watch(watched, function(val){
    console.log("watched", val)
  }, true);

}
return {
  scope:{
    searchString: '='
  },
  link: link,
  templateUrl:'drpdwn.html'
}
})

Edit:

if I call $interval() within link function it works as expected. is this intentional behaviour?

Answer 1

It doesn't get called because there is never a digest triggered on the scope.

Try this instead, using ng-model:

http://plnkr.co/edit/qMP5NDEMYjXfYsoJtneL?p=preview

function link(scope, element, attrs){
  scope.searchString = 'bumba';
  scope.$watch('searchString', function(val){
    console.log("watched", val)
    val = val.toLowerCase();
    val = val.replace(/ */g,'');
    var regex = /^[a-z]*$/g;
    if (regex.test(val)) {
      scope.searchString = val;
      // TODO something....
    }
  }, true);

Template:

<input class="form-control"  ng-model="searchString" id="focusedInput"  type="text" placeholder="something" />
<div></div>

Answer 2

You are setting the the value you want in the watcher function.

function watched(){
    element[0].firstChild.value = 'bumba'; //this gets called once
    return element[0].firstChild.value;
}

is essentially the same return value as

function watched() {
    return 'bumba';
}

So the watcher will always return the same value.

If you want to initialize the value. Do it outside of the watcher:

element[0].firstChild.value = 'bumba'; //this gets called once
function watched(){
    return element[0].firstChild.value;
}