How to implement an ng-change for a custom directive

I have a directive with a template like

<div>
    <div ng-repeat="item in items" ng-click="updateModel(item)">
<div>

My directive is declared as:

return {
	templateUrl: '...',
	restrict: 'E',
	require: '^ngModel',
	scope: {
        items: '=',
		ngModel: '=',
		ngChange: '&'
	},
	link: function postLink(scope, element, attrs) 
    {
		scope.updateModel = function(item)
        {
             scope.ngModel = item;
             scope.ngChange();
        }
	}
}

I would like to have ng-change called when an item is clicked and the value of foo has been changed already.

That is, if my directive is implemented as:

<my-directive items=items ng-model="foo" ng-change="bar(foo)"></my-directive>

I would expect to call bar when the value of foo has been updated.

With code given above, ngChange is successfully called, but it is called with the old value of foo instead of the new updated value.

One way to solve the problem is to call ngChange inside a timeout to execute it at some point in the future, when the value of foo has been already changed. But this solution make me loose control over the order in which things are supposed to be executed and I assume that there should be a more elegant solution.

I could also use a watcher over foo in the parent scope, but this solution doesn't really give an ngChange method to be implmented and I have been told that watchers are great memory consumers.

Is there a way to make ngChange be executed synchronously without a timeout or a watcher?

Example: http://plnkr.co/edit/8H6QDO8OYiOyOx8efhyJ?p=preview

If you require ngModel you can just call $setViewValue on the ngModelController, which implicitly evaluates ng-change. The fourth parameter to the linking function should be the ngModelCtrl. The following code will make ng-change work for your directive.

link : function(scope, element, attrs, ngModelCtrl){
    scope.updateModel = function(item) {
        ngModelCtrl.$setViewValue(item);
    }
}

In order for your solution to work, please remove ngChange and ngModel from isolate scope of myDirective.

Here's a plunk: http://plnkr.co/edit/UefUzOo88MwOMkpgeX07?p=preview

tl;dr

In my experience you just need to inherit from the ngModelCtrl. the ng-change expression will be automatically evaluated when you use the method ngModelCtrl.$setViewValue

angular.module("myApp").directive("myDirective", function(){
  return {
    require:"^ngModel", // this is important, 
    scope:{
      ... // put the variables you need here but DO NOT have a variable named ngModel or ngChange 
    }, 
    link: function(scope, elt, attrs, ctrl){ // ctrl here is the ngModelCtrl
      scope.setValue = function(value){
        ctrl.$setViewValue(value); // this line will automatically eval your ng-change
      };
    }
  };
});

More precisely

ng-change is evaluated during the ngModelCtrl.$commitViewValue() IF the object reference of your ngModel has changed. the method $commitViewValue() is called automatically by $setViewValue(value, trigger) if you do not use the trigger argument or have not precised any ngModelOptions.

I specified that the ng-change would be automatically triggered if the reference of the $viewValue changed. When your ngModel is a string or an int, you don't have to worry about it. If your ngModel is an object and your just changing some of its properties, then $setViewValue will not eval ngChange.

If we take the code example from the start of the post

scope.setValue = function(value){
    ctrl.$setViewValue(value); // this line will automatically evalyour ng-change
};
scope.updateValue = function(prop1Value){
    var vv = ctrl.$viewValue;
    vv.prop1 = prop1Value;
    ctrl.$setViewValue(vv); // this line won't eval the ng-change expression
};

After some research, it seems that the best approach is to use $timeout(callback, 0).

It automatically launches a $digest cycle just after the callback is executed.

So, in my case, the solution was to use

$timeout(scope.ngChange, 0);

This way, it doesn't matter what is the signature of your callback, it will be executed just as you defined it in the parent scope.

Here is the plunkr with such changes: http://plnkr.co/edit/9MGptJpSQslk8g8tD2bZ?p=preview

Samuli Ulmanen and lucienBertin's answers nail it, although a bit of further reading in the AngularJS documentation provides further advise on how to handle this (see https://docs.angularjs.org/api/ng/type/ngModel.NgModelController).

Specifically in the cases where you are passing objects to $setViewValue(myObj). AngularJS Documentatation states:

> When used with standard inputs, the view value will always be a string (which is in some cases parsed into another type, such as a Date object for input[date].) However, custom controls might also pass objects to this method. In this case, we should make a copy of the object before passing it to $setViewValue. This is because ngModel does not perform a deep watch of objects, it only looks for a change of identity. If you only change the property of the object then ngModel will not realize that the object has changed and will not invoke the $parsers and $validators pipelines. For this reason, you should not change properties of the copy once it has been passed to $setViewValue. Otherwise you may cause the model value on the scope to change incorrectly.

For my specific case, my model is a moment date object, so I must clone the object first before then calling setViewValue. I am lucky here as moment provides a simple clone method: var b = moment(a);

link : function(scope, elements, attrs, ctrl) {
    scope.updateModel = function (value) {
        if (ctrl.$viewValue == value) {
            var copyOfObject = moment(value);
            ctrl.$setViewValue(copyOfObject);
        }
        else
        {
            ctrl.$setViewValue(value);
        }
    };
}

The fundamental issue here is that the underlying model does not get updated until the digest cycle that happens after scope.updateModel has finished executing. If the ngChange function requires details of the update that is being made then those details can be made available explicitly to ngChange, rather than relying on the model updating having been previously applied.

This can be done by providing a map of local variable names to values when calling ngChange. In this scenario, you can mapping the new value of the model to a name which can be referenced in the ng-change expression.

For example:

scope.updateModel = function(item)
{
    scope.ngModel = item;
    scope.ngChange({newValue: item});
}

In the HTML:

<my-directive ng-model="foo" items=items ng-change="bar(newValue)"></my-directive>

See: http://plnkr.co/edit/4CQBEV1S2wFFwKWbWec3?p=preview