What does Angular classify as a watcher?

Question

What does Angular classify as a watcher?

What qualifies as "watchers" within Angular? Are watchers the only type of monitoring mechanism, or do other Angular elements like ngModel also serve as watchers?

Am I overlooking something crucial? For example, do watchers play a role in enabling directives such as ngModel to function properly?

update: Is there a method to verify the presence of watchers? During testing, how can I determine when to trigger scope.$digest()?

javascript angularjs watch angularjs-watch

Answer 1

Answer №1

watchers is essentially dirty checking, which tracks the old and new values.

During each digest cycle, these watchers are evaluated. They can be a combination of scope variables or any expression. Angular gathers all these watchers on every digest cycle and maintains them inside an array called $$watchers. To check how many watchers are present, you can use console.log($scope.$watchers) within your controller.

Markup

<body class="container" ng-controller="myCtrl">
  Hello {{test}}
  I'm testing to show $$watchers {{test1}}
  <div ng-show="show">Showing this div using ng-show</div>
  <div ng-if="show">Showing this div using ng-show</div>
  Watcher is not compulsory that should be scope variables {{'1'}}
</body>

Plunkr

In the above code snippet, try to guess how many watchers there are. Although there are only 3 {{}} interpolation directives visible, if you inspect the console with $scope.$$watchers, it will display 5 watchers.

Why does it show 5 watchers when we can only see 3? This is because we have used the ng-show and ng-if directives, which internally create $watch on the provided expression in their attributes. These expressions get evaluated during each digest cycle.

You can also create custom watchers using $watch (deep/simple watch) & $watchGroup

Additionally, you can utilize watchers with $attrs.$observe, which functions similarly to $watch but specifically for interpolation variables.

$attrs.$observe('test',function(value){
    alert('')
});

Many Angular directives internally employ watchers such as ng-repeat, ng-show, ng-if, ng-include, ng-switch, ng-bind, interpolation directive {{}}, filters, etc. These directives incorporate watchers to manage two-way binding effectively.

Answer 2

watchers is essentially dirty checking, which tracks the old and new values.

During each digest cycle, these watchers are evaluated. They can be a combination of scope variables or any expression. Angular gathers all these watchers on every digest cycle and maintains them inside an array called $$watchers. To check how many watchers are present, you can use console.log($scope.$watchers) within your controller.

Markup

<body class="container" ng-controller="myCtrl">
  Hello {{test}}
  I'm testing to show $$watchers {{test1}}
  <div ng-show="show">Showing this div using ng-show</div>
  <div ng-if="show">Showing this div using ng-show</div>
  Watcher is not compulsory that should be scope variables {{'1'}}
</body>

Plunkr

In the above code snippet, try to guess how many watchers there are. Although there are only 3 {{}} interpolation directives visible, if you inspect the console with $scope.$$watchers, it will display 5 watchers.

Why does it show 5 watchers when we can only see 3? This is because we have used the ng-show and ng-if directives, which internally create $watch on the provided expression in their attributes. These expressions get evaluated during each digest cycle.

You can also create custom watchers using $watch (deep/simple watch) & $watchGroup

Additionally, you can utilize watchers with $attrs.$observe, which functions similarly to $watch but specifically for interpolation variables.

$attrs.$observe('test',function(value){
    alert('')
});

Many Angular directives internally employ watchers such as ng-repeat, ng-show, ng-if, ng-include, ng-switch, ng-bind, interpolation directive {{}}, filters, etc. These directives incorporate watchers to manage two-way binding effectively.

Answer 3

Answer №2

Observers, as outlined in the documentation you're referencing, are mechanisms within Angular designed to monitor a variable or function result in a two-way binding fashion throughout any given digest cycle, regardless of what triggers that cycle.

In my view, anything in Angular that can respond to ANY event qualifies as an "observer."

The typical way to implement an observer is by using: $scope.watch(...)

However, it's important to minimize the use of observers whenever possible.
They require their callback functions to run at EACH digest cycle for dirty checking, which can greatly impact performance.

ng-model does not directly correlate with the concept of observers.
ng-model simply facilitates binding between a view and controller variable, distinguishing itself as a separate concept.

Answer 4

Observers, as outlined in the documentation you're referencing, are mechanisms within Angular designed to monitor a variable or function result in a two-way binding fashion throughout any given digest cycle, regardless of what triggers that cycle.

In my view, anything in Angular that can respond to ANY event qualifies as an "observer."

The typical way to implement an observer is by using: $scope.watch(...)

However, it's important to minimize the use of observers whenever possible.
They require their callback functions to run at EACH digest cycle for dirty checking, which can greatly impact performance.

ng-model does not directly correlate with the concept of observers.
ng-model simply facilitates binding between a view and controller variable, distinguishing itself as a separate concept.

Answer 5

Answer №3

Let's dive into the concepts of $watch(), $digest(), and $apply() with a practical example:

<div ng-controller="myController">
{{data.time}}

<br/>
<button ng-click="updateTime()">Update Time - Angular Click Event</button>
<button id="updateTimeButton">Update Time</button>
</div>
<script>
var module = angular.module("myapp", []);
var myController1 = module.controller("myController", function($scope) {
    $scope.data = { time : new Date() };
    $scope.updateTime = function() {
        $scope.data.time = new Date();
    }
    document.getElementById("updateTimeButton")
            .addEventListener('click', function() {
        console.log("Update time clicked");
        $scope.data.time = new Date();
    });
});
</script>

In this demonstration, we bind the $scope.data.time variable to an interpolation directive to display its value in the HTML page. This binding automatically creates an internal watch on the $scope.data.time variable.

The example includes two buttons. The first button triggers the $scope.updateTime() function through an ng-click event listener. Upon clicking this button, AngularJS invokes $scope.$digest() to update data bindings accordingly.

However, the second button features a regular JavaScript event listener within the controller function. Although both buttons essentially perform the same task, the absence of $scope.$digest() after executing the event listener for the second button results in the updated data not being displayed. In other words, clicking the second button updates the $scope.data.time variable but fails to reflect the changes visually.

To resolve this issue, one can include a $scope.$digest() call at the end of the button event listener as follows:

document.getElementById("updateTimeButton")
    .addEventListener('click', function() {
console.log("Update time clicked");
$scope.data.time = new Date();
$scope.$digest();
});

An alternative approach involves utilizing the $apply() function within the button event listener:

document.getElementById("updateTimeButton")
    .addEventListener('click', function() {
$scope.$apply(function() {
    console.log("Update time clicked");
    $scope.data.time = new Date();
});
});

Take note of how $scope.$apply() is invoked inside the button event listener and the update operation on $scope.data.time is encapsulated within the function passed to $apply(). Once the $apply() call completes, AngularJS internally triggers $digest() to ensure all data bindings are updated accordingly.

Answer 6

Let's dive into the concepts of $watch(), $digest(), and $apply() with a practical example:

<div ng-controller="myController">
{{data.time}}

<br/>
<button ng-click="updateTime()">Update Time - Angular Click Event</button>
<button id="updateTimeButton">Update Time</button>
</div>
<script>
var module = angular.module("myapp", []);
var myController1 = module.controller("myController", function($scope) {
    $scope.data = { time : new Date() };
    $scope.updateTime = function() {
        $scope.data.time = new Date();
    }
    document.getElementById("updateTimeButton")
            .addEventListener('click', function() {
        console.log("Update time clicked");
        $scope.data.time = new Date();
    });
});
</script>

In this demonstration, we bind the $scope.data.time variable to an interpolation directive to display its value in the HTML page. This binding automatically creates an internal watch on the $scope.data.time variable.

The example includes two buttons. The first button triggers the $scope.updateTime() function through an ng-click event listener. Upon clicking this button, AngularJS invokes $scope.$digest() to update data bindings accordingly.

However, the second button features a regular JavaScript event listener within the controller function. Although both buttons essentially perform the same task, the absence of $scope.$digest() after executing the event listener for the second button results in the updated data not being displayed. In other words, clicking the second button updates the $scope.data.time variable but fails to reflect the changes visually.

To resolve this issue, one can include a $scope.$digest() call at the end of the button event listener as follows:

document.getElementById("updateTimeButton")
    .addEventListener('click', function() {
console.log("Update time clicked");
$scope.data.time = new Date();
$scope.$digest();
});

An alternative approach involves utilizing the $apply() function within the button event listener:

document.getElementById("updateTimeButton")
    .addEventListener('click', function() {
$scope.$apply(function() {
    console.log("Update time clicked");
    $scope.data.time = new Date();
});
});

Take note of how $scope.$apply() is invoked inside the button event listener and the update operation on $scope.data.time is encapsulated within the function passed to $apply(). Once the $apply() call completes, AngularJS internally triggers $digest() to ensure all data bindings are updated accordingly.

What does Angular classify as a watcher?

Answer №1

Answer №2

Answer №3

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