The $scope.$watch function does not activate with each individual change

Question

The $scope.$watch function does not activate with each individual change

Yesterday, I realized that in my angularJS 1.4.8 application, the $scope.$watch doesn't trigger on every change causing a bug to occur.

Is there a way to make it work on every change immediately? For example, in this code snippet, I want the function in watch to be triggered upon every change in the message:

(function(){

  angular.module('myApp', [])
  .controller('myAppController', myAppController)

  function myAppController($scope){
    console.log('controller loading !');
    $scope.message = 'message1';

    $scope.$watch('message', function(newMessage){
      console.log('newMessage', newMessage)
    });

    function changeMessage(){
      $scope.message='hi';
      $scope.message='hi12';
    }

    changeMessage(); 
  }

})();

The console output will be:

controller loading !
newMessage hi22

Plunker link available: https://plnkr.co/edit/SA1AcIVwr04uIUQFixAO?p=preview

Edit: I am curious if there are any alternative methods besides wrapping the change with a timeout and using scope.apply. In my original code, there are multiple instances where I change the scope property and I would like to find a solution that does not require this for every change.

javascript angularjs

Answer 1

Answer №1

The reason for this occurrence is that the watch function will only trigger if the value is modified "in between" digest loops.

In your case, your function is altering the message value within the same function, which means it will all be processed in a single digest loop. Therefore, when Angular moves on to the next loop, it will only recognize the last changed value, which in this scenario is hi22.

Here's an informative article that explains this behavior in detail.

Answer 2

The reason for this occurrence is that the watch function will only trigger if the value is modified "in between" digest loops.

In your case, your function is altering the message value within the same function, which means it will all be processed in a single digest loop. Therefore, when Angular moves on to the next loop, it will only recognize the last changed value, which in this scenario is hi22.

Here's an informative article that explains this behavior in detail.

Answer 3

Answer №2

Revamp your changeMessage method to incorporate the $scope.$apply function. This will guarantee that any modifications made are accurately displayed and Angular is notified of the alterations made to the variable.

changeMessage() {
   setTimeout(function () {
        $scope.$apply(function () {
          $scope.message = "Timeout called!";
        });
    }, 2000);
}

Answer 4

Revamp your changeMessage method to incorporate the $scope.$apply function. This will guarantee that any modifications made are accurately displayed and Angular is notified of the alterations made to the variable.

changeMessage() {
   setTimeout(function () {
        $scope.$apply(function () {
          $scope.message = "Timeout called!";
        });
    }, 2000);
}

Answer 5

Answer №3

When the value is changed within the same digest cycle, the watcher does not trigger and retains the last value. However, by using $timeout, we can change the value of $scope.message in the next digest cycle, allowing the watcher to capture the change as expected.

Here is a simple test example:

 $scope.$watch(function(){
  console.log('trigger');
  return $scope.message;
},
  function(newMessage){
  console.log('newMessage', newMessage)
});

function changeMessage(){
  $scope.message='hi';

  $timeout(function(){
    $scope.message='hi12';
  });      
}

Output:

controller loading !
 trigger
 newMessage hi
 trigger
 trigger
 newMessage hi12
 trigger

Answer 6

When the value is changed within the same digest cycle, the watcher does not trigger and retains the last value. However, by using $timeout, we can change the value of $scope.message in the next digest cycle, allowing the watcher to capture the change as expected.

Here is a simple test example:

 $scope.$watch(function(){
  console.log('trigger');
  return $scope.message;
},
  function(newMessage){
  console.log('newMessage', newMessage)
});

function changeMessage(){
  $scope.message='hi';

  $timeout(function(){
    $scope.message='hi12';
  });      
}

Output:

controller loading !
 trigger
 newMessage hi
 trigger
 trigger
 newMessage hi12
 trigger

Answer 7

Answer №4

It is unnecessary to wrap changeMessage in both setTimeout and $apply simultaneously. If you want to delay execution, simply use:

function changeMessage(){
    $timeout(function(){
        $scope.message = 'message';
    }/* or specify time here, it doesn't matter */);
}

Alternatively, you can use:

function changeMessage(){
    $scope.message = 'message';
    $scope.$apply();
}

Both approaches eventually trigger $rootScope.$digest. For more details, visit:

Answer 8

It is unnecessary to wrap changeMessage in both setTimeout and $apply simultaneously. If you want to delay execution, simply use:

function changeMessage(){
    $timeout(function(){
        $scope.message = 'message';
    }/* or specify time here, it doesn't matter */);
}

Alternatively, you can use:

function changeMessage(){
    $scope.message = 'message';
    $scope.$apply();
}

Both approaches eventually trigger $rootScope.$digest. For more details, visit:

Answer 9

Answer №5

$watch() is only triggered between every $digest().

For a detailed explanation about the $apply() and $digest()

In this scenario, the $scope.message is being updated in the current $digest() cycle.

To change this behavior, you can apply each new value to the $scope using $apply(), as suggested by @Ajinkya. However, setting a timeout of 2000ms does not always guarantee that it will be executed after the $digest(). Angular provides a built-in timeout function which can be used instead.

(function(){
  
  angular.module('myApp', [])
  .controller('myAppController', myAppController)
  
  function myAppController($scope, $timeout){
    console.log('controller loading !');
    $scope.message = 'message1';
    
    $scope.$watch('message', function(newMessage){
      console.log('newMessage', newMessage)
    });
    
    function changeMessage(){
    setTimeout(function () {
        $scope.$apply(function () {
          $scope.message='hi12';
        });
    }, 2000);
      
    }
    
    changeMessage(); 
  }
  
})();

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>

<div ng-app="myApp" ng-controller="myAppController"></div>

Solution

The most effective approach is to utilize the built-in $timeout function without specifying the time in milliseconds.

By doing this, Angular ensures that the $timeout will always run after the latest $digest(). Additionally, there is no need to use $scope.$apply() as the $timeout already triggers a $digest(), whereas $scope.$apply() initiates a new $digest() cycle manually.

(function(){
  
  angular.module('myApp', [])
  .controller('myAppController', myAppController)
  
  function myAppController($scope, $timeout){
    console.log('controller loading !');
    $scope.message = 'message1';
    
    $scope.$watch('message', function(newMessage){
      console.log('newMessage', newMessage)
    });
    
    function changeMessage(){
        $timeout(function () {
            $scope.message='hi12';
        });
      
    }
    
    changeMessage(); 
  }
  
})();

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>

    <div ng-app="myApp" ng-controller="myAppController"></div>

Answer 10

$watch() is only triggered between every $digest().

For a detailed explanation about the $apply() and $digest()

In this scenario, the $scope.message is being updated in the current $digest() cycle.

To change this behavior, you can apply each new value to the $scope using $apply(), as suggested by @Ajinkya. However, setting a timeout of 2000ms does not always guarantee that it will be executed after the $digest(). Angular provides a built-in timeout function which can be used instead.

(function(){
  
  angular.module('myApp', [])
  .controller('myAppController', myAppController)
  
  function myAppController($scope, $timeout){
    console.log('controller loading !');
    $scope.message = 'message1';
    
    $scope.$watch('message', function(newMessage){
      console.log('newMessage', newMessage)
    });
    
    function changeMessage(){
    setTimeout(function () {
        $scope.$apply(function () {
          $scope.message='hi12';
        });
    }, 2000);
      
    }
    
    changeMessage(); 
  }
  
})();

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>

<div ng-app="myApp" ng-controller="myAppController"></div>

Solution

The most effective approach is to utilize the built-in $timeout function without specifying the time in milliseconds.

By doing this, Angular ensures that the $timeout will always run after the latest $digest(). Additionally, there is no need to use $scope.$apply() as the $timeout already triggers a $digest(), whereas $scope.$apply() initiates a new $digest() cycle manually.

(function(){
  
  angular.module('myApp', [])
  .controller('myAppController', myAppController)
  
  function myAppController($scope, $timeout){
    console.log('controller loading !');
    $scope.message = 'message1';
    
    $scope.$watch('message', function(newMessage){
      console.log('newMessage', newMessage)
    });
    
    function changeMessage(){
        $timeout(function () {
            $scope.message='hi12';
        });
      
    }
    
    changeMessage(); 
  }
  
})();

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>

    <div ng-app="myApp" ng-controller="myAppController"></div>

The $scope.$watch function does not activate with each individual change

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Solution

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