$apply already in progress

The error $apply already in progress in AngularJS occurs when you attempt to manually trigger a $digest cycle using $scope.$apply() while AngularJS is already in the middle of running a $digest cycle. AngularJS runs its $digest cycle automatically when certain actions (like model updates, events, or external asynchronous functions) trigger the view to be updated. If you manually call $apply() while AngularJS is already processing the $digest cycle, it will throw this error to prevent an infinite loop or conflicting changes.

Step 1: Understanding $apply and $digest in AngularJS

In AngularJS, $digest is the process where the framework checks all watched variables (scope variables bound to the view) to see if their values have changed. When a change is detected, Angular updates the view to reflect the model’s new state.

• $scope.$apply(): This method is used to trigger a $digest cycle manually when changes happen outside of Angular’s normal context (for example, changes made in third-party libraries or asynchronous events like HTTP requests).
• $digest cycle: Angular automatically runs this cycle when the view needs to be updated, either after user interactions, timeouts, or events.

If you call $scope.$apply() inside the middle of a $digest cycle, AngularJS will throw the $apply already in progress error to prevent the cycle from triggering multiple times.

Step 2: Causes of $apply already in progress

Here are the common reasons why you might encounter this error:

1. Calling $apply() Inside Another $digest Cycle: When AngularJS is already running its $digest cycle, triggering $apply() will cause AngularJS to enter another $digest cycle within the ongoing one, which leads to this error.
2. Calling $apply() Inside setTimeout or setInterval: If you use $scope.$apply() inside a setTimeout() or setInterval() function, and these are triggered during the $digest cycle, Angular will detect that $apply() is being invoked while a $digest cycle is already running.
3. Using $apply() Inside an Asynchronous Callback: If you invoke $apply() inside an asynchronous callback (like in an HTTP request or a callback from a third-party library) that gets executed while the $digest cycle is ongoing, the error will occur.
4. Nested $apply() Calls: If $apply() is called inside a function that’s itself inside a $scope.$apply() or $timeout(), this will lead to nested $apply() calls, causing the error.

Step 3: How to Fix the $apply already in progress Error

1. Avoid Calling $apply() Inside Another $apply() or $digest Cycle

Ensure you don’t call $apply() from within another $apply() or $digest cycle. For example, if you are calling $apply() manually after an event or timeout, make sure it’s not happening during an already-running cycle.

// Incorrect: Calling $apply inside another $apply
$scope.$apply(function() {
  $scope.$apply(function() {
    $scope.someVar = 'value'; // This will throw $apply already in progress error
  });
});

2. Use $evalAsync() for Asynchronous Updates

If you need to update the model asynchronously but don’t want to trigger an additional $digest cycle, use $scope.$evalAsync(). This method schedules an expression to be evaluated in the next $digest cycle, rather than immediately triggering a new cycle.

$scope.someFunction = function() {
  $scope.$evalAsync(function() {
    // Update the model asynchronously without triggering an additional $digest cycle
    $scope.someVar = 'new value';
  });
};

Using $evalAsync() ensures that the update is handled in the next $digest cycle rather than causing nested cycles.

3. Use $timeout() Instead of $apply()

Instead of manually calling $apply(), you can use $timeout() to delay the execution of the code and allow Angular to run the $digest cycle before running the code. $timeout() automatically triggers the $digest cycle and handles this for you.

$timeout(function() {
  $scope.someVar = 'new value'; // $timeout ensures digest cycle handling
}, 0);

This ensures that Angular will handle the update properly without conflicting with the existing $digest cycle.

4. Check for Nested $timeout() Calls

Ensure that $timeout() isn’t being used in a nested manner, which could cause the $apply already in progress error.

$timeout(function() {
  $scope.someVar = 'new value';
  $timeout(function() {
    // This could cause a nested $apply problem
    $scope.someVar2 = 'another value';
  }, 0);
}, 0);

5. Make Sure $apply() Is Not Called in Event Listeners

If $apply() is called inside an event listener (such as $scope.$on()), and the event is triggered while the $digest cycle is already running, you’ll get the $apply already in progress error.

$scope.$on('someEvent', function(event, data) {
  $scope.$apply(function() {
    // This will cause $apply already in progress error if event is triggered inside an ongoing digest cycle
    $scope.someVar = data;
  });
});

To fix this, use $evalAsync() or $timeout() instead of $apply().

6. Use $rootScope for Global Changes

For global changes to the model, instead of using $apply() directly on a local scope, consider using $rootScope to trigger changes across the application.

$rootScope.$apply(function() {
  // Update model globally
  $scope.someVar = 'new value';
});

This helps avoid conflicts between nested $apply() calls on individual controllers or scopes.

Step 4: Debugging the Issue

1. Log the Execution Flow: Add console.log() statements in your $apply() and $digest code to ensure that it is not being triggered unexpectedly or in an overlapping context.

console.log('Before $apply');
$scope.$apply(function() {
  console.log('Inside $apply');
  $scope.someVar = 'new value';
});
console.log('After $apply');

This will help you identify where the $apply() call is being made and how the cycle is running.

2. Track $digest Cycle Triggers: Check whether the $digest cycle is running multiple times and whether any asynchronous code is involved in the triggering. You can use $scope.$$phase to check if a $digest cycle is already in progress.

if (!$scope.$$phase) {
  $scope.$apply(function() {
    $scope.someVar = 'new value';
  });
}

This ensures that $apply() is only called if the $digest cycle is not already in progress.

Step 5: Final Considerations

1. Avoid Manual $apply() in Most Cases: AngularJS’s automatic handling of $digest cycles works in most situations. Use $apply() sparingly and prefer $timeout(), $evalAsync(), or $scope.$watch() when appropriate.
2. Async Code Handling: When using third-party libraries, AJAX requests, or setTimeout, make sure that the model changes are handled asynchronously to avoid disrupting AngularJS’s digest cycle.
3. Event Handling: When updating scope variables in response to events or callbacks, make sure the $digest cycle is triggered correctly without causing conflicts by using $evalAsync().