$applyAsync() vs $digest() – When to use which?

In AngularJS, $applyAsync() and $digest() are two methods used to trigger the AngularJS digest cycle, which is responsible for updating the views when the model changes. Both methods play a critical role in managing changes to the scope, but they serve different purposes and should be used in different scenarios. Understanding when to use each can significantly improve the performance and behavior of your AngularJS application.

$applyAsync()

$applyAsync() is a method introduced in AngularJS to allow you to schedule an evaluation to be run asynchronously. When you call $applyAsync(), AngularJS schedules the execution of a function during the next $digest cycle, but it does so asynchronously, which means it allows for other JavaScript tasks to complete first, and then it triggers the digest cycle.

Usage of $applyAsync()

Asynchronous Nature: $applyAsync() is useful when you want to delay the evaluation of expressions until after the current JavaScript execution stack has been cleared. This avoids unnecessary $digest cycles while still ensuring that changes to the scope are reflected in the view.
Batching Updates: If you need to make several changes to the scope and you don’t want to trigger a $digest cycle every time, $applyAsync() is ideal because it batches the changes together and only triggers the digest cycle once, after the current stack of synchronous code has been processed.
Performance Optimization: Using $applyAsync() can help improve performance in applications where frequent updates occur, such as when handling events that happen quickly in succession (e.g., mouse movements, scrolling, etc.).

Example of $applyAsync()

javascriptCopyEdit$scope.updateData = function() {
  // Updates to the scope model
  $scope.someModel = 'Updated Data';
  // Schedule the digest cycle asynchronously
  $scope.$applyAsync(function() {
    console.log('Digest cycle triggered asynchronously');
  });
};

When to Use $applyAsync()

When you need to update the scope asynchronously: Use $applyAsync() when the changes to the scope do not need to be reflected immediately, and you want to defer the digest cycle until after the current execution context is complete.
When you want to reduce the number of digest cycles: If you’re performing multiple scope changes in quick succession, $applyAsync() can help ensure that only one digest cycle is triggered, improving performance.

$digest()

$digest() is a method that manually triggers the digest cycle. This method checks all the watchers on the scope to see if the model has changed and needs to be updated in the view. Unlike $apply() or $applyAsync(), $digest() does not initiate a full $apply process, meaning it won’t evaluate expressions outside the current scope or run any $apply() or $applyAsync() hooks.

Usage of $digest()

Immediate Updates: $digest() is typically used when you need to trigger a digest cycle immediately, without scheduling it asynchronously. It ensures that all watchers in the scope are evaluated, and if any changes are detected, the view will be updated right away.
Manual Triggering: $digest() is useful when you have a custom event or interaction that needs to be processed immediately, and you want AngularJS to check the scope for changes right away.
Limited Scope: $digest() only checks the watchers for the current scope and its children. If you need to include parent scopes or global expressions, you would need to call $apply() instead.

Example of $digest()

javascriptCopyEdit$scope.updateData = function() {
  // Updates to the scope model
  $scope.someModel = 'Updated Data';
  // Manually trigger the digest cycle to reflect changes immediately
  $scope.$digest();
};

When to Use $digest()

When you need to immediately trigger a digest cycle: Use $digest() when you want to manually trigger the update process for the current scope (and its children) immediately, without waiting for AngularJS to decide when to run it.
In custom event handling or non-AngularJS code: If you are working with external libraries or custom events (like jQuery events), and you need to inform AngularJS that changes have been made to the scope, $digest() can help trigger the digest cycle.

Differences Between $applyAsync() and $digest()

Feature	$applyAsync()	$digest()
Purpose	Schedules the evaluation of changes in the scope asynchronously.	Manually triggers a digest cycle immediately.
Asynchronous	Yes, it runs asynchronously in the next digest cycle.	No, it runs immediately on the current scope.
Use Case	When you want to delay scope updates or batch them together.	When you need to trigger a digest cycle immediately for the current scope.
Digest Cycle	Triggers a digest cycle after the current stack of JavaScript code completes.	Immediately triggers the digest cycle for the scope.
Scope Update	Changes to the scope are reflected when the digest cycle runs.	Changes are reflected immediately after the `$digest()` call.
Performance	More optimized as it batches updates and reduces unnecessary digest cycles.	Can be performance-heavy if called too often because it triggers the digest cycle immediately.

When to Use Each:

Use $applyAsync() when:
- You want to batch multiple updates together and delay the digest cycle.
- You need to reduce the number of digest cycles, improving performance in cases with frequent updates.
- You don’t need to reflect the changes immediately but want to ensure they are processed in the next digest cycle.
Use $digest() when:
- You need to trigger the digest cycle immediately and ensure the scope updates are reflected right away.
- You are working in a situation where AngularJS is unaware of changes to the scope (e.g., interacting with third-party libraries or custom events).