Using $scope.$evalAsync() for async updates

In AngularJS, $scope.$evalAsync() is a powerful method used to schedule an asynchronous update to the scope model. It allows you to evaluate expressions and update the scope in a way that avoids triggering multiple digest cycles. This is particularly useful in scenarios where you need to perform asynchronous tasks but want to ensure that the view is updated efficiently.

Understanding $scope.$evalAsync()

$scope.$evalAsync() is used to schedule an expression (or function) to be executed in the next digest cycle, which happens after AngularJS has completed all its current tasks. Essentially, it queues an expression to be evaluated asynchronously, avoiding potential issues with timing and ensuring that the digest cycle runs only once for multiple changes.

Unlike $scope.$apply() or $scope.$digest(), which explicitly trigger the digest cycle, $evalAsync() schedules an update to occur within the next digest cycle, ensuring that AngularJS doesn’t needlessly trigger multiple digest cycles for the same set of changes.

This method is particularly helpful when you need to execute code that interacts with AngularJS models (e.g., updating a scope property) but you’re outside of AngularJS’s context, such as when working with third-party libraries, setTimeout, or promises.

Basic Syntax

$scope.$evalAsync(expression);

• expression: The expression or function to be evaluated within the next digest cycle. This can be a string containing an expression or a function that will be executed.

Use Cases for $scope.$evalAsync()

1. Avoiding Digest Loops in Asynchronous Operations

When working with asynchronous code (e.g., API calls, timers, etc.), you might need to update the scope after the operation is completed. Using $scope.$evalAsync() allows AngularJS to schedule the update during the next digest cycle, rather than triggering unnecessary digest cycles directly within the async callback.

Example:

angular.module('app', [])
  .controller('AsyncController', function($scope, $timeout) {
    $scope.data = "Initial Data";

    // Simulating an async operation (e.g., API call)
    $timeout(function() {
      // After the async operation completes, update the scope
      $scope.$evalAsync(function() {
        $scope.data = "Updated Data";
      });
    }, 1000);
  });

In this example, $timeout simulates an asynchronous operation. Once the operation completes, $scope.$evalAsync() schedules the update to the $scope.data value. This ensures that AngularJS will handle the scope update during the next digest cycle.

2. Preventing Unnecessary Digest Cycles

In some cases, if you were to directly update $scope properties inside a setTimeout or promise callback, AngularJS may trigger multiple digest cycles, potentially causing performance issues. $scope.$evalAsync() helps prevent these unnecessary cycles.

Example:

angular.module('app', [])
  .controller('TestController', function($scope, $timeout) {
    $scope.counter = 0;

    // Simulating an async operation that updates the model
    $timeout(function() {
      $scope.counter++;
    }, 1000);

    // Using $evalAsync to update after some external operation
    $timeout(function() {
      $scope.$evalAsync(function() {
        $scope.counter++;
      });
    }, 1500);
  });

Without $evalAsync(), each update to $scope.counter would trigger its own digest cycle. By using $evalAsync(), AngularJS batches the changes and evaluates them in a single digest cycle, ensuring better performance.

3. Updating Scope Models After DOM Updates

Sometimes, you may need to interact with the DOM or perform certain actions after AngularJS has already updated the view. $scope.$evalAsync() ensures that updates to the scope are queued and handled in the next digest cycle after the current one is complete.

For example, after interacting with third-party libraries that manipulate the DOM directly, you can use $scope.$evalAsync() to update the model and ensure that the view is synchronized with the scope.

Example:

angular.module('app', [])
  .controller('DOMController', function($scope, $timeout) {
    $scope.message = "Hello World";

    // Using $evalAsync to update the scope after DOM manipulation
    $timeout(function() {
      $scope.$evalAsync(function() {
        $scope.message = "Updated after DOM manipulation!";
      });
    }, 2000);
  });

In this case, $timeout simulates a DOM update after two seconds. Using $evalAsync() ensures that the scope is updated in the next digest cycle, keeping the model and view in sync.

How $scope.$evalAsync() Works Internally

AngularJS manages an internal digest cycle that checks for changes in the scope and updates the view accordingly. The $evalAsync() method adds an expression or function to a queue, which will be evaluated when AngularJS runs its next digest cycle. This process helps AngularJS efficiently handle updates without unnecessary reflows, improving performance.

It’s important to note that $evalAsync() does not trigger a new digest cycle immediately. It schedules the update to happen during the next digest cycle, which means that the update won’t occur right away but will be done at the appropriate time in AngularJS’s execution flow.

Differences Between $evalAsync(), $apply(), and $digest()

• $apply(): Triggers a full digest cycle, meaning AngularJS will check the entire scope for changes. This is useful when you are working outside AngularJS’s context (like in a third-party library) and need to manually inform AngularJS that the scope has changed.
• $digest(): Starts a digest cycle manually. It checks for changes in the scope and updates the view. Unlike $apply(), $digest() only checks the current scope and its children, not the entire scope tree.
• $evalAsync(): Schedules an expression to be evaluated during the next digest cycle. Unlike $apply() and $digest(), $evalAsync() doesn’t immediately trigger a digest cycle. It only schedules the update to be processed later, which is more efficient for asynchronous tasks.

Best Practices for Using $scope.$evalAsync()

• Use $evalAsync() for Asynchronous Updates: When working with third-party libraries or any other asynchronous task, always use $evalAsync() to ensure that updates to the scope happen during the next digest cycle without unnecessarily triggering multiple cycles.
• Avoid Multiple $digest() or $apply() Calls: Instead of calling $apply() or $digest() in multiple places, try to use $evalAsync() when possible. This minimizes the number of digest cycles triggered by your application and improves performance.
• Avoid Direct Manipulation of the DOM: AngularJS provides a mechanism to manage the DOM and scope efficiently. If you need to interact with the DOM or use third-party libraries, always ensure that the updates are properly synchronized with AngularJS’s digest cycle using $evalAsync().