Flickering issues due to slow digest cycle

In AngularJS, flickering issues often arise due to slow digest cycles, causing delayed updates to the UI. This happens when AngularJS takes too long to process scope changes, leading to visible delays in rendering.

In this guide, we will cover:
What causes a slow digest cycle.
How it leads to flickering in UI updates.
Best practices to fix flickering issues.

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1. Understanding the Digest Cycle in AngularJS

How AngularJS Updates the UI

AngularJS uses a digest cycle to detect changes and update the UI. When a change is made to a model, the digest cycle:
Checks for changes in all watched variables ($scope.$watch()).
Updates the DOM with the new values.
If changes trigger more updates, the digest cycle re-runs until the UI stabilizes.

When the Digest Cycle Becomes Slow

Too many watchers ($watch()) slow down digest cycles.
Large datasets processed in ng-repeat cause heavy computation.
Excessive use of $digest() or $apply() manually can overload updates.
External event listeners (e.g., setTimeout, setInterval) triggering unnecessary digest cycles.

If the digest cycle takes too long, UI updates become laggy, leading to flickering issues.

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2. Common Causes of Flickering in AngularJS

Heavy Use of $watch() in Controllers

Using too many $watch() expressions forces AngularJS to track a large number of variables, slowing down the digest cycle.

Bad Example (Too Many Watchers)

app.controller("MyController", function ($scope) {
    $scope.$watch("user.name", function (newValue) {
        console.log("User name changed:", newValue);
    });

    $scope.$watch("user.age", function (newValue) {
        console.log("User age changed:", newValue);
    });

    $scope.$watch("user.email", function (newValue) {
        console.log("User email changed:", newValue);
    });
});

Problem: Every time any of these variables change, AngularJS runs multiple checks, increasing digest time and causing flickering.

Solution: Use $watchGroup() Instead

app.controller("MyController", function ($scope) {
    $scope.$watchGroup(["user.name", "user.age", "user.email"], function (newValues) {
        console.log("User details changed:", newValues);
    });
});

Reduces the number of watchers, improving performance.

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Large Datasets in ng-repeat

Using ng-repeat with a large dataset can cause UI flickering due to frequent DOM updates.

Bad Example (Slow Rendering)

<div ng-repeat="item in largeDataset">
    <p>{{ item.name }}</p>
</div>

Problem:

• If largeDataset has thousands of items, the digest cycle has to track every item and update them one by one, slowing UI updates.

Solution: Use track by to Optimize ng-repeat

<div ng-repeat="item in largeDataset track by item.id">
    <p>{{ item.name }}</p>
</div>

Prevents unnecessary DOM re-renders, reducing flickering.

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Excessive $digest() or $apply() Calls

Calling $digest() or $apply() too frequently triggers unnecessary UI updates, leading to flickering.

Bad Example (Forcing Digest Too Often)

app.controller("MyController", function ($scope, $interval) {
    $interval(function () {
        $scope.counter++;
        $scope.$apply(); // Forces UI update every second
    }, 1000);
});

Problem:

• Every second, $apply() forces a full digest cycle, even if no real update is needed.

Solution: Use $timeout Instead of $apply()

app.controller("MyController", function ($scope, $timeout) {
    function updateCounter() {
        $scope.counter++;
        $timeout(updateCounter, 1000);
    }
    updateCounter();
});

Uses AngularJS’s built-in $timeout(), preventing excessive digest cycles.

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Using $watch() on Complex Objects

Watching an entire object causes frequent UI updates, even if only a small part of the object changes.

Bad Example (Watching an Entire Object)

$scope.$watch("user", function (newValue) {
    console.log("User data changed:", newValue);
}, true); // Deep watch

Problem:

• AngularJS deeply checks every property in the user object, even if only one property changes.

Solution: Watch Specific Properties Instead

$scope.$watch("user.name", function (newValue) {
    console.log("User name changed:", newValue);
});

Improves performance by reducing unnecessary checks.

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Handling External API Calls Efficiently

If your app frequently makes API calls (e.g., polling data), it can trigger unnecessary digest cycles, leading to flickering.

Bad Example (Frequent API Calls)

setInterval(function () {
    $http.get("/api/data").then(function (response) {
        $scope.data = response.data;
    });
}, 2000);

Problem:

• setInterval() keeps making requests even if the scope is destroyed, causing performance issues.

Solution: Use $interval and Cancel on $destroy

var interval = $interval(function () {
    $http.get("/api/data").then(function (response) {
        $scope.data = response.data;
    });
}, 2000);

$scope.$on("$destroy", function () {
    $interval.cancel(interval);
});

Prevents unnecessary API calls when the controller is destroyed.

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3. Best Practices to Fix Flickering Issues

Use One-Time Binding (::) Where Possible

If a variable doesn’t need to be updated, use one-time binding to avoid unnecessary digest cycles:

<h1>{{ ::user.name }}</h1>

Prevents AngularJS from checking user.name on every cycle.

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Use ng-if Instead of ng-show/ng-hide

• ng-show and ng-hide keep elements in the DOM, even if hidden.
• ng-if removes elements completely, reducing the digest load.

Bad Example (Using ng-show)

<div ng-show="isVisible">
    <p>This text is always in the DOM.</p>
</div>

Better Solution (Using ng-if)

<div ng-if="isVisible">
    <p>This text is removed from the DOM when hidden.</p>
</div>

Reduces the number of DOM elements AngularJS needs to track.