flutter programming and coding

How to Analyze and Debug ANR in a Flutter App?

If you’ve been developing Flutter apps, you may have encountered ANR (Application Not Responding) issues that cause your app to freeze and become unresponsive. ANR can be frustrating for both developers and users, but fear not! In this comprehensive guide, we’ll explore what ANR is, why it happens, and how to analyze and debug ANR in a Flutter app. Let’s dive in!

Understanding ANR in Flutter Apps

ANR stands for “Application Not Responding,” and it occurs when the main thread of your app becomes blocked for an extended period. The main thread is responsible for handling user interactions, rendering UI, and processing tasks like network requests and database operations. When the main thread is blocked, the app appears unresponsive, and users might see a system dialog asking if they want to close the app.

In Flutter, ANR issues typically arise from tasks that consume too much time on the main thread. Common causes include intensive computations, time-consuming network requests, and poorly optimized UI rendering.

Common ANR Causes in Flutter Apps

Before we dive into debugging ANR issues, let’s identify some common causes of ANR in Flutter apps:

  1. Long-running computations on the main thread
  2. Blocking network operations
  3. Inefficient database queries
  4. Excessive UI rendering
  5. Deadlock in synchronized blocks

Debugging ANR in a Flutter App

Now that we understand ANR and its common causes, let’s explore how to analyze and debug ANR issues in a Flutter app:

1. Using Flutter Performance Tools

Flutter provides built-in performance tools that can help you identify ANR issues. To use these tools, run your app in profile mode and then access the performance overlay. You can enable the performance overlay by shaking your device or emulator or by clicking the “Toggle Performance Overlay” button in the Flutter Inspector.

The performance overlay displays key performance metrics, including frames per second (FPS), memory usage, and GPU usage. By monitoring these metrics, you can identify areas of your app that might be causing ANR.

2. Using Systrace

Systrace is a powerful tool provided by Android that helps in tracing and debugging performance issues. It can be used to analyze ANR problems as well. To generate a Systrace report for your Flutter app, follow these steps:

  # Connect your device or emulator
  adb devices
  # Start tracing
  adb shell "su root && perfetto -o /data/misc/perfetto-traces/trace"
  # Reproduce the ANR in your app
  # Stop tracing
  adb shell "su root && pkill -SIGINT perfetto"
  # Pull the trace file to your computer
  adb pull /data/misc/perfetto-traces/trace ~/trace

Once you have the trace file, you can open it in Chrome by navigating to chrome://tracing/. Systrace provides a detailed view of CPU and thread activity, which can help you pinpoint the exact cause of ANR.

3. Identifying Expensive Operations

ANR issues often arise from operations that consume excessive time on the main thread. To identify such operations, use Flutter’s built-in timeline feature. By recording and analyzing the timeline, you can pinpoint expensive operations that lead to ANR.

  import 'dart:async';
  import 'package:flutter/foundation.dart';
  void main() {
    // Start recording the timeline
    // Perform expensive operations here
    // Stop recording the timeline

After running the app with the timeline recording, open the Flutter Inspector, click on the “Timeline” tab, and load the recorded timeline file. Analyzing the timeline will help you find the bottlenecks causing ANR.

Frequently Asked Questions (FAQs)

1. Why does ANR happen in Flutter apps?

ANR occurs when the main thread of the app is blocked for an extended period due to tasks that consume too much time. This can happen because of long-running computations, blocking network operations, inefficient database queries, excessive UI rendering, or deadlock in synchronized blocks.

2. How to prevent ANR in Flutter apps?

To prevent ANR, avoid performing long-running tasks on the main thread. Offload intensive computations and network operations to background threads or isolates. Optimize UI rendering to minimize the time spent on the main thread. Use asynchronous programming for time-consuming operations like file I/O and database queries.

3. How can Systrace help in debugging ANR?

Systrace provides a detailed view of CPU and thread activity during the app’s execution. By analyzing Systrace, you can identify which threads are blocking the main thread and causing ANR. This helps you pinpoint the exact cause of the issue and optimize your app accordingly.


ANR can be a challenging issue to deal with in Flutter apps, but armed with the right tools and knowledge, you can effectively analyze and debug ANR problems. Using Flutter’s built-in performance tools, Systrace, and timeline recording, you can identify and resolve bottlenecks that cause ANR and create a smooth and responsive user experience.

Happy coding!

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