Inline Anonymous Function In Flutter

Shashank Shrivastav is an ambitious Flutter developer who had been working on a shift managment app in a paregaon village (Maharashtra) . The app’s interface required several nested widgets, multiple states, and various asynchronous data fetching. Things were going smoothly until one day Shashank noticed a strange issue — his widgets were being rebuilt unnecessarily, causing performance issues, and a key part of the UI was showing incorrect values.

Shashank had a complex widget tree inside a StatefulWidget. He used an API call to fetch some data, and based on that data, a Text widget would update to show the count of tasks remaining for the day. However, the UI wasn't reflecting the correct count, even though he was sure the API was returning the right values.

Frustrated, Shashank decided to take a break, grabbing a cigarette to refresh his mind and clear his thoughts. After the break, feeling more focused, he returned to his code, hoping the issue might be something simple he had overlooked. But even with a fresh perspective, the same error persisted—his widget still wasn’t showing the correct count.

On top of that, his widget seemed to be rebuilding multiple times, even when there was no apparent state change. This was making the app sluggish, and Shashank couldn’t pinpoint the cause of the problem.

The Solution: Inline Anonymous Functions to the Rescue

Frustrated but determined, Shashank decided to introduce inline anonymous functions to debug the issue. He wanted to print the state and data returned by the API at different points in the widget tree.

Using inline anonymous functions in Flutter to print and return widgets is a flexible technique mainly useful for quick debugging and short-lived logic. However, it should be used with care to avoid cluttering your code and causing unnecessary rebuilds. When debugging, understanding where and when to use these functions can help you quickly identify issues, but it’s equally important to use more advanced debugging tools as your app grows in complexity.

What is an Inline Anonymous Function (Lambda)?

An inline anonymous function (lambda) is a function that is defined without a name and can be immediately invoked in the place where it is defined. In Dart (the language used by Flutter), you can create an anonymous function using the following syntax:

() {
  // Function body
  return someValue;
}()

• The () before the function body defines the anonymous function.
• The () after the function body immediately invokes it.
• The function can return any value, including widgets.

So shashank in build() method, he wrapped part of the widget tree in an anonymous function:

@override
Widget build(BuildContext context) {
  return Center(
    child: () {
      // Fetch and print the data for debugging
      int taskCount = fetchTaskCountFromApi();
      print('Task count fetched: $taskCount');

      // Return the widget based on the fetched data
      return Text('Tasks remaining: $taskCount');
    }(),
  );
}

Why It Worked for Him:

• Quick Debugging of State and API Data: By printing the task count directly inside the anonymous function, Shashank could instantly verify that the correct value was being fetched from the API. He saw the console output and realized that his state was fine — the problem was that the widget was rebuilding too often.
• Understanding Rebuilds: Shashank added additional print statements in different parts of the widget tree to check how often certain parts were rebuilt. This helped him identify that a parent widget was triggering unnecessary rebuilds, which was impacting the child widget’s performance.

With this quick inline debugging, Shashank figured out that he was incorrectly placing his API call inside the build() method, causing it to be called on every rebuild. He fixed the issue by moving the API call into initState() and then set the state only when new data was fetched.

When a Developer Will Use Inline Anonymous Functions to Debug

1. Tracking Widget Construction
2. Logging Values Temporarily
3. Debugging State

Conclusion:

Shashank realized that using inline anonymous functions can be a powerful tool for quick debugging, especially when you’re stuck and need to understand state management or lifecycle issues. It gave him immediate insights into the widget rebuilding problem without cluttering his code with too many separate methods.

However, he also understood that this technique was best used for temporary debugging, and for longer-term solutions, properly structured functions and logging would be a better approach.

Feel free to test the code on dart pad.

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