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4 Flutter facts I got wrong until I wrote the tests

2026-10-11 · Manish Kumar Panday

I've been building DartIsCool, a free animated course that explains how Flutter works inside. My rule for the Flutter section was simple: no claim goes on a page unless a flutter test proves it. 317 tests later, four of my "facts" turned out to be wrong or incomplete.

1. Without itemExtent, one jump can build thousands of list items

ListView.builder(
  controller: controller,
  itemCount: 10000,
  itemExtent: 80, // without this line, see below
  itemBuilder: (context, i) => ListItem(i),
);

controller.jumpTo(400000); // item 5000 at the top

I expected a lazy list to build only what's on screen. With every item 80 px tall and the list jumping to item 5,000, the test counted the itemBuilder calls: 5,000 without itemExtent, 18 with it (prototypeItem also gave 18). Without a fixed extent, Flutter has to lay out every item between the old and new position to know where item 5,000 starts.

2. popUntil with a typo doesn't throw — it empties the stack

Navigator.of(context).popUntil(ModalRoute.withName('/hmoe')); // meant '/home'

No route matches, so popUntil keeps popping — past /home, past the first route — and leaves an empty navigator, without any exception. Worth a guard in code review.

3. Two identical const widgets aren't always the same object

final a = const Text('Hi');
final b = const Text('Hi');
print(identical(a, b)); // false in debug and flutter test, true in release

In debug builds Flutter's widget-creation tracking quietly records where each widget was written, so two const widgets on different lines differ. In release they're canonicalised into one object. If you reason about rebuilds using identical, test in the mode you care about.

4. --dart-define is not a place for secrets

const apiKey = String.fromEnvironment('API_KEY');

Once your code actually uses apiKey, the value is compiled into the release binary and can be read straight out of it. (If you only use, say, its length, the compiler folds it away — which is why "I checked and it wasn't there" can mislead you.) Anything you ship can be extracted; keep real secrets on a server.

Each of these comes with an animation you can drive with your own input — keys and State, Row's flex layout, a frame timeline, lazy list building, the Navigator stack, deep links — in the six lessons of Level 12:

https://dartiscool.flutterdev.in/flutter/f01-widgets-elements.html

The same site also has a problem-solving level — about 90 classic interview problems grouped by pattern, from Two Sum to N-Queens, each animated and solved in tested Dart: https://dartiscool.flutterdev.in/problems/p01-arrays-hashing.html

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