Allow debugDefaultTargetPlatformOverride to be resetted with addTearDown

[passsy]

void main() {
  testWidgets('reset debugDefaultTargetPlatformOverride with tearDown', (tester) async {
    debugDefaultTargetPlatformOverride = TargetPlatform.macOS;
    addTearDown(() {
      debugDefaultTargetPlatformOverride = null;
    });
  });
}

produces

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following assertion was thrown running a test:
The value of a foundation debug variable was changed by the test.

When the exception was thrown, this was the stack:
#0      debugAssertAllFoundationVarsUnset.<anonymous closure> (package:flutter/src/foundation/debug.dart:30:7)
#1      debugAssertAllFoundationVarsUnset (package:flutter/src/foundation/debug.dart:32:4)
#2      TestWidgetsFlutterBinding._verifyInvariants (package:flutter_test/src/binding.dart:861:12)
#3      AutomatedTestWidgetsFlutterBinding._verifyInvariants (package:flutter_test/src/binding.dart:1274:11)
#4      TestWidgetsFlutterBinding._runTestBody (package:flutter_test/src/binding.dart:848:7)
<asynchronous suspension>

The test description was:
  reset debugDefaultTargetPlatformOverride with tearDown
════════════════════════════════════════════════════════════════════════════════════════════════════

I was expecting addTearDown to be the correct solution because setting the override to null might not be executed at the end of the test when the test fails.

In fact - without addTearDown - all remaining tests will fail with the exception above when this test fails, because it never executes debugDefaultTargetPlatformOverride = null;.

void main() {
  testWidgets('reset debugDefaultTargetPlatformOverride with tearDown', (tester) async {
    debugDefaultTargetPlatformOverride = TargetPlatform.macOS;
    // ... test implementation
	expect(true, isFalse);
	
    // might not be called
    debugDefaultTargetPlatformOverride = null;
  });

  testWidgets('default target platform is android', (tester) async {
    // fails with exception although this test is 100% correct
    expect(true, isTrue);
  });
}

How to fix

_verifyInvariants should be called after executing all addTearDown.

---

[✓] Flutter (Channel master, 3.1.0-0.0.pre.2434, on macOS 12.5.1 21G83
    darwin-arm, locale en-DE)
    • Flutter version 3.1.0-0.0.pre.2434 on channel master at
      /opt/homebrew/Caskroom/flutter/3.0.4/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision c8569b6e98 (6 days ago), 2022-08-23 13:50:09 -0700
    • Engine revision c9d0a01281
    • Dart version 2.19.0 (build 2.19.0-127.0.dev)
    • DevTools version 2.16.0

[huycozy]

@passsy: Thanks for filing the issue. This issue is reproducible on the latest stable and master channels with provided sample code.

console error logs

00:02 +0: reset debugDefaultTargetPlatformOverride with tearDown                                                                                     
══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following assertion was thrown running a test:
The value of a foundation debug variable was changed by the test.

When the exception was thrown, this was the stack:
#0      debugAssertAllFoundationVarsUnset.<anonymous closure> (package:flutter/src/foundation/debug.dart:34:7)
#1      debugAssertAllFoundationVarsUnset (package:flutter/src/foundation/debug.dart:37:4)
#2      TestWidgetsFlutterBinding._verifyInvariants (package:flutter_test/src/binding.dart:888:12)
#3      AutomatedTestWidgetsFlutterBinding._verifyInvariants (package:flutter_test/src/binding.dart:1335:11)
#4      TestWidgetsFlutterBinding._runTestBody (package:flutter_test/src/binding.dart:875:7)
<asynchronous suspension>

The test description was:
  reset debugDefaultTargetPlatformOverride with tearDown
════════════════════════════════════════════════════════════════════════════════════════════════════
00:02 +0 -1: reset debugDefaultTargetPlatformOverride with tearDown [E]                                                                              
  Test failed. See exception logs above.
  The test description was: reset debugDefaultTargetPlatformOverride with tearDown

flutter doctor -v

[✓] Flutter (Channel stable, 3.0.5, on macOS 12.2.1 21D62 darwin-x64, locale en-VN)
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    • Engine revision e85ea0e79c
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[✓] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
    • Android SDK at /Users/huynq/Library/Android/sdk
    • Platform android-33, build-tools 31.0.0
    • ANDROID_HOME = /Users/huynq/Library/Android/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jre/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.12+0-b1504.28-7817840)
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[✓] IntelliJ IDEA Community Edition (version 2022.1.1)
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[✓] Connected device (3 available)
    • SM T225 (mobile) • R9JT3004VRJ • android-arm64  • Android 11 (API 30)
    • macOS (desktop)  • macos       • darwin-x64     • macOS 12.2.1 21D62 darwin-x64
    • Chrome (web)     • chrome      • web-javascript • Google Chrome 103.0.5060.114

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[✓] Flutter (Channel master, 3.1.0-0.0.pre.2572, on macOS 12.5 21G72 darwin-x64, locale en-EE)
    • Flutter version 3.1.0-0.0.pre.2572 on channel master at /Users/huynq/Documents/GitHub/flutter_master
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision cfee9c8467 (25 minutes ago), 2022-08-29 21:50:23 -0400
    • Engine revision 0bf86adeff
    • Dart version 2.19.0 (build 2.19.0-149.0.dev)
    • DevTools version 2.16.0

[✓] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
    • Android SDK at /Users/huynq/Library/Android/sdk
    • Platform android-33, build-tools 31.0.0
    • ANDROID_HOME = /Users/huynq/Library/Android/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jre/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.12+0-b1504.28-7817840)
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[✓] Xcode - develop for iOS and macOS (Xcode 13.3)
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    • Build 13E113
    • CocoaPods version 1.11.3

[✓] Chrome - develop for the web
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[✓] Connected device (3 available)
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    • macOS (desktop)  • macos       • darwin-x64     • macOS 12.5 21G72 darwin-x64
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[DenisBogatirov]

Same issue

[passsy]

My current workaround is a custom addFlutterTearDown which only works with my custom testWidgets2 method

/// A special version of [addTearDown] that is executed before [testWidgets] completes.
///
/// Only to be used in conjunction with [robotTest].
///
/// It is required for configurations that need to be reset before the flutter test finishes, like:
///
/// - [debugDefaultTargetPlatformOverride]
/// - [debugImageOverheadAllowance]
/// - [debugInvertOversizedImages]
/// - [debugOnPaintImage]
/// - [debugNetworkImageHttpClientProvider]
void addFlutterTearDown(dynamic Function() callback) {
  if (Invoker.current == null) {
    throw StateError('addFlutterTearDown() may only be called within a test.');
  }

  final list = Zone.current[#flutter_test.teardowns] as List?;
  if (list == null) {
    throw StateError('addFlutterTearDown() may only be called within using testWidgets2');
  }
  list.add(callback);
}

import 'package:test_api/src/backend/invoker.dart';

@isTest
void testWidgets2(
  String description,
  WidgetTesterCallback callback, {
  bool? skip,
  Timeout? timeout,
  bool semanticsEnabled = true,
  dynamic tags,
}) {
  final List<dynamic Function()> flutterTearDowns = [];

  try {
    testWidgets(
      description,
      (tester) async {
        return runZoned(
          () async {
            try {
              await callback(tester);
            } finally {
              await Invoker.current!.runTearDowns(flutterTearDowns);
              flutterTearDowns.clear();
            }
          },
          zoneValues: {
            #flutter_test.teardowns: flutterTearDowns,
          },
        );
      },
      skip: skip,
      timeout: timeout,
      semanticsEnabled: semanticsEnabled,
      tags: tags,
    );
  } finally {
    flutterTearDowns.clear();
  }
}