向 TargetPlatform 枚举添加 'linux' 和 'windows'

Summary

Two new values were added to the TargetPlatform enum that could require additional cases in switch statements that switch on a TargetPlatform and don’t include a default: case.

Context

Prior to this change, the TargetPlatform enum only contained four values, and was defined like this:

enum TargetPlatform {
  android,
  fuchsia,
  iOS,
  macOS,
}

A switch statement only needed to handle these cases, and desktop applications that wanted to run on Linux or Windows usually had a test like this in their main() method:

// Sets a platform override for desktop to avoid exceptions. See
// https://flutter.dev/desktop#target-platform-override for more info.
void _enablePlatformOverrideForDesktop() {
  if (!kIsWeb && (Platform.isWindows || Platform.isLinux)) {
    debugDefaultTargetPlatformOverride = TargetPlatform.fuchsia;
  }
}

void main() {
  _enablePlatformOverrideForDesktop();
  runApp(MyApp());
}

Description of change

The TargetPlatform enum is now defined as:

enum TargetPlatform {
  android,
  fuchsia,
  iOS,
  linux, // new value
  macOS,
  windows, // new value
}

And the platform test setting debugDefaultTargetPlatformOverride in main() is no longer required on Linux and Windows.

This can cause the Dart analyzer to give the missing_enum_constant_in_switch warning for switch statements that don’t include a default case. Writing a switch without a default: case is the recommended way to handle enums, since the analyzer can then help you find any cases that aren’t handled.

Migration guide

In order to migrate to the new enum, and avoid the analyzer’s missing_enum_constant_in_switch error, which looks like:

warning: Missing case clause for 'linux'. (missing_enum_constant_in_switch at [package] path/to/file.dart:111)

or

warning: Missing case clause for 'windows'. (missing_enum_constant_in_switch at [package] path/to/file.dart:111)

Modify your code as follows:

Code before migration:

void dance(TargetPlatform platform) {
  switch (platform) {
    case TargetPlatform.android:
      // Do Android dance.
      break;
    case TargetPlatform.fuchsia:
      // Do Fuchsia dance.
      break;
    case TargetPlatform.iOS:
      // Do iOS dance.
      break;
    case TargetPlatform.macOS:
      // Do macOS dance.
      break;
  }
}

Code after migration:

void dance(TargetPlatform platform) {
  switch (platform) {
    case TargetPlatform.android:
      // Do Android dance.
      break;
    case TargetPlatform.fuchsia:
      // Do Fuchsia dance.
      break;
    case TargetPlatform.iOS:
      // Do iOS dance.
      break;
    case TargetPlatform.linux: // new case
      // Do Linux dance.
      break;
    case TargetPlatform.macOS:
      // Do macOS dance.
      break;
    case TargetPlatform.windows: // new case
      // Do Windows dance.
      break;
  }
}

Having default: cases in such switch statements isn’t recommended, because then the analyzer can’t help you find all the cases that need to be handled.

Also, any tests like the one referenced above that set the debugDefaultTargetPlatformOverride are no longer needed for Linux and Windows applications.

Timeline

Landed in version: 1.15.4
In stable release: 1.17

References

API documentation:

Relevant issues:

Relevant PR: