packages/flutter/test_private/README.md

# Private Test Runner

These are tests of private interfaces that can't easily happen in the regular
flutter tests due to problems with test and implementation interdependence.

This gets around the problem of parts existing in more than one library by
making a copy of the code under test.

The test script `bin/test_private.dart` tests private interfaces by copying the
code under test into a temporary workspace. The test is then free to make the
copied flutter source into a "part" of its own library by declaring a library
and using the `part` directive with a relative path to include the parts. This
way the test and the private interface are part of the same library, and the
private interface can be accessed by the test.

The tests are run like so:

```shell
dart run bin/test_private.dart
```

One limitation is that the copied private API needs to be separable enough to be
copied, so it needs to be in its own separate files.

To add a private test, add a manifest file of the form (assuming
"my_private_test" is the name of the test) to the [test](test) subdir:

```json
{
  "tests": [
    "my_private_test.dart"
  ],
  "pubspec": "my_private_test.pubspec.yaml",
  "deps": [
    "lib/src/subpackage/my_private_implementation.dart",
  ]
}
```

It will copy the files in `deps` relative to the `packages/flutter` directory
into a similar relative path structure in the test temporary directory tree. It
will copy the `pubspec` file into `pubspec.yaml` in the test temporary
directory, and copy all of the `tests` into the top of the test temporary
directory tree.

Each test gets its own temporary directory tree under a generated temporary
directory in the system temp dir that is removed at the end of the run, or under
the path given to `--temp-dir` on the command line. If a temporary directory is
given explicitly, it will not be deleted at the end of the run.