RFC: re-write container API

[mdjastrzebski]

Summary

Current container API has following issues:

• it returns composite element, while we try to avoid return such components
• it is not behaving the same as RTL API of the same name, as RTL returns an element containing the rendered elements

Some users have been trying to use it in their component tests in a following way:

test('renders correctly', () => {
  const { container } = render(<Test />);
  expect(container).toBeEmptyElement()  // this should fail, but passes
});

Suggested solution

1. Rename container to UNSAFE_root to allow easy migration for users that might have valid uses for container API.

• Potential valid uses seem to be custom queries atm.
• Make container log deprecation message pointing to UNSAFE_root

3. Create a new root return from render (+ screen.root) that would expose root host component. This will allow to simplify writing component test, eg.:

test('renders correctly', () => {
  render(<Test />);
  expect(screen.root).toBeEmptyElement()
});

Related Issues

This should be released as part of v12.

• Original discussion: Container API is broken #1289
• Issue that triggered the dicussion toBeEmptyElement isn't working as expected testing-library/jest-native#141

[mdjastrzebski]

@thymikee @AugustinLF @pierrezimmermannbam @MattAgn looks good, or do you see something worth changing?

[pierrezimmermannbam]

No this seems good to me!

[MattAgn]

Seems good to me too