Concerning React 18 and migration to createRoot

[wijnbladh]

Helloes,

Here are same observations from my brief investigation on how this library could be updated to React 18 and most notably the createRoot mechanic. Hopefully these remarks can be useful if that journey ever comes around.

1. Changing the render method invocation in inject method of ReactDomFactory.js to createRoot is quite straightforward, but we will probably need to stash the returned root values somewhere and use them later for unmounting in dispose

2. Unit tests might break badly since in React 18 the rendering is no longer an instant synchronous operation and the unit tests seem to expect this, consider for example:

// tests/Bootstrapper.spec.js, it('should render a component', (done) => { ...
...
const app = new App(containerBuilder.build(), () => {
    const componentLookup = node.innerHTML.match(/\[component MockComponent\]/g);
    expect(componentLookup).not.toEqual(null);
    expect(componentLookup.length).toEqual(1);

    done();
});
...

The second parameter to App is a callback that probably gets invoked once react-habitat thinks things are done. It works fine pre-createRoot. However, swicthing to createRoot breaks this test since nothing will appear in node at the time when this callback gets executed. Wrapping the expects into a timeout makes the test pass, but the root cause probably needs to be fixed elsewhere in the library:

// tests/Bootstrapper.spec.js, it('should render a component', (done) => { ...
...
const app = new App(containerBuilder.build(), () => {
    setTimeout(() => {
        const componentLookup = node.innerHTML.match(/\[component MockComponent\]/g);
        expect(componentLookup).not.toEqual(null);
        expect(componentLookup.length).toEqual(1);

        done();
    }, 0);
});
...

Additionally, when observing the unit test output in a Karma-browser window I can see the component render. To me these findings suggest that it is the no-longer-synchronous React 18 rendering that is the problem here.

Good luck!