servo/tests/wpt/web-platform-tests/common/worklet-reftest.js

// Imports code into a worklet. E.g.
//
// importWorklet(CSS.paintWorklet, {url: 'script.js'});
// importWorklet(CSS.paintWorklet, '/* javascript string */');
function importWorklet(worklet, code) {
    let url;
    if (typeof code === 'object') {
      url = code.url;
    } else {
      const blob = new Blob([code], {type: 'text/javascript'});
      url = URL.createObjectURL(blob);
    }

    return worklet.addModule(url);
}

async function animationFrames(frames) {
  for (let i = 0; i < frames; i++)
    await new Promise(requestAnimationFrame);
}

async function workletPainted() {
    await animationFrames(2);
}

// To make sure that we take the snapshot at the right time, we do double
// requestAnimationFrame. In the second frame, we take a screenshot, that makes
// sure that we already have a full frame.
async function importWorkletAndTerminateTestAfterAsyncPaint(worklet, code) {
    if (typeof worklet === 'undefined') {
        takeScreenshot();
        return;
    }

    await importWorklet(worklet, code);
    await workletPainted();
    takeScreenshot();
}