> On May 30, 2019, at 11:01 PM, Ryosuke Niwa <[email protected]> wrote:
>
> I’m gonna give you a game changing function:
>
> function listenForEventOnce(target, name, timeout) {
> return new Promise((resolve, reject) => {
> const timer = timeout ? setTimeout(reject, timeout) : null;
> target.addEventListener(name, () => {
> if (timer)
> clearTimeout(timer);
> resolve();
> }, {once: true});
> });
> }
>
> You can then write a test like this:
> await listenForEventOnce(document.body, 'load');
> // do stuff after load event.
>
> await listenForEventOnce(document.querySelector('input'), 'focus');
> await listenForEventOnce(visualViewport, 'scroll', 5000);
> // After the input element is focused, then the visual viewport scrolled or 5
> seconds has passed.
Ryosuke++.
Just FYI, if you’re writing LayoutTests, we’ve got something very similar in
LayoutTests/media/video-test.js:
function waitFor(element, type) {
return new Promise(resolve => {
element.addEventListener(type, event => {
consoleWrite(`EVENT(${event.type})`);
resolve(event);
}, { once: true });
});
}
And:
function sleepFor(duration) {
return new Promise(resolve => {
setTimeout(resolve, duration);
});
}
And also:
function shouldReject(promise) {
return new Promise((resolve, reject) => {
promise.then(result => {
logResult(Failed, 'Promise resolved incorrectly');
reject(result);
}).catch((error) => {
logResult(Success, 'Promise rejected correctly');
resolve(error);
});
});
}
So you could also do:
await Promise.race(waitFor(document.body, ‘load’),
shouldReject(sleepFor(5000)));
Although we’d probably want a new function, “rejectIn(duration)”, and then it’d
be:
await shouldResolve(Promise.race(waitFor(document.body, ‘load’),
rejectIn(5000)));
But all that said, I agree that wrapping events in Promises makes it very easy
to write readable, single `async` function test cases. A+++, would write test
again.
-Jer
> - R. Niwa
>
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