"Inject" callback with async/await

Question

I'm refactoring some promise code to using async/await, and it works great except for one thing.

Something that a Promise lets you do is sort of "inject" the callback wherever you'd like. Ex:

public DoSomethingAsync() {
   return new Promise<void>(done => {
      // do some stuff
      done();
   });
}

Or even maybe the callback is wrapped up in another callback:

public DoSomethingAsync() {
   return new Promise<void>(done => {
      const somethingDeferred = GetSomethingDeferred();
      somethingDeferred.onsuccess = () => {
         // some success code
         done();
      };
   });
}

There is a similar problem with the "reject" handler.

Is there a way to refactor these to async/await? Or are they stuck wrapped up in a new Promise... block?

I think it's possible that the first example can be reduced all the way to simply:

public async DoSomethingAsync() {
   // do some stuff
}

Because the callback will automatically be called afterwards anyway. But the second example seems much more problematic.

score 3 · Accepted Answer · edited May 23 '17 at 12:30

Is there a way to refactor these to async/await?

No.

Or are they stuck wrapped up in a new Promise... block?

Yes. You need to promisify these functions that are asynchronous but don't return a promise already using the new Promise constructor (or a helper function that can shorten the repetitive code), there is no way around that. You'd rewrite your second example to

public async DoSomethingAsync() {
    await new Promise<void>((resolve, reject) => {
        const somethingDeferred = GetSomethingDeferred();
        somethingDeferred.onsuccess = resolve;
        somethingDeferred.onerror = reject;
    });
    // some success code
    return …;
}

or, with a helper function:

function toPromise(somethingDeferred) {
    return new Promise<void>((resolve, reject) => {
        somethingDeferred.onsuccess = resolve;
        somethingDeferred.onerror = reject;
    });
}

public async DoSomethingAsync() {
    await toPromise(GetSomethingDeferred());
    // some success code
    return …;
}

"Inject" callback with async/await

1 Answers1