Promise chains are not executed sequentially using Q promise library

问题

I am performing multiple batched asynchronous operations in my code. Although operations within a batch should execute asynchronously batches should be executed synchronously one after another.

Here is a jsfiddle I created. Look at the console as all output is there. And here is the code for convenience:

asyncChain(10, 'FIRST CHAIN')
.then(function () {
  asyncChain(10, 'SECOND CHAIN');
})
.then(function(){
  asyncChain(10, 'THIRD CHAIN');
});

function asyncChain(n, msg) {
  var promiseChain = Q.fcall(function () {
    10;
  });
  console.log('starting:' + msg);
  for (var i = 0; i < n; i++) {
    promiseChain = promiseChain.then(asyncOperation(i, msg));
  }
  console.log('returning' + msg);
  return promiseChain;
}

function asyncOperation(i, msg) {
  var d = Q.defer();

  setTimeout(function () {
    console.log('resolving for #' + i + msg);
    d.resolve(i);
  }, 300 + Math.random()*1000);

  return d.promise;
}

Basically these are 3 batched promise operations I expect to be finished one after another. Meaning the output for this sample would be something like this:

starting FIRST CHAIN
returning FIRST CHAIN
resolving 1..10 FIRST CHAIN

starting SECOND CHAIN
returning SECOND CHAIN
resolving 1..10 SECOND CHAIN
and so on

I tried using all() method instead of then() but it stopped execution after first chain. Am I missing something obvious?

Thanks for any advice.

Cheers

回答1:

Am I missing something obvious?

Yes. For then to resolve the promise with another promise, you have to return that other promise to it. Your function does just start another asyncChain, but returns undefined from the callback which resolves the promise immediately.

asyncChain(10, 'FIRST CHAIN').then(function () {
  return asyncChain(10, 'SECOND CHAIN');
}).then(function(){
  return asyncChain(10, 'THIRD CHAIN');
});

来源：https://stackoverflow.com/questions/18933903/promise-chains-are-not-executed-sequentially-using-q-promise-library

标签

javascript

promise

deferred