Can I make a synchronous promise in the JavaScript Q library?

Question

I want to do something like the following:

delay( 2500 )
  .then( function () { console.log( "Step 1 done" ) } )
  .then( delay( 7500 ) )
  .then( function () { console.log( "Step 2 done" ) } );

So implementation of delay has been demonstrated many times before:

function delay( ms ) {
  var deferred = Q.defer();
  setTimeout( deferred.resolve, ms );
  return deferred.promise;
}

But if I run the above in node.js I get:

... delay of 2500ms
Step 1 done
Step 2 done
... delay of ~7500ms

rather than what I expect to see:

... delay of 2500ms
Step 1 done
... delay of 7500ms
Step 2 done

In the examples provided on https://github.com/kriskowal/q/wiki/Examples-Gallery I can't find any examples of synchronous functions (functions that return a value without any callbacks involved) chained with promise functions.

有关如何将同步操作与异步承诺混合的任何想法？

我试过了:

function synchronousPromise() {
  var deferred = Q.defer();
  console.log( "Synchronous function call" );
  deferred.resolve();
  return deferred.promise;
}

delay( 2500 )
  .then( function(){synchronousPromise()} )
  .then( function(){delay( 7500 )} )
  .then( function(){synchronousPromise()} );

这输出:

... delay of 2500ms
Time now is 2013-06-20
Time now is 2013-06-20
... delay of 7500ms

..仍然不是我想要实现的目标.

Answer 1

如果要链接回调,则必须从其中一个回调中返回一个新的promise对象.在你的第一个例子中,你写

.then( delay( 7500 ) )

这意味着你要传递一个promise对象.then,而不是一个函数.根据Promise/A +提案(后面的Q),必须忽略所有非函数参数.所以,基本上就像你写的一样:

delay( 2500 )
  .then( function () { console.log( "Step 1 done" ) } )
  .then( function () { console.log( "Step 2 done" ) } );

相反,传递调用delay并返回promise对象的函数:

delay( 2500 )
  .then( function () { console.log( "Step 1 done" ); } ) 
  .then( function () { return delay( 7500 ); } )
  .then( function () { console.log( "Step 2 done" ); } );

现在,只有delay在第二个回调中返回的promise对象被解析后,才会调用最后一个回调.