多次解决承诺

Question

I am building a module using Promises , where I make multiple http calls on multiple urls , parse the responses and then again make more http calls.

c = new RSVP.Promise({urls:[]}) //Passing a list of urls
c.then(http_module1) // Call the http module
.then(parsing_module) // Parsing the responses and extract the hyperlinks
.then(http_module2) // Making http requests on the data produced by the parser before.
.then(print_module) // Prints out the responses.

The problem is that - If I use a promise, I can not parse the modules unless all the http requests are made. This is because - Once a promise has been resolved or rejected, it cannot be resolved or rejected again.

Build my own version of promises or is there an alternate approach?

Answer 1

You can write functions that return handles to your promises and create reusable parts that are still chainable. For example:

function getPromise(obj){
   return new RSVP.Promise(obj);
}
function callModule(obj){
   return getPromise(obj).then(http_module1);
}

var module = callModule({urls:[]})
  .then(getFoo())
  .then(whatever());

  //etc

Answer 2

There are libraries that support such kind of pipes/streams, you don't need to build such yourself.

Yet the task seems to be doable with promises as well. Just don't use a single promise for an array of urls, but multiple promises - one for each url:

var urls = []; //Passing a list of urls
var promises = urls.map(function(url) {
    return http_module1(url) // Call the http module
      .then(parsing_module) // Parsing the responses and extract the hyperlinks
      .then(http_module2) // Making http requests on the data produced by the parser before.
      .then(print_module); // Prints out the responses.
});

This will run all of them in parallel. To wait until they have ran, use RSVP.all(promises) to get a promise for the results, see also https://github.com/tildeio/rsvp.js#arrays-of-promises