promise链式

var getJSON = function(url) {
  var promise = new RSVP.Promise(function(resolve, reject){
    var client = new XMLHttpRequest();
    client.open("GET", url);
    client.onreadystatechange = handler;
    client.responseType = "json";
    client.setRequestHeader("Accept", "application/json");
    client.send();

    function handler() {
      if (this.readyState === this.DONE) {
        if (this.status === 200) { resolve(this.response); }
        else { reject(this); }
      }
    };
  });

  return promise;
};

getJSON("/posts.json").then(function(json) {
  // continue
}, function(error) {
  // handle errors
});

Chaining

One of the really awesome features of Promises/A+ promises are that they can be chained together. In other words, the return value of the first resolve handler will be passed to the second resolve handler.

If you return a regular value, it will be passed, as is, to the next handler.

getJSON("/posts.json").then(function(json) {
  return json.post;
}).then(function(post) {
  // proceed
});

The really awesome part comes when you return a promise from the first handler:

getJSON("/post/1.json").then(function(post) {
  // save off post
  return getJSON(post.commentURL);
}).then(function(comments) {
  // proceed with access to post and comments
});

转自: https://github.com/tildeio/rsvp.js