Update Flux Dispatcher.dispatch and waitFor examples

The previous examples didn't properly work when 1) a Store callback does waitFor on Stores that haven't been reached yet and 2) a Store callback waits on another Store that is already waiting. The updated example uses constructs Promises up front and then asynchronously resolves them.

Author: joshma

docs/docs/flux-todo-list.md

@@ -74,13 +74,6 @@ var _addPromise = function(callback, payload) {
  }));
 };
 
-/**
- * Empty the queue of callback invocation promises.
- */
-var _clearPromises = function() {
- _promises = [];
-};
-
 var Dispatcher = function() {};
 Dispatcher.prototype = merge(Dispatcher.prototype, {
 
@@ -99,12 +92,27 @@ Dispatcher.prototype = merge(Dispatcher.prototype, {
  * @param {object} payload The data from the action.
  */
  dispatch: function(payload) {
- _callbacks.forEach(function(callback) {
- _addPromise(callback, payload);
+ // First create array of promises for callbacks to reference.
+ var _resolves = [];
+ var _rejects = [];
+ _promises = _callbacks.map(function(_, i) {
+ return new Promise(function(resolve, reject) {
+ _resolves[i] = resolve;
+ _rejects[i] = reject;
+ });
+ });
+ // Dispatch to callbacks and resolve/reject promises.
+ _callbacks.forEach(function(callback, i) {
+ // Callback can return an obj, to resolve, or a promise, to chain.
+ // See waitFor() for why this might be useful.
+ Promise.resolve(callback(payload)).then(function() {
+ _resolves[i](payload);
+ }, function() {
+ _rejects[i](new Error('Dispatcher callback unsuccessful'));
+ });
  });
- Promise.all(_promises).then(_clearPromises);
+ _promises = [];
  }
- 
 }); 
 
 module.exports = Dispatcher; 
@@ -583,18 +591,18 @@ Adding Dependency Management to the Dispatcher
 
 As I said previously, our Dispatcher implementation is a bit naive. It's pretty good, but it will not suffice for most applications. We need a way to be able to manage dependencies between Stores. Let's add that functionality with a waitFor() method within the main body of the Dispatcher class. 
 
-We'll need another public method, waitFor(). 
+We'll need another public method, waitFor(). Note that it return a Promise that can in turn be returned from the Store callback.
 
 ```javascript
  /**
  * @param {array} promisesIndexes
  * @param {function} callback
  */
  waitFor: function(promiseIndexes, callback) {
- var selectedPromises = _promises.filter(function(/*object*/ _, /*number*/ j) {
- return promiseIndexes.indexOf(j) !== -1;
+ var selectedPromises = promiseIndexes.map(function(index) {
+ return _promises[index];
  });
- Promise.all(selectedPromises).then(callback);
+ return Promise.all(selectedPromises).then(callback);
  }
 ```