Power of Promises- September 2011

Who is this guy? Brian Arnold Support Lead & Software Engineer at SitePen, Inc. Lead Engineer on SitePens Dojo Tutorials eﬀort Lead Engineer on SitePens Training Workshops© SitePen, Inc. All Rights Reserved

Asynchronicity is hard General feedback after having taught Deferreds and promises to several classes: The distinction between a Deferred and promise isnt clear Most people dont realize they can make their own Thinking asynchronously is hard at rst© SitePen, Inc. All Rights Reserved

Whats the distinction? A promise is something that represents a value returned from an asynchronous operation Rewritten in Dojo 1.5, dojo.Deferred is an implementation of the Promises/A CommonJS proposal: http://wiki.commonjs.org/wiki/Promises/A© SitePen, Inc. All Rights Reserved

The result of then The return of promise.then(someCallback) is a new promise for the return value of someCallback When creating our own APIs we can leverage this functionality, returning the returned promise from .then // Make a request that returns metadata and an array of users var def = dojo.xhrGet({url: "/users", /*...*/}); // Create a new promise for just the users var users = def.then(function(data){ return data.users; }); // Hand back our users return users;© SitePen, Inc. All Rights Reserved

dojo.when (Deferred.when) Handles either promises or values First two arguments promiseOrValue callback When given a promise, it returns a promise for the return of the callback, like .then does© SitePen, Inc. All Rights Reserved

Use Case: Twitter Store API to implement: getIdentity: A way for the store to determine a tweets ID get: A means to retrieve a full set of details for a tweet query: Search Twitter for the provided term and return an array of results© SitePen, Inc. All Rights Reserved

Twitter Store: getIdentity A very trivial piece of our store, but still useful Given a tweet, return its id_str property dojo.declare("my.TwitterStore", null, { idProperty: "id_str", getIdentity: function(/*Object*/ tweet){ // summary: // Returns a tweets identity // tweet: Object // The tweet to get the identity from return tweet[this.idProperty]; }, // ... get and query ... });© SitePen, Inc. All Rights Reserved

Twitter Store: get Store will cache tweets When get is called: If cached, return tweet If not, well retrieve the tweet, but use our own intermediary Deferred to control what it means to be a successful retrieval© SitePen, Inc. All Rights Reserved

Twitter Store: get get: function(/*String*/ id){ // summary: // Gets the full details for a tweet // See https://dev.twitter.com/docs/api/1/get/statuses/show/ %3Aid for details // id: // The identifier as expected by Twitter // Check our index, and if we have it, return it var tweet = this.tweetIndex[id]; if (tweet) { return tweet; } // ...© SitePen, Inc. All Rights Reserved

Twitter Store: get var def = new dojo.Deferred(); // Our intermediary Deferred // Retrieve our tweet var req = dojo.io.script.get({ url: tweetUrl, callbackParamName: callback, content: { id: id } });© SitePen, Inc. All Rights Reserved

Twitter Store: get // Using .then, well set up a graceful handler for success, // as well as what to do with an error req.then( dojo.hitch(this, function(tweet){ if ("error" in tweet) { def.reject(tweet); } else { var id = this.getIdentity(tweet); this.tweetIndex[id] = tweet; def.resolve(tweet); } }), dojo.hitch(def, reject) ); // Hand back our promise return def.promise; } // end get© SitePen, Inc. All Rights Reserved

Twitter Store: query query: function(/*String*/query, /*Object?*/ options) { var count = options.count || this.count, searchUrl = options.searchUrl || this.searchUrl, search; // Execute our search search = dojo.io.script.get({ url: searchUrl, callbackParamName: callback, content: { q: query, rpp: count } }); // Hand back a new promise that is *just* for the results array return dojo.store.util.QueryResults( search.then( dojo.hitch(this, _processTweets) ) ); }© SitePen, Inc. All Rights Reserved

Twitter Store: _processTweets _processTweets: function(/*Object*/ res) { // Twitter search results have a property called // `results`, which is just an array of objects var tweets = res.results; // Iterate over our tweets, cleaning a bit. // We are very intentionally *not* saving to tweets index, // as the details for each tweet in search results are sparse. for (var i = 0, l = tweets.length; i < l; ++i) { this._scrubTweet(tweets[i]); } // Hand back the tweets array return tweets; }© SitePen, Inc. All Rights Reserved

Thats it! Thats really all there is to implementing a custom Dojo Object Store Dojo Object Stores are one of the easiest ways that you can start using promises very powerfully right now in your code Being able to create Deferreds and return the promise property allows us to create powerful APIs that allow for clean separation of concerns© SitePen, Inc. All Rights Reserved

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Power of Promises- September 2011

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