[SOUND] In this lecture, we're going to talk about coding and dealing with asynchronous behavior with Promises and the $q service. Before Promises, we had callbacks, and we actually still do. The way you would execute asynchronous behavior is, you would have some asynchronous function, and you would pass as an argument a function value that would get executed once the asynchronous behavior was complete. The problem with this model is that there's not really an easy, straightforward way to task the results of the asyncFunction back to the caller. Now this is especially if the real recipient of the result is a few layers away. In other words, one service can call some other service, and that service may call your synchronized function. The result should really propagate all the way to the top function, if that's what you choose, but it's very hard to achieve this with a callback approach. In addition, what would happen if you needed to chain a few asynchronous pieces together? For example, when asyncFunction1 would complete, instead of returning the value, you would still have to call asyncFunction2. When that would complete, you would finally move asyncFunction3, and only then deal with all the results combined together. Right off the bat, you can see that it's very hard to read this code, and that's not even showing the error handling. With error handling, this would look like a complete mess, also, what if we wanted all these async functions to execute in parallel? And then, if that recall succeeded, execute our logic, but even if one failed, fail all of them and execute some error handling. That's not a very trivial thing to accomplish with a callback approach, but with a Promise API, we can actually get all these things accomplished fairly easily. The Promise API is actually part of the new ES6 API, however, not all browsers implement ES6 API yet, so Promise object might not be available. But AngularJS implements its own Promise API that's very similar to the one that's coming in ES6, so what is a Promise? A promise is an object which can be passed around or returned that holds references to the outcome of asynchronous behavior. In Angular, promises are created through the special service called the dollar sign q service. Let's take a look at some templates as to how you would go about doing that So here we have an asynchronous function. And the very first thing we would do is call the $q.defer method. That service creates an object that represent the asynchronous environment with all the hooks into it including the promise object. Then after we implement some behavior that object has a special method called resolve that marks a successful completion of our execution and then wraps the data for the promise to retrieve later on. In this case we're wrapping the result object or some result as part of our resolve method call. If something goes wrong we could call deferred.reject, which will mark unsuccessful completion and will also wrap some data for the promise. In this case it will probably be some error object or some error message. Last but certainly not least, we have to return our promise to the caller of this function. So therefore we say return deferred.promise;. That sends back the promise object to the caller, which is really a hook back to the life cycle of this entire process. Now remember that even though this sample code looks synchronous. And it probably is looking at it right now. The parts that ultimately call deferred.resove and deferred.reject can be part of something that truly runs asynchronously. For example if we wanted to simulate that in this example, we could wrap the highlighted lines of code in a setTimeoutFunction The next step is for the caller to call our asynchronous function and to capture a reference to the promise object. You can clearly see here that a promise is something I could pass around into other API's map application. Then wherever my application is appropriate. I could call the then function on my promise and extract the results or handle the error. Note that the then function takes two arguments. Both of which are functions themselves. The argument that the promise API will send into these functions is what we resolve the rejected, the deferred object with accordingly. Obviously here I'm referring to the result object and the error object which could be just a simple string. The then function is also chainable because it itself returns a promise. We'll demonstrate the power of this chainability in example part 2 of this lecture. The $q Service also has the capabilities to resolve multiple promises asynchronously, so no promise has to wait for another to complete in order to even start running. The $q.all method accomplishes that, plus the ability to have a central place to handle all the results or to handle any errors that come from any of the promises. In part two this lecture, let's go back to the code editor and see some examples of how to use this API.
