Making Our Code Asynchronous

If we have another look at the recipes in the OkHttp documentation, 0:00 we recall that we are currently using this Synchronous Get Recipe in our code. 0:05 With some additional knowledge now about Threads, Concurrency, and Asynchronous 0:10 processing, we'll want to switch to this other recipe, the Asynchronous Get. 0:14 Notice here in the description that it will download a file on a worker thread 0:23 and get called back when the response is readable. 0:27 Let's update our code to be more like this Asynchronous model, 0:30 and then we'll talk about this call back idea. 0:33 So let's head back to Android Studio. 0:37 And main activity low, close the log. 0:42 So inside on create, we can keep a lot of our code the same. 0:46 The first difference is here, the response = call.execute. 0:50 This is the synchronous method. 0:57 We can delete that now. 0:59 So let's ignore this try catch block just for a moment, and here, 1:01 under where our call is initialized, 1:06 we want to add a new line to use the Asynchronous method, enqueue. 1:08 So call.enqueue. 1:13 Notice the parameter for this method is a callback object. 1:18 Type in new callback and hit enter to generate the callback. 1:21 Isn't auto complete great? 1:27 Well, sometimes, so this should look familiar. 1:29 We have an anonymous interclass of type callback. 1:32 It has two methods that we need to override, onFailure and onResponse. 1:36 Fortunately, good naming conventions have been used here, 1:41 making it easy to imagine what they're used for. 1:44 Let's turn back to our try-catch block. 1:46 We need to cut and paste it, but where to put it. 1:51 Since we're checking in there about a successful response, 1:54 it needs to go into the onResponse method. 1:57 Fortunately, that method is called with a response parameter already. 2:01 So we do not even need to change our code. 2:05 So we'll cut our try catch and 2:07 put it here, Into our response. 2:11 Our code is updated to make Asynchronous calls. 2:19 So let's try it out. 2:22 And another error. 2:31 Let's go into Logcat and see what it says. 2:34 So here we get a java.lang security exception permission denied, 2:47 missing internet permission. 2:50 Yes, we forgot to get permission for 2:53 our app to use the internet, that is a common mistake. 2:55 And can easily be fixed with a single permission line in AndroidManifest.xml. 2:59 So let's close down our Logcat again, go to manifest, android manifest, 3:04 and up here towards the top do uses permission. 3:11 We want Android Permission. 3:18 Internet. 3:25 And closer tag. 3:28 Great. 3:30 And let's run our app again. 3:32 Nice, no errors, and our emulator shows the default Hello World. 3:37 Let's take another look and Logcat and see our data. 3:43 And there it is, awesome, we'll talk about this 3:46 format of data called JSON in a little bit. 3:51 If you're not getting the data displayed here, you may want to debug your code and 3:56 execute it line-by-line to see what is happening. 4:00 Maybe adding additional log statements inside the on response method would help. 4:02 One common mistake is to have extra spaces, missing slashes, or 4:07 forgetting a comma when we built the URL. 4:11 So let's go back into main activity, close that down, and 4:14 take a look herein our callback. 4:17 What exactly is happening in our code here? 4:20 The on queue method executes the call, but does so by putting it into a queue. 4:22 This queue is provided by OkHttp and 4:28 executes the calls Asynchronously in the order they are added to the queue. 4:32 Since this is the only order in the queue it gets executed right away. 4:38 At that point, the call is executing in the background and 4:43 we are able to continue to execute code on the main UI thread, like normal. 4:46 We don't have any other code, but we could add another log statement down here. 4:51 Let's go ahead and do that to see the order of the log statements. 4:56 Log d tag, okay, and we'll do main UI code is running. 5:04 Hooray. 5:14 So, if this were Asynchronous code, it would process through log the response and 5:18 then log this new statement, but if we run our code now, And 5:23 look in Logcat, we see that our statement is displayed before the response. 5:30 This illustrates that the main UI thread 5:37 keeps working while the background thread is still working. 5:39 So what is a callback and how is it working? 5:43 Callbacks are very common in Java and Android development. 5:46 Asynchronous processing makes use of this, because it's the communication bridge 5:50 between a background worker thread and the main thread. 5:55 What's happening is this. 5:58 When we call the enqueue() method, 6:00 we're saying hey, call us back when you're done, here's how to do that. 6:01 We can imagine we are giving the OkHttp code a phone number to call us back on 6:06 when they're finished getting data from the web. 6:11 This callback is an object defined by the OkHttp Library. 6:13 OkHttp then says, 6:18 okay, as long as you give me a correct callback object, I'll use it. 6:20 When the call finishes in the background, 6:25 the onReponse method of this product is called. 6:28 Then the code we write inside it will be executed. 6:32 If there's an error the onFailure method would be called, 6:36 this could be an instant later or possibly a long time. 6:40 Generally, there's a limit on network connections, but 6:44 in theory, this background thread could run indefinitely. 6:47 As I mentioned, we'll talk about the JSON data in a little bit. 6:52 First though, let's make our code more robust by handling potential errors. 6:56