Using the Loader

We just finished creating our loader, but we still haven't seen how to use it. 0:00 But before we get to that, 0:04 it looks like I forgot to delete this percentProgress line. 0:06 So let's do that. 0:09 Okay, now it's time to get back to deleting our AsyncTaskClass. 0:12 Over in MainActivity, let's start by deleting our AsyncTaskClass. 0:16 And let's also delete this new MyAsyncTask line. 0:23 Next, we need to implement a few callbacks to help us interact with our loader. 0:32 Specifically, we need to implement LoaderCallbacks, and 0:37 make sure we're picking the option from the support library. 0:42 And let's use a bitmap array for our type parameter again. 0:47 Then let's use Alt+Enter to implement the three required methods. 0:53 The first method is onCreateLoader. 1:00 In this method, we need to create a new instance of our loader and return it. 1:03 So since our loader requires a URL as a parameter, 1:08 let's first cut and paste the URL code into the top of this method. 1:11 Then, right below the URL, let's return a new instance of our loader. 1:23 return new MyAsyncTaskLoader, and 1:29 passing in this for the context and url for the URL. 1:33 I'd love to get rid of this return null as well, but 1:43 even though our URL isn't malformed, we've still gotta catch the exception, 1:46 which means we need to have a return down here. 1:51 Well. Moving on to onLoadFinished, 1:54 this method is called when your work is done and is where we should update the UI. 1:57 So let's first update the data, our adapter is based on. 2:02 myAdapter.bitmaps = data. 2:08 And then let's notify our adapter that the data has been updated. 2:13 myAdapter.notifyDataSetChanged. 2:18 Nice. 2:21 The last method is onLoaderReset. 2:22 If your loader's been reset, then it no longer contains any data. 2:26 So if you've got any references to data in your loader, 2:30 you'd want to clean those up in this method. 2:33 Since we don't have any way to reset our loader, we're fine leaving this empty. 2:36 Now that we've implemented our loader callbacks, 2:41 all that's left is to start our loader. 2:43 At the bottom of onCreate. 2:45 Let's call getSupportLoaderManager().initLoader(). 2:51 Then let's pass in an arbitrary number for the ID, I'll choose 1. 3:01 And an arguments bundle, which we don't have, so let's just pass in null. 3:07 And finally a LoaderCallbacks object, 3:12 which we've implemented right here in this activity. 3:14 So let's just pass in this. 3:17 And before we forget, let's make sure to show our progress bar while we're loading 3:22 and hide it once we're done. 3:26 So at the top of OnCreateLoader, 3:28 let's add progressBar.setVisibility to View.VISIBLE. 3:32 And then at the top of onLoadFinished, Let's set it to gone. 3:39 progressBar.setVisibility(View.GONE); All right, time for the moment of truth. 3:45 Let's run the app and see what happens. 3:55 There's our progress bar, and now we've got our images. 4:02 And if we rotate the app, Awesome, it's the same images! 4:06 So instead of re-downloading all of those images, we just used our cached images and 4:13 made for a much better user experience. 4:18 When you've gotta do something in the background and 4:22 you're worried about the activity lifecycle, just use a loader. 4:24 Since a loader will stay alive through a rotation, it's a great 4:28 place to do your loading and keep any data safe from being destroyed in an activity. 4:30 If you'd like to learn more about loaders, check out the teacher's notes below. 4:35 And as always, if you've got any questions or would like to try your hand at 4:39 answering someone else's question, be sure to check out the community. 4:42 Until next time. 4:46