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Extending the Thread Class
6:10In this video we’ll see a new way to run code on a Thread, and start downloading a playlist instead of just one song!
Using a runnable with a thread is
an easy way to route some of your app's
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heavier traffic off of the main thread,
but it's not the only way.
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There's actually two ways we can
run code on a separate thread.
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We can pass in a runnable to a new thread
object, which we're currently doing,
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or we can extend the thread class and
override its run method.
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Let's refactor our code to
instead use this second method
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of extending the thread class.
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Let's start by creating a new
class called DownloadThread.
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And let's make it extend the thread class.
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Then let's override the run method
of our new DownloadThread class by
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hitting Ctrl+O to bring up the override
dialog and then selecting the run method.
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Then instead of calling super.run, let's
call our downloadSong method instead.
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Let's cut the method out of MainActivity,
And
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paste it into the bottom of
our DownloadThread class.
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Then let's use Alt+Enter to
create a new tag constant,
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just like we did in MainActivity.
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.class.getSimpleName.
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Next, let's call downloadSong
instead of super.run.
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Lastly, back in MainActivity and
our download button's onClick listener,
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let's get rid of our runnable object,
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And use our new DownloadThread class
instead of the normal thread class.
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Looks good.
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Let's quickly test our app to make
sure it still works the same.
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And success.
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It works just like it did before.
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This code is exactly the same as
when we were using a runnable and
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passing it to a new thread.
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All right, let's take a minute to see
what's going on behind the scenes.
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Let's start by opening the Android
device monitor by clicking up here.
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On the left,
we see a list of running processes,
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and if we expand this list, look at that,
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there's our process right down here.
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Let's click on it and
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then click this button to show all
the threads in our app's process.
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Then if we go back to our app and
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we click on the Download button,
there's our thread.
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And if we click it a bunch,
a bunch of threads.
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If you're ever curious about what's
going on inside your device,
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the Android device monitor is
a great place to start looking.
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Now instead of downloading a single song,
let's say we want to download a playlist.
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In fact, there's an array of songs in
the teacher's notes for us to use.
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Let's go back to the code and
create a new class called Playlist.
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Then, let's copy and
paste the songs array into our new class.
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Making the songs array public static
means we'll be able to access it from
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anywhere in our app.
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Let's close this file and
head over to our DownloadThread class.
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Let's also minimize the log.
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The last thing we need to do is
call the downloadSong method for
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each song in our playlist.
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Sounds like a good place for
a for each loop.
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Inside the run method, lets type for
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(String song: Playlist.songs) and
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then let's add our curly brackets
around the download song method.
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Before we test this, let's take a minute
to talk about what would happen if we
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instead added our loop around the code and
our download button's onClick method.
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If we looped here,
we would create a new thread for
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each song, and all the songs would
be downloading at the same time.
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You'd think this would be great, but by
trying to download all the songs at once,
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we'd be giving our users a bad experience.
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They want to listen to their playlist as
soon as possible, and we've only got so
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much bandwidth we can use for downloads.
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So if we're spreading that bandwidth
out over the whole playlist,
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they'll have to wait a long time
before they can listen to any song.
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It would be better if we downloaded
the songs one at a time.
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This way, our users can start
enjoying their music a lot quicker.
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All right, now let's test our app and
see what happens.
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I'll click the download button and
in ten seconds, we can start
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to see our songs finish downloading one
after the other, just like we wanted.
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We've got our playlist and we're
downloading the songs one at a time, but
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there's still more work to be done.
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In the next video, we'll learn about some
potential errors and how to fix them.
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