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Services Are Not Threads
7:39In this video we’ll add a separate Thread to our Service and drive home the point that Services are not Threads!
We just got an ANR by trying to do
our downloading in our service.
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But didn't we just say that services
are an application component
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that can perform long running
operations in the background?
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What gives?
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Well, by default,
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everything in an app runs on the main
thread, including our service.
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So even if a service is performing long
running operations in the background
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by default it will be doing those long
running operations on the main thread.
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If we want our service to do
work on a separate thread,
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we need to create that
separate thread ourselves.
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Seems simple enough.
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Now let's make our download service do
the downloading on a separate thread.
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Let's close the Android manifest and
start with some planning.
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On the left is the code from when we
created our own thread and main activity.
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We created, named, and started
the thread in the on create method.
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Then, in the on click method of our
button for each song we obtained and
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set a message and
then sent that message using our handler.
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Now we need to replicate
this in our service.
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Just like an activity, a service also
has a life cycle and life cycle methods.
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And one of these methods is the good or
onCreate method.
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So we'll create, name and
start our thread, in the onCreate method.
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And since we've now replaced this
message part with starting our service,
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we should now be obtaining and
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sending messages to our handler over
here in the onStartCommand method.
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All right.
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Let's start and download service and
remove the download song method.
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We'll now be using our
download handler and
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it still has its download
song method from before.
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Let's also remove the call
to download the song.
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Then let's override our
services on create method.
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Let's place our cursor
above onStartCommand and
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use control O to override
the onCreate method.
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And let's delete this call to super.
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We don't need it.
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Now we need to create, name and start our
thread just like we did in our activity.
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Let's start by creating a new download
thread at the top of the on create method.
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Download thread thread
equals new download thread.
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Then on the next line,
let's set the name of our thread.
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Thread.setName DownloadThread.
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And lastly let's start our thread,
thread.start.
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Next up,
we need to send a message to our thread
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each time on start command is called.
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Under where we declare a song,
let's obtain a message, and
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set it to a new message variable.
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Message message equals message.obtain.
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Then let's attach our song
variable to our message,
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message.obj equals song,
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and send the message using our handler,
which we currently don't have access to.
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Let's quickly create a field for
our download handler.
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private download handler
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mhandler, then let's give it
a value at the bottom of on Create
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mhandler equals thread but mhandler.
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Lastly, back in on StartCommand,
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let's send our message,
mHandler.sendMessage(message);,
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great work!
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Now that we're doing our download and
a separate thread,
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again we shouldn't have any
issues with the app being slow or
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not responding, but
let's test it out just to be sure.
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Let's click the download button and
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yikes the app crashed If
we look through the log.
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And filter by errors.
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It looks like we're getting a null pointer
exception because we're trying to invoke
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the virtual method,
send message on a null object reference.
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And if we click on the link
to jump to the error
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it looks like that null
object is our handler.
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Just like last time this
is because our main thread
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is ahead of our download thread.
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The main thread has already
finished the onCreate method and
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moved on to the onStartCommand method all
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before our download thread has
even created our download handler.
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To fix this,
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let's make our main thread wait until
the handler exists before we assign it.
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Up in the on create method
above the assignment
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to mHandler,let's type while thread.m
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handler equals null and add our brackets.
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We shouldn't have to worry
about a null handler any more.
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Using an empty while loop like this
definitely isn't the best practice.
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But it's the best we can do
with what we've learned so far.
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Now let's test the app again and
hopefully everything will work.
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Also make sure you have
the Android device monitor open so
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we can see what happens when
we kill our app's process.
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And if we click the download button,
it seems to be working.
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And if we wait ten seconds,
We can see our downloads start coming in.
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Nice.
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Now let's switch to the Android device
monitor and kill our apps process
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and if we check the log and
scroll up a little bit,
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we can see a scheduling restart of
crashed service and there's our service.
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It looks like it'll restart in 47 seconds.
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So let's wait for 47 seconds and
see what happens.
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And look at that, our process came back.
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And check it out.
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Our downloads are happening again.
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What's going on here?
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Let's find out in the next video.
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