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Alright, so it's time to start creating

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our app to download that RSS feed, and

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display the data onto the screen. Now the

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app itself is going to be pretty basic

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but it will demonstrate some very

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important concepts, including performing

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operations on a separate thread, so that

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the app doesn't appear to freeze while

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it's performing a long-running task. Now

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although it's getting rarer these days,

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you may have actually seen android pop

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up

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ANR dialogues - ANR, by the way, stands

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for application not responding, although

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Google have changed the way that the

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dialogues appear. In fact, I can make one

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appear quite easily, by doing something

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incredibly foolish with a few lines of

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code. I'm actually going to add a loop to

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the onCreate method. What I'll do

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first, though, is I'm going to start my

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emulator, so that it's ready. And we're

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going to type in some code, while that's

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actually starting. So again, we're

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going to add this loop - we're going to

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add this after the setContentView line,

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and the loop's just going to say while

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parentheses true, open the code block and

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then we're going to put val x, lower case

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val x equals 1, and that's actually it.

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Now, in case it's not obvious, don't ever

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do anything like this. What I've done is

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created a loop that just goes round and

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round forever, but doesn't do anything

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other than give x the value of 1. So it's

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a really pointless thing to do, but it

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will prevent the onCreate method from

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finishing, and will make the app freeze.

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So when I go ahead now and run this on

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my emulator -

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give that a moment to compile.

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Now we don't even get to see the usual

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Hello World that Android Studio includes

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in the default layout, and that's because

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the display doesn't appear until after

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onCreate terminates. So if I come over to

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the app now. If I click on the back

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button to stop the app, there's a short

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pause, and what should happen is the

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Android framework will now detect that

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the app's frozen, and you can see that

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that it's popped up on the screen now and,

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at this point here, I can either wait or

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close the app. And sometimes the app

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really is doing something and you might

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decide to wait, but I'm going to close

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this one because it's never going to be

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doing anything useful. So I'm gonna click

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on Close app. So that was an extreme

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example, but you can see that Android

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doesn't like it when an app takes over

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the device, and doesn't finish its job in

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a reasonable amount of time. And there's

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a very good reason for that,

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which is that the user interface is

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shared by all apps. So there's only one

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screen on the device, so if one app is

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taking it over and it refuses to

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release it, then the device, effectively,

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can't be used for anything else.

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Now if you remember from the

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discussion of the activity lifecycle,

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when another activity is started, it

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doesn't become visible until the on

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Pause method of the current activity

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finishes. Now this activity with the

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infinite loop will never finish,

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so if Android doesn't provide that

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dialog as a way to kill the app, then the

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device, effectively, couldn't do anything

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else. Now remember that activities are

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being interrupted all the time. One thing

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that often interrupts what you're doing

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on a phone, is an incoming phone call. If

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your app prevents users from answering

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the phone, well frankly, they're not going

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to be very impressed. In fact, that won't

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actually happened. Android is now

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cleverer than that, and will let things

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like an incoming phone call take over,

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but the point still holds. If the user

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wanted to make a phone call, they'd have to

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close our rogue app first, and we've just

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seen that it doesn't close until we tell

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Android to kill it. So all of this is

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really, really should hopefully

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emphasize the point that it's important

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that we think about what our apps are

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actually doing, and we don't perform long

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running tasks on the main thread. Doing

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so will block the user interface. Now

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you'll find the documentation referring

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to the UI thread,

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which means the thread that the user

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interface is running on. So when we do

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something like downloading data over the

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Internet, we don't really have any

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control over how long the process will

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take. Sometimes the Internet can be very

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slow, and sometimes even sites such as

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Apple's can go down and become

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unavailable. Now that would obviously

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cause our download to take a long time,

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and could cause our app to freeze while

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it's waiting for that download to

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complete. So the way to cope with this is

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to run the download on a separate thread.

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Now think of a thread as a way of

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running code at the same time as other

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code is also running. In fact, with modern

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multiprocessor or multi-core devices,

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that's exactly what does happen. The

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thread will run on another processor or

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core, while the user interface continues to

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execute. Now writing multi-threaded

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applications is notoriously difficult,

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and even experienced programmers

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struggle to get it right. But fortunately,

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Android provides a class that takes care

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of all of the complexity for us. So we

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can use an AsyncTask to perform the

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download on a separate thread, and then

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get notified when the download is

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finished. So the download will happen in

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the background, and the user interface

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throughout the UI thread, will receive

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the results of the download. So the

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download will happen asynchronously,

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which just means in the background on a

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separate thread, and our app can get on

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with other things while that's going on.

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Now we can even switch to another app

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and do something else while an async

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task is running. This app won't allow

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that, but we'll see ways to make that

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possible later in the course. So

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basically, we tell the Android framework

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to get on with the task, then forget

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about it until that task finishes. So

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that's the theory, at least. Unfortunately,

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though, using an AsyncTask to

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communicate with the UI thread in Kotlin,

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isn't quite as easy as it sounds. Now I'm

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going to go ahead and use one anyway, but

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in subsequent videos, we're going to

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delete all this code and do it a bit

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differently. Now there are a couple of

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reasons for doing this. One is that most

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of the google examples are still written

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in Java, and probably will be for a while

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yet. So that means that you're going to

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be seeing the the Java way of doing

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things, and the Kotlin way's often

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very different. So this example will show

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just how different it can be. Now the

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other reason is that Android Studio will

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actually convert Java code to Kotlin for

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us. In fact, that's how I produced his

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first lot of code that we're going to

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see. I had to do a bit of tweaking but,

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ultimately, that did work. However, what

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it produces is basically Java code,

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written with a Kotlin syntax. Now you

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will find Kotlin code that's been

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produced in this way, and you may even

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end up having to maintain some. But

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hopefully, by the time you've worked

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through these next few videos, you'll be

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better positioned to spot code like that,

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and more importantly, you won't write any

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more yourself. Alright, so let's start.

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First thing I'm going to do is delete

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that rogue while loop that we don't

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want. So I'm going to come over here and

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delete those three lines. Now most of the

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time, new classes go into their own

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Kotlin file, but this class is going to

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access properties of our MainActivity,

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so therefore, I'm going to create it as

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an inner class. So normally, I'd

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right-click on the package and choose

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New then Kotlin Class, and that would

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create a new class. But here we don't

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want to do that, because we're going to

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create the class inside the Main

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Activity class. So I'm going to call this

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one

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download data, and it's just the same as

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any other Kotlin class, except that it

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can only be used by MainActivity. So in

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other words, it's not visible to any

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other class in the program. So I come

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down here and add this code. I'm going to

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clean that space up and then below the

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onCreate function, I'm going to type

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private inner class DownloadData : I'm

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gonna type AsyncTask. Press Enter there,

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and I'm going to add the diamonds and then

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put string - or add a diamond operator,

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I should say - String comma Void comma

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String. Then at the right of the diamond

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operator I'm going to add parentheses.

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Then add the left and right curly braces.

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Now at the moment, we've got an error

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there, but what I've done is, I've created

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a new class that extends the AsyncTask

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class.

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Now extending an existing class, which is

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also known as sub-classing, lets you take

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advantage of all the processing in the

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existing class. So here what we're doing,

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is taking advantage of the asynchronous

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processing that the Android framework

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provides. And all we have to do is

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provide the code for the task that we

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want to perform, and then let Android

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take care of all the multi-threading

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complexity for us. Now we need to set up

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three parameters when we create an Async

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Task, so that Android knows what type

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of task we're dealing with. Now here we're

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saying that the information that we'll

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provide to the task, will be of type

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string. So we're going to pass in the URL,

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in the RSS feed, and we're going to store

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that as a string. This is that first

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parameter here, first argument - string. Now

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the second parameter's used if we want

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to display a progress bar - that's this

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one.

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Now our download is quite small, so there

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won't be time for a progress bar to be

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displayed. Now if we were downloading

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large amounts of data, that could perhaps

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take a few minutes or more to download,

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then it might be a good idea to give the

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user some idea of how long it'll take,

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and also how far through we were. But

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as we're not going to do that, I'm

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making the second type here void, to show

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that we're not actually using that. Now

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onto the third parameter - this is the

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type of the result that we want to get

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back. So all our XML will be a string, so

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we're using string again for the third

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parameter type. Now you can find out more

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information on the AsyncTask class in

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Google's documentation. Let's just

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open it up briefly. I'm just going to

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paste this over the top of our RSS feed.

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Now at the moment, as I mentioned, a lot

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of Google's examples, and including this

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one, as at the time of recording this

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video, is still in Java and not Kotlin,

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unfortunately, and that's one reason why

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Kotlin programmers need to have some

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familiarity with the Java language. Now

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we will look at how Android Studio can

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make life a lot easier for us, so don't

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think, by any means, that you need to be

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an expert Java programmer to work

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through this course. Alright, so that's

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the three parameters that I've talked

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about here, that we need to define when

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we setup an AsyncTask,

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so that Android knows what type of task

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we're dealing with. The first parameter,

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again, will be a string containing, or

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holding, the address of the RSS feed.

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We're not using the second parameter at

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all, and the third will be the string

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containing all the XML that we've

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downloaded. Now Android Studio's

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actually showing an error here,

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and that's because if we're going to be

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using an AsyncTask, then there are some

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functions that we need to implement. Well,

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actually, there's only one function that

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we must implement, and that's the doin

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Background function. We can hover

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over there - you can see that it's telling

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us that it's really looking for this

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doInBackground. It's basically saying

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that the class DownloaData is not

295
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abstract and does not implement an

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abstract base class member. But as it

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turns out, we're also going to implement

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another function, though. Because we want

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to get data back when the task completes,

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we need to implement the on post

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execute function. Android will call that

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function when the job's done. Without

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that, there wouldn't be a way for us to

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know that the task had finished, and

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therefore we wouldn't get the result. Now

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we can get Android Studio to create the

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base stubs for the methods, by using ctrl

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o to get a list of the methods that are

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available to override or implement. You

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do want to make sure that your cursor is

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in the inner class for that to happen,

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and once you do that, you can just do a

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ctrl o, and that brings up a list of

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potential functions that can be

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overridden. And, of course, we saw that in

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an earlier video when we overrode the

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methods in the Activity Lifecycle. so as

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you can see, there are a few methods

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available, and if we're updating a

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progress bar then we want to do that in

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the on, we want to use rather, the on

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ProgressUpdate function, and the on

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PreExecute function would be used to set up

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the progress bar, for example, but can

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also be used with setting up anything

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else that's needed for our task to run.

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Now both of those methods are called on

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the main UI thread, otherwise they

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wouldn't be able to display anything. So

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it's important not to do anything like

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trying to start the download in onPre

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Execute. Now the other method we want,

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though, is onPostExecute, which also runs

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on the main UI thread, once the

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background process has completed. So,

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therefore, we're going to choose both

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doInBackground

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and we're gonna choose

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onPostExecute, which was already selected,

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and then we're going to click on Ok. And

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you can see that those two functions

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have now been created, and the error

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message has disappeared. Alright, so I'm

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going to start the code to do the

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downloading in the next video. But what

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I'm going to do for now is just put some

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logging into these functions, so that we

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can see what's going on and get a feel

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for how the AsyncTask works. So we're

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going to need a log tag for each of our

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classes. So I'm going to come up, firstly,

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to our MainActivity, and the first line

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after the definition for our Main

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Activity, I'm going to type private val,

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TAG in uppercase, equals double quotes

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MainActivity, in double quotes. So we can

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now use that tag when logging in Main

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Activity's onCreate function, and we can go

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ahead and do that. We can type log.d

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parentheses and tag comma,

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and we can just type onCreate called. Let's

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do the same for the DownloadData inner

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class. So I'm going to come down into the

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first, or the first line after the definition,

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and we're going to type private val TAG

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equals DownloadData. Alright, so now that

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I've added those tags, we'll finish the

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video here. In the next video, we'll start

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continuing on with this program, and

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we're going to start, in that next video,

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by looking at the warnings that Android

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Studio are popping up. So I'll see you in

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the next video.