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Okay we're now ready to run the app and

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investigate the various stages of the

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activity lifecycle. Now before I do that,

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though, I wanted to talk briefly about

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the panes down here, these various

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windows that you can pop up, and that's

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because you can actually reposition them,

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using the cog wheel over here on the

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right hand side. So I'm going to come

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over here and choose Floating mode from

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the pop-up menu. Once we do that, you can

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see we've got just a little window that

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is now separate from the main Android

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Studio window, and we can move that

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around. So that can be really handy if

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you've got a single monitor, because you

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can display that and the emulator at the

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same time, or we can actually see a lot

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more screen real estate which we're

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going to be doing here in this video. But

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it's also useful if you've got two

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monitors, because you can put logcat

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on one window and the main programming

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code, your designer and so forth, and see

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the code on the other monitor, so quite

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handy. Alright, so let's run this app. So

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I'm going to leave this logcat open

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here, run our app. I'll just need to click

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that to bring that back on the screen

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again, and I'll also bring the emulator

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over. So there's our app, and you can see

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over here now in logcat, that the

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onCreate method's been called, up here,

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then the on Start and then the onResume.

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And that's what we would've expected

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from the activity lifecycle. Now

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because this was the first time the app

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was launched, there's no state to be

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restored, so consequently onRestore

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InstanceState wasn't called. Alright,

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so let's add some text now and click the

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button a few times and keep an eye on

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the logcat, because what

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we're going to do is try rotating it

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into landscape mode. But first, I'm going

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to enter these three items; bread - and you

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can see onClick has been called in

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the logcat there - milk, cheese. So we've

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got three items there, and what I'm going

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to do is make sure there's some text in

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the editText and then we're going to move

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it into landscape mode. So I'm going to

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type in cream here, then move over to

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landscape. Then you can see we've still got

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cream showing there, and we can see that

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over here now, the various events that

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were called. We had our onClick method

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called three times, which you would have

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expected for the three items because I

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didn't click the button the fourth time.

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But then what happened was, when the

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device was rotated,

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the onPause method was called, followed

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by onSaveInstanceState, and then on

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Stop and then onDestroy. And you may

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have seen that the app briefly vanished

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from the device screen, briefly, while it

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was rotating into landscape mode. Now

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because it was Android destroying the

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activity, and it was in the foreground,

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Android then recreates the activity

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again. So that's where the onCreate, on

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Start is actually called. And because

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we've got now some state because on

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SaveInstanceState was called prior to

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going into landscape mode, as a result, on

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RestoreInstanceState is called automatically

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by Android. And then finally,

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we've got on the onResume method being

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called as well. And after that the app's

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once again running in the foreground,

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which it's doing there now. So let's add a

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comment down here, just so we know where we're

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at. We'll type in Back Button, because what

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we're about to do is now select the back

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button to move back. Firstly, I'm going to

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remove the keyboard by pressing back, so

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we're not expecting anything to be

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called there. But now if I actually go

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back again,

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it's gonna close the app. And what's

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happened here, you can see that when I've

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actually done that, we've got onPause

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called, onStop called and then on

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Destroy called,

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and obviously at that point the app was

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then closed. Note that there wasn't a

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call this time, to onSaveInstanceState,

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and that's because the state won't be

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restored when the app's relaunched.

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Because Android only restores state if

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it was responsible for destroying the

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activity. Now if the user chooses to

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close the app, then they wouldn't expect

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it to start up with the data already in

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the editText from the last run. With

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that said though, they may however

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expect to see the bread, milk and cheese

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in the TextView, but if you wanted to

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save those results, then onStop would be

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the place to do that. Now because the

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logcat

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gets cleared when we run an app from

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Android Studio, what I'm going to do is

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use the emulator's app screen to launch

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our button click counter app again. So

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I'm gonna type in a comment though, into

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logcat first, and just put launching app, and

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then I'm going to come back here and

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we're going to launch the app. I'm going to

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do that sideways because I'm still in

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landscape mode, buttoncounterapp, I'm

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gonna click on that. There's our button

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counter app running again now,

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and looking at the log, you can see that

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we've got onCreate, onStart and on

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Resume called, so that's good. Then what

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we'll do now is, we'll just go back to

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portrait mode.

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There's portrait mode again there now,

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just so it's a little bit easier. We can

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see a bit more of the log and

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the application running on our

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emulator as well. And we can see now,

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that by doing that this second time,

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we've got onCreate called because I

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went back to portrait mode onStart. And

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onSaveInstanceState, by the way, was

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called just prior to, or just after we

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rotated the device back to portrait. So

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while it was still in that

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process of moving back to portrait, on

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SaveInstanceState was called. Then

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consequently down here, onStart, on

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RestoreInstanceState was called then

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onResume, and then we're back with

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the app running in portrait mode again.

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Alright, you can sort of see there, the lifecycle

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is being fairly consistent there. We'll type

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in Locking Screen next, so I'm gonna type

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Locking Screen into logcat, and what

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we're gonna do now is just that. We're going

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to briefly lock the screen, and on an

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emulator, we can do that by the power

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button. So if we just press the power

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button quickly, and just then release it

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fairly quickly, then that locks it, just

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like a real Android device would.

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If we hold it down it'll shut it

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down. So I'm just going to click it

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briefly, and there's our lock, so

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the application is now going to lock,

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bearing in mind we've got some extra

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bits and pieces showing here as well

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which we're ignoring. But the ones that

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we want to see here is the onPause

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called, onSaveInstanceState called and

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then onstop called. So note that the

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user hasn't exited the app as such. They've 

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just switched off the screen, and that's

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why Android's calling onSaveInstance

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State after onPause. And onStop's then

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called and our app's now no longer running

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in the foreground. So I'm gonna add

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another comment down here now, and we'll

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put Unlock Screen, and we're gonna use

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the power button to unlock the screen. So

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I'm gonna click it once quickly again. So

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just like a normal Android app, we now need
180

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to swipe to get back to where we were

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again.

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Alright, so at this point you might be

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wondering, the screen was locked,

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when the screen was locked,

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we had onSaveInstanceState called. So why

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wasn't onRestoreInstanceState called

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when we unlocked the screen? Well there's

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no reason to call it, and

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that's because the app, or the

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activity rather, wasn't destroyed,

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it was just stopped. So in other words, no

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state was lost and there's no reason to

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restore it. We can actually check that

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though. What we can do, is we can go ahead

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and enter our items again. So I can enter

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bread, milk, cheese, and we're going into

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cream again but this time we're not

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going to tap the button like we did last

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time. It'll lock the screen again, and you

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can see what's happened here.

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We've got, I think this is more Android

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Studio functionality problems

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here, because we've got the Unlock Screen

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and you saw that when I went back to it,

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nothing seemed to be working for a

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while. We weren't getting any entries into

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here, and then when I've locked the screen,

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we've suddenly got all these entries

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coming back through again. So the point I

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want to make here, though, is that after

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we unlocked the screen, we actually did

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get onRestart called and we got onStart

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called, and then we also got onResume

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called. All three of them were called very

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quickly, as you can see here by the time,

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over to the left-hand side. But the point

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was that onRestoreInstanceState wasn't

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00:07:57,020 --> 00:07:59,060
called when we unlocked the screen. And

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again, the reason for that is that from

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Android Studio, there actually wasn't a

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reason to call it, and that's because

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the activity wasn't destroyed.

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It was just stopped, so no state was lost

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and there's no reason to restore it. You

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00:08:11,300 --> 00:08:12,380
also saw, even though I've locked the

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screen now, we did get the onClick calls

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successfully being called when we

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clicked the buttons, as we would expect,

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but they weren't updated in the log. I

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suspect that's some sort of issue with

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00:08:22,070 --> 00:08:23,930
logcat, probably because I'm running a

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00:08:23,930 --> 00:08:26,000
beta version of Android Studio. You can

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see that they were actually called, and

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then when I locked the screen, we got on

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Pause called and then onSaveInstanceState

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called and then as well as onStop. So at

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this point, we've now locked the screen,

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and again onSaveInstanceState's called

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00:08:38,299 --> 00:08:40,429
because Android might destroy the

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00:08:40,429 --> 00:08:42,260
activity when the screen's locked.

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Now if updates start downloading or

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some other app wakes up, memory may get

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00:08:46,520 --> 00:08:48,620
low. Because our app is no longer in the

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foreground, it's fair game for Android to

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destroy it if it needs extra resources.

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00:08:52,370 --> 00:08:55,190
So that's the reason that onSave

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00:08:55,190 --> 00:08:57,440
InstanceState is actually called here, when

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00:08:57,440 --> 00:08:58,649
the screen's actually locked.

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00:08:58,649 --> 00:09:01,290
Now with that said, though, unless Android

250
00:09:01,290 --> 00:09:03,180
does destroy the activity, then there's

251
00:09:03,180 --> 00:09:05,009
no need for it to restore the state. So

252
00:09:05,009 --> 00:09:06,509
that's way, in this case, it didn't call

253
00:09:06,509 --> 00:09:08,959
the onRestoreInstanceState when we

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00:09:08,959 --> 00:09:11,069
unlocked the screen. So the same thing

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00:09:11,069 --> 00:09:12,389
should happen now if I go to unlock the

256
00:09:12,389 --> 00:09:16,230
screen again. We've still got our items

257
00:09:16,230 --> 00:09:19,019
here so nothing's been destroyed, but our

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00:09:19,019 --> 00:09:20,879
log is not working at the moment but,

259
00:09:20,879 --> 00:09:22,740
hopefully, that'll catch back up.

260
00:09:22,740 --> 00:09:24,839
But in any event, I can assure you like

261
00:09:24,839 --> 00:09:27,240
it happened last time, that the on

262
00:09:27,240 --> 00:09:29,939
RestoreInstanceState won't be called, and

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00:09:29,939 --> 00:09:31,920
that's because the application was

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00:09:31,920 --> 00:09:34,019
stopped and it wasn't destroyed. So it's

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00:09:34,019 --> 00:09:35,399
just there as we left it, in other words.

266
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We can see that by the three entries

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00:09:37,259 --> 00:09:38,879
that we've got here, and also the entry

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00:09:38,879 --> 00:09:40,740
still showing in the editText. So

269
00:09:40,740 --> 00:09:42,929
again, if Android did destroy the

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00:09:42,929 --> 00:09:45,119
activity, then we wouldn't expect to see

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00:09:45,119 --> 00:09:47,639
our bread, milk and cheese here, because

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00:09:47,639 --> 00:09:49,529
normally on the onRestoreInstance

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00:09:49,529 --> 00:09:52,829
States is called, this isn't actually

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00:09:52,829 --> 00:09:54,209
saved because we haven't written any

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00:09:54,209 --> 00:09:55,499
code to do that yet. The only thing that

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00:09:55,499 --> 00:09:58,439
is normally saved is this option,

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00:09:58,439 --> 00:10:01,470
whatever we've typed in the editText. So

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00:10:01,470 --> 00:10:03,179
again, unless Android does destroy the

279
00:10:03,179 --> 00:10:04,439
activity there's no need for it to

280
00:10:04,439 --> 00:10:06,480
restore the state, so that's why in this

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00:10:06,480 --> 00:10:08,730
case, onRestoreInstanceState wasn't being

282
00:10:08,730 --> 00:10:10,649
called. It wasn't called last time and

283
00:10:10,649 --> 00:10:13,050
wasn't called this time either. But again,

284
00:10:13,050 --> 00:10:14,579
we can't really confirm that right now

285
00:10:14,579 --> 00:10:17,249
because Android Studio seems to be

286
00:10:17,249 --> 00:10:19,920
playing up a little bit, and it's not

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00:10:19,920 --> 00:10:21,540
actually showing us all the logs. So I

288
00:10:21,540 --> 00:10:24,720
can see that it seems to just have been

289
00:10:24,720 --> 00:10:26,610
stalled there and it's not actually updating,

290
00:10:26,610 --> 00:10:27,870
and I can confirm that by trying to add

291
00:10:27,870 --> 00:10:30,899
a click on the button now. We've added

292
00:10:30,899 --> 00:10:32,459
that here but we're not getting any log

293
00:10:32,459 --> 00:10:34,259
intries in the logcat. So clearly a bug

294
00:10:34,259 --> 00:10:36,959
with Android Studio at the moment, but

295
00:10:36,959 --> 00:10:38,339
anyway, you've seen that work earlier in

296
00:10:38,339 --> 00:10:39,870
the video. So that's actually the

297
00:10:39,870 --> 00:10:42,480
activity lifecycle. So that technique, by

298
00:10:42,480 --> 00:10:43,949
the way, of adding logging to all the

299
00:10:43,949 --> 00:10:46,439
methods is great to see, or for seeing

300
00:10:46,439 --> 00:10:48,720
what's going on, and when various methods

301
00:10:48,720 --> 00:10:50,879
are called. Now it doesn't just apply to

302
00:10:50,879 --> 00:10:53,399
the lifecycle methods either. There's

303
00:10:53,399 --> 00:10:55,350
all sorts of situations where we have to

304
00:10:55,350 --> 00:10:57,230
provide callbacks for Android to call.

305
00:10:57,230 --> 00:10:59,220
Adding logging to them so that you can

306
00:10:59,220 --> 00:11:01,230
see exactly when Android does call them,

307
00:11:01,230 --> 00:11:03,029
is a very good way to develop your

308
00:11:03,029 --> 00:11:05,999
understanding of how Android works. Alright,

309
00:11:05,999 --> 00:11:07,829
so just to show you this before I finish

310
00:11:07,829 --> 00:11:09,779
this video, that the logcat should still

311
00:11:09,779 --> 00:11:11,819
be working. I believe it's a problem to

312
00:11:11,819 --> 00:11:12,420
do with

313
00:11:12,420 --> 00:11:14,520
Android Studio, probably linking to the

314
00:11:14,520 --> 00:11:15,750
emulator. So if I go to close this

315
00:11:15,750 --> 00:11:19,560
emulator down, hopefully, what we'll see

316
00:11:19,560 --> 00:11:21,990
is some errors pop up because I want to see one

317
00:11:21,990 --> 00:11:23,550
of those new log entries actually just

318
00:11:23,550 --> 00:11:25,200
come through. At the moment they haven't

319
00:11:25,200 --> 00:11:27,120
come back through, since we locked the

320
00:11:27,120 --> 00:11:29,040
screen for the second time. I'm gonna

321
00:11:29,040 --> 00:11:33,480
just try this again, start our API 26

322
00:11:33,480 --> 00:11:35,670
emulator. At some point, we'll probably

323
00:11:35,670 --> 00:11:44,530
see these entries.

324
00:11:44,530 --> 00:11:47,080
So we're still not getting any logging

325
00:11:47,080 --> 00:11:49,240
output there. So what I might do is I'm

326
00:11:49,240 --> 00:11:52,360
just gonna close down Android Studio, but

327
00:11:52,360 --> 00:11:54,840
at some point what we do here,

328
00:11:54,840 --> 00:11:58,210
we should find this gets fixed up. This

329
00:11:58,210 --> 00:12:00,400
will be a good thing to show you if you're

330
00:12:00,400 --> 00:12:01,900
in a similar situation where you're

331
00:12:01,900 --> 00:12:03,040
getting a lock up and you're not seeing

332
00:12:03,040 --> 00:12:05,470
logcat working. The combination of these

333
00:12:05,470 --> 00:12:07,620
things should actually fix the problem.

334
00:12:07,620 --> 00:12:09,700
Alright, so I'm starting that again. I'm

335
00:12:09,700 --> 00:12:12,310
gonna go and open logcat, and as we would

336
00:12:12,310 --> 00:12:15,130
expect now, there's no logcat entry. We'll

337
00:12:15,130 --> 00:12:18,430
wait till it initializes. Alright, I can see

338
00:12:18,430 --> 00:12:19,630
we've got the three entries showing

339
00:12:19,630 --> 00:12:21,250
there, from our most recent call, and if I

340
00:12:21,250 --> 00:12:23,170
just go back to our emulator again and

341
00:12:23,170 --> 00:12:27,640
if I just run this again. So there's some

342
00:12:27,640 --> 00:12:29,080
sort of temporary issue there, probably

343
00:12:29,080 --> 00:12:30,310
again, relating to the fact this is a

344
00:12:30,310 --> 00:12:32,740
beta version of Android Studio where

345
00:12:32,740 --> 00:12:34,570
it's not doing the right thing. You can

346
00:12:34,570 --> 00:12:35,680
see we've now got the log entries

347
00:12:35,680 --> 00:12:37,600
working again. So what I want to do is

348
00:12:37,600 --> 00:12:40,360
just do that last bit of, that

349
00:12:40,360 --> 00:12:41,590
last testing and just to show that it

350
00:12:41,590 --> 00:12:44,080
is working. So if I type in these three

351
00:12:44,080 --> 00:12:47,230
entries again, so bread, milk and

352
00:12:47,230 --> 00:12:54,820
cheese, cheese, and if I type cream, but this

353
00:12:54,820 --> 00:12:56,200
time I'm not going to add that and I'm

354
00:12:56,200 --> 00:12:59,440
going to lock the device. So you can see

355
00:12:59,440 --> 00:13:01,540
the device was locked. We had on

356
00:13:01,540 --> 00:13:04,030
Pause called, onSaveInstanceState

357
00:13:04,030 --> 00:13:05,920
called and onStop called.

358
00:13:05,920 --> 00:13:08,140
So hopefully now, when i unlock it, we

359
00:13:08,140 --> 00:13:09,460
should see what I originally wanted to

360
00:13:09,460 --> 00:13:12,610
show you, was the onRestoreInstanceState not

361
00:13:12,610 --> 00:13:14,230
being called but being able to see the

362
00:13:14,230 --> 00:13:16,420
other entry. So let's try that, and you

363
00:13:16,420 --> 00:13:17,350
can see there, that's really what I

364
00:13:17,350 --> 00:13:19,060
wanted to show you early on, so clearly

365
00:13:19,060 --> 00:13:21,250
that was a bug with Android Studio. So

366
00:13:21,250 --> 00:13:23,920
onSaveInstanceState was called prior

367
00:13:23,920 --> 00:13:25,450
to us locking the screen, as well as on

368
00:13:25,450 --> 00:13:27,940
Stop and then, of course, previously onPause,

369
00:13:27,940 --> 00:13:30,580
but once we came back after unlocking

370
00:13:30,580 --> 00:13:32,950
the screen, we only got onRestart, on

371
00:13:32,950 --> 00:13:34,870
Start and onResume. We didn't get the on

372
00:13:34,870 --> 00:13:37,140
RestoreInstanceState being called,

373
00:13:37,140 --> 00:13:39,490
because unless Android does destroy

374
00:13:39,490 --> 00:13:40,660
the activity, there's no need to

375
00:13:40,660 --> 00:13:42,610
actually call that onRestoreInstance

376
00:13:42,610 --> 00:13:44,470
State. And again, if that was true we'd

377
00:13:44,470 --> 00:13:46,330
expect to still see our list, and cream

378
00:13:46,330 --> 00:13:47,860
in the editText when we unlocked

379
00:13:47,860 --> 00:13:49,780
the device, and you can see here, that

380
00:13:49,780 --> 00:13:51,070
clearly, it hasn't been destroyed because

381
00:13:51,070 --> 00:13:53,560
we can still see those items. Alright,

382
00:13:53,560 --> 00:13:54,880
so we got there in the end. We had a few

383
00:13:54,880 --> 00:13:56,380
problems with the software but we

384
00:13:56,380 --> 00:13:58,210
managed to get it working.

385
00:13:58,210 --> 00:14:00,520
So that's the activity lifecycle. Now

386
00:14:00,520 --> 00:14:02,350
that technique, adding logging to all the

387
00:14:02,350 --> 00:14:04,029
methods, is great for seeing what's going

388
00:14:04,029 --> 00:14:06,399
on and also when various methods are

389
00:14:06,399 --> 00:14:08,140
called. It doesn't just apply to the

390
00:14:08,140 --> 00:14:10,270
lifecycle methods either, because there's

391
00:14:10,270 --> 00:14:11,950
all sorts of situations where we have to

392
00:14:11,950 --> 00:14:13,720
provide callbacks for Android to call.

393
00:14:13,720 --> 00:14:16,089
And adding logging to them so you can

394
00:14:16,089 --> 00:14:18,279
see exactly when Android does call them,

395
00:14:18,279 --> 00:14:19,959
is very good way to develop your

396
00:14:19,959 --> 00:14:22,330
understanding of how Android works. Al

397
00:14:22,330 --> 00:14:23,620
right, so there's a few important

398
00:14:23,620 --> 00:14:25,209
implications of what we've looked at

399
00:14:25,209 --> 00:14:27,670
with the activity lifecycle. We're going

400
00:14:27,670 --> 00:14:31,140
to discuss those in the next video.