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All right.
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So in the last lesson, we managed to get this far. We've got a to-do list where we're able to
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add items through the UIAlertController, and we've also got this table view that allows us to check
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and uncheck items in our to-do list.
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Okay. So I mentioned in the last episode that I was going to show you how we actually have some bugs in
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our app.
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And the problem occurs when your app actually disappears from the memory of the phone.
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So if we have a look inside app delegate, now we've not really talked about many of these delegate methods,
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but this is a good time to do so because we're going to use it to show what's actually happening
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when your app is going into the background.
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So the first thing here, this method where it says didFinishLaunchingWithOptions.
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This is a place--this gets called when your app gets loaded up, so it doesn't matter if the rest of your
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app is going to crash.
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This is the first thing that happens and this happens before the viewDidLoad inside the first view
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controller or the initial view controller.
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So if we make a print statement here and says didFinishLaunchingWithOptions, so that we can see when
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this gets called. And the next thing is this one which is applicationWillResignActive.
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And this tends to get triggered when, say, something happens to the phone while the app is open, i.e., in
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the foreground.
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So, say, if the user receives a call,
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this is the method where you can do something to prevent the user losing data.
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So, for example, say, if they were filling in a form in your app, and in the middle of it, they get a call
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from somebody.
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Now, it would be really annoying if by the time they get back to your app that your app was actually been terminated
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and they've lost all of their data.
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So that's another time point.
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Now, this is another one that we're going to look at today.
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Now, this is applicationDidEnterBackground. And this happens when your app disappears off the screen,
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so when you press the home button, for example, or when you open up a different app. It's like a deck of
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card, there's another card that's being placed on top of the deck,
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so your app is no longer visible and it's entered the background.
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Now, the other one that is super important is the applicationWillTerminate.
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And this is the point where basically your app is going to be terminated.
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Now, this can be user-triggered or it can be system-triggered.
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So, say, if the user has pressed the home button to quit your app, and they decide to go into a different
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app,
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If this app happens to be a really resource intensive app, say, a game like Hearthstone or something, then
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it might reclaim a lot of the resources that are being hogged by your app.
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Even though it's in the background, it might still have processes that are running and is using up the
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precious memory of the iPhone.
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So in those cases, when the resources gets reclaimed from your app, then it goes from being in the background
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to being terminated or assassinated, basically, by the operating system.
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Now, let's run our app again and see when each of these messages gets triggered.
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So the first thing that happens is didFinishLaunchingWithOptions,
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and that happens at the moment when your app first loads up. And then the next thing is that if we press
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the home button, your application goes into the background.
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And similarly, if I double click and go to a different app, your application also enters the background
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because it's no longer the thing that you see on the screen.
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So the next one is how do you terminate your app, right?
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Well, this can happen because the operating system triggers it because it needs to reclaim those resources
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that precious memory or the user might trigger themselves by force quitting your app.
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So if you double click on your home button, now sometimes it doesn't work very well in the simulator,
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so just bear with it, but try it a few times and you should get to this type of screen.
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And this is a place where you can browse through all of the apps that are still in the background.
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But you can also hit it and swipe it up to quit, to force quit that app.
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And that's the moment where you'll get "applicationWillTerminate."
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And that is this delegate function.
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Now, some of the logical things you might want to do, for example, is that, you know, in your form-filling
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application, you'll probably want to add some safe method to your "applicationWillTerminate" or even
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better to applicationDidEnterBackground because it's such bad user experience to go back into your
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app and find that all of the data that you've been entering for the last five minutes has just now disappeared.
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It's a sure way to get bad reviews.
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So your app has a distinct lifecycle where it goes from being launched to being visible on the screen
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to entering the background, and finally, being destroyed and the operating system reclaiming the resources
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from it.
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So just as we have life cycles, we were born, we live, and we die,
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so do apps.
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So, now that we've cleared up what most of these delegate methods do, I want to show you where the problem
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lies in our app.
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Now, we know that in previous modules, I tend to get rid of the bezel because that allows us to have a
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bigger screen.
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But in this case, because we're using the home button quite a few times,
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so if you want your bezels to come back, you can always go to Window and Show Device Bezels to toggle
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that.
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Okay, so here's our app as it appears when it first loads up. And I'm going to add a new to-do item.
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And when I hit Add Item, you'll see it appear in our table view
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because we're adding it to our array and we're reloading the table view.
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Now, if I hit the home button and my app enters the background, it still keeps a hold of that data in
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the memory of the phone.
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So when I reopen it, you can see that it still exists.
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Now, the problem occurs when the app doesn't just enter the background, but actually terminates.
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So all of scenarios where this might happen is, A: you're operating system trying to reclaim resources
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because the user is trying to use like a really memory intensive app,
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second option is if the user force quit your app.
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Third option: if you installed an update to your app.
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Fourth option: if the user updated their operating system. There are so many cases where your app is probably
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going to end up terminating.
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And when that happens,
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so let's try that again.
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Let's terminate our app. Application will terminate,
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right? So, now it's gone from the memory of the phone.
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And if we click on it again, you can see that new item that we added is now gone.
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And this is because we don't have any form of persistent data storage.
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We're only adding and appending to our array and that array is only going to be held in memory for as
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long as this TodoListViewController exists.
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And it's going to disappear and be destroyed if the application gets terminated.
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So this is why we need persistent memory storage.
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But there's actually quite a few options you have in terms of how you want to persist your data.
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So in the next lesson, we're going to talk about one of the easiest ways of persisting small bits of
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data, and that's through using NSUserDefaults.
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So I'll see you there.