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In your system you can talk about the life cycles that we have in view control.

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There are many many different life cycles and they essentially define how a viewpoint or a goes through

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its different stages.

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So this is I will try to explain as many of them as I can for you.

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The school two weeks.

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And I'm going to make a new application let's call this one our view control are View.

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That's right the whole thing View control life cycle is so like that it's and I'm sure by now you have

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definitely seen or have used the view that the law is the default thing that we always have in here.

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Now beyond that there are so many many other wants and what I'm good at doing this lesson is I'm right

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all of them down.

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And then we're going to go ahead and print something in each of them to find out which one comes first

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and then they're going to reorder them so we get a sense of what's happening.

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So another one is actually called viu will appear.

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So the mule will appear and that is super dart of view will appear because it's a subclass of the few

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control of course.

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So let's see what you will appear does.

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And what's the difference between the ones that they say view something something load or view something

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and appears.

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The difference between these two is this Indos view is low that essentially it means all of your objects

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are created but they are not visible to user or they're not going to be visible just yet in the ones

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that they say View will appear or view did appear.

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It means that something is going to be visible or something has become visible some little right in

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here.

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You will soon be shown.

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And this is often useful for are useful for van right before transitioning into another view controller.

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This would be very useful for the moment that you're going to another view control and you want to have

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something happening and other lifecycle that we have in here is viewed with a Ph.

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And obviously the name of it is suggesting what it does.

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This is essentially for in your view has loaded all the objects are initialized and now it is being

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shown on the screen to your user.

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So you might be wondering what's the difference between viewed it appear and view lot.

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And what should I use in view that appear.

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So when you look at the new law essentially everything that you want has been initialized and allocated

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but they are not being visible yet.

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And there might be a little time between that and when they are being you know visible.

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So for instance if you want to start a timer start a timer or run an animation.

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This would be a much better place because there it will happen once things are being shown on the screen.

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Now in saying that there is a little bit of a downside and something that you have to be careful about

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what happens just once only once in the life cycle of your application.

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So if you load an app and you go from one view control to either one that only happens the very first

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time you're going to that view control both views that appear happens every time that that you counter

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is being shown.

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So if I go to my storyboard

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and imagine I have this main view controller that has a background with a color such as says I have

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a new installation of exclude all my colors are gone.

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So I have to remake them so they have this fun with some sort of a green background.

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And I have another view controller on this one I'm going to maybe make it a little bit pink.

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OK.

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So this is my other one.

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So the moment the app runs this view is going to be loaded.

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And that's the last time that that cycle happens.

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And then when I have a Segway and I go to the other one whenever I go to the ping point of view loads.

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Now if I had a button to go back when I come back to the green one it is not viewed that load anymore.

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It is the view it appears that happens over and over and over.

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So keep in mind that the view that happens only once.

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Why would it appear.

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Happens each time that you are land in this view controller.

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So that's something that you have to keep in mind because sometimes you know you shouldn't try to initialize

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or allocate something in the view that appear simply because we run multiple times.

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If you do that now as you can imagine view you appear simply happens after view will appear.

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And there's another one that happened somehow between these two or somewhere between these two.

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And that is the one that they call view tape.

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Lay out some views.

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So this is superdad view that lay out some views.

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This is essentially where all the pieces of the view are laid out and what laying out means is something

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that they're going to talk about it a lot in the coming lessons in Arthur lay out but essentially means

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this if you have an idea let's go ahead and say by then and this button is meant to be in this particular

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place with a bad going for it I can see it.

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And this is not currently being designed for iPhone 8 and you're running your app on let's say iPhone

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X..

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So what should it do and where should it be laid out.

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So someone else will decide about that.

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And that is this lifecycle.

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This is the one that this is so based on the constraints which currently we have none of based on the

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constrains or some other program programming you know aspect of the of the application virtually place

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this button and the moment that that decision is made for everyone.

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This function essentially has been finished viewed did lay out of use.

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So we talked about how viewable up here often happens right before transitioning to another view controller.

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So if that is true.

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So what happens to all the Wii controller the one that we just moved out of it so far that we have another

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one called view did disappear and that is superb view disappear.

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True.

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So this function as you can imagine happens right then a view is disappearing from the stack and we

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are moving to another view controller and it's a very useful place to get rid of continuous work for

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instance or continuous tasks.

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Finally have a default thing that is often called on the receive memory warming and that is superdog

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did receive memory warning and if if this function happens it means that your memory has exceeded the

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available memory of the device you're running on and you might want to do something about it if you

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have a cache of so many images you might want to empty some of it or you might want to do something

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that doesn't let your application crash.

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To be honest this has never happened to me because most devices they have about four to eight gigabytes

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of memory and is very difficult to kind of crush them.

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So with that in mind let's go ahead and write a little print in each of these so we know where we are

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as the as the fronts.

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So I'm going to say print what they want to print.

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The View will appear.

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I'm going to put this one here say this is if you did disappear.

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This is art.

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Did you receive memory warning.

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And I will show you a way to essentially simulate this.

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So to force it to happen the you did lay out some views of fact and copy that.

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Put it in here and say you did appear and that the default one which is view that last so I'm going

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to copy that.

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And view the notes of one's feet on the application.

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We should be able to see all of these.

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And then we're going to re format this code so that we know which one happens then because I want to

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test the view did disappear.

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I'm going to go to my main a storyboard.

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I'm going to get rid of that segue and say My boss then takes me to the other view controller.

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We're not doing anything and ping one for now.

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So you only care about the green one.

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So let's run this application.

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And now that been loaded.

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We have these four happening here.

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That last view will appear view it lay on abuse and view it appear.

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And these four are happening in the exact same order that we expected them to happen.

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So if you go in here the first thing that's happening is that the view is loading.

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So I'm going to put this one at the very top.

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It doesn't really matter but just for the sake of presentation I'm doing this.

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So the very first thing is that the view did load.

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So every PC is now sitting next to each other and is properly allocated initialized.

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The next thing is that the view will appear soon after that is the view is laying out the fuse.

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So after you will appear at The View laid out the south views and then the view did appear which is

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one.

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So here

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now the next thing is you want to see when does The View disappear happen.

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And that is if I tap on this button and says view it disappear because we went into the ping fine so

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that one is also successfully happening and to do the receive memory warning like I said is a feature

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here that you can actually simulate that simulates memory warning.

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And if you press that it says did receive memory warning you can actually add code in here that does

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something beyond the memory warning happens.

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Maybe like I said emptying the cash getting rid of some some data reading your values or whatever is

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the way to do that.

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So that's about the lifecycle of our of your controller.

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I think this little text in here best describes it that it goes from view that low that allocates initializes

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everything.

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Then it goes to you will appear where everything is ready to be shown right before you contort is shown

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and that you have to keep in mind is not necessarily about the second view controller and so on.

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Even the very first controller has to be shown at some point at the beginning.

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So a viewer will appear.

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It happened right before then.

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There is a lot then the subfields are being properly laid out based on the constrains for auto lay out

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or their program and take values.

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Once all of these things are ready the view will be in a state that it has appeared The View did appear

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happens that's when you see things.

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And then we go from one view to either one view disappears.

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And of course you might get a memory warning.

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So let's keep all of these in mind when we come to our apps.

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We will be using these very important topic is you have to keep in mind that the view that load happens

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once and that is when your view actually loads all of its objects but all the other ones happen multiple

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times depending on your navigation between different view controllers and so on and so forth.

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So let's keep that in mind on this.