All language subtitles for 4. Stateful Widget Lifecycle Methods

Now in the last lesson we used our scratch.dart file to learn more about futures and async and await.
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But we don't need this file anymore so you can either delete it or you can keep it around for reference
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in the future.
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It won't affect your project.
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Now let's head back into our loading screen and if we take a look at the code now, it should make a bit
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more sense to you.
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You can see that we're creating a new geolocator object that comes from geolocator.dart. And then
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we're using that object to get the current position. We're calling getCurrentPosition
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and this is a method that's inside geolocator. And we provide the amount of accuracy we want for this
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location.
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And we're going to choose low so that we don't use up all the battery. And because it takes time to get
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the current position
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this method is asynchronous and it will happen independently of whatever else you decide to try and
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do. But because in most cases we can only proceed forwards with for example printing the position we
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got back or with using that position in getting some weather data,
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then we're adding the async keyword to modify our function. And that gives us access to the await keyword
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which basically says wait until this completes before you continue doing anything. If we didn't have
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this await keyword, then we could still have a position but it won't be an actual position.
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It will be a future position.
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It's just like that order number.
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It will have a value in the future
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once this process completes. But at the point where this code is called or when it's triggered, this is
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just a receipt.
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It's not an actual coffee.
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And right now if I was to print out this position variable, it won't be an actual position that actually
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comes out. And we can show that by changing this from running scratch.dart to main.dart.
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And now if I press run and I go ahead and click on get location, then you can see that what's being printed
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in my console on the main.dart, so we can close down scratch.dart
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if you want, is an instance of future position. This is kind of like you promise your friends to go and
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grab them a coffee and all you did is come back with the receipt.
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Now they're not going to want the receipt.
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They're going to want the actual coffee.
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So that's why we're adding that await keyword in front of the method call to say that, wait for this
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to finish before you assign this value to the position.
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So that way we actually get an actual position or an actual coffee, rather than just the promise of a
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coffee that's going to be there in the future.
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And now if I click on get location, you can see the actual location gets printed rather than just a future.
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So it's all very well and good that we can get the location when we press on this button.
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But what if we wanted the location as soon as our screen loads up? Because in our weather app, as soon
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as we open up the app, it's going to trying and detect our location and based off that location, it's gonna
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get the weather.
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It doesn't make sense to force the user to press a button to say get weather or get location,
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that seems a bit extra. So how would I call get location if I wanted it to happen as soon as my screen
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loads up? Well in order to do this
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we have to learn about widget lifecycles.
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We know that stateless widgets are basically just like very simple Lego blocks right?
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You can't saw them in half,
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you can't change them.
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You can't do anything with them unless you decide to destroy them and create a new one.
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And you have to keep destroying and creating new ones every time you want a change in a stateless widget.
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So for these widgets, their lifecycle methods are very simple. There's only one that you should be concerned
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about and that's the build method. When the widget gets built
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this method will be called and inside here,
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you will create whatever appearance or widget you want to show up on screen.
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Now on the other hand, we also have our stateful widgets and we know that these stateful widgets can
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be combined and we can track the state using a state object.
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Now that state object is there keep track of variables such as what is the configuration of my widgets,
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what are the properties of my widgets.
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And I can change all of those variables by using a set state and it will update my app. Now
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in this case, the state object actually lives a lot longer and so it's got more lifecycle methods.
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There is an init state, which gets triggered when that state initially gets initialized.
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There's of course the build method which gets triggered when the widgets are actually built and will
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show up on screen. And then there's a deactivate method which gets called when that stateful widget
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gets destroyed.
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So just as we humans are born and we grow up we go through different life stages and then we also die,
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so do our stateful widgets. But we can tap into each of those stages in the lifecycle if we wanted different
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things to happen at various times.
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Now I just want to show you when these lifecycle methods will get triggered. And to do so, I'm pulling
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up that previous file that we had, the navigation demo that we used earlier on.
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Now you don't have to write any of the code,
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I just want to show you what actually happens.
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It's enough to just follow along. Here we still got our screen 1 which is a stateless widget.
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We've got our screen 2 which is currently a stateful widget. And I've made our app start out at a screen
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1 which is the first route.
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So when I run my app, I head over to the screen 1 which only has a single button that pushes me onto
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screen 2.
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Now once I'm on screen 2, I want to be able to show you when these lifecycle methods actually get called.
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Inside our state object, I'm going to tap into a number of methods that come from the parent class, the
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state class. One of those is the initState method.
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And if we click on this initState, hold down CONTROL + J or CONTROL + Q, you can see that this
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is called when an object is inserted into the tree.
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And this means that when we create our stateful widget as soon as it's inserted into the tree, it's
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going to call initState.
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So this is the first thing that happens.
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So inside here I'm going to add a print statement saying 'initState called'. And that will print whenever
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this method gets triggered. And it will get triggered automatically at a particular stage in the life
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of this state object.
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Now the second one that gets triggered,
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so the second point in the lifecycle, is going to be the build method.
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Now inside here, I'm also going to add a print statement saying 'build called'.
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And finally there's also another method that comes from the parent class that I want to show you which
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is called deactivate. And deactivate will get triggered when this stateful widget gets destroyed.
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So let's add a print statement in here as well.
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Now let's go ahead and click run for our changes to go through. And in our console you can see that as
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soon as I click on this button, I should head over to screen 2.
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Now as soon as that stateful widget is created and inserted into the widget tree, initState gets
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called.
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So that gets printed in here.
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And then as soon as it builds all of the widgets inside the screen which is just my button and my scaffold
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etc. then we get billed called. Now every time I make a change in the screen or I change one of the properties
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that the widgets depend on, then build will get called again and again and again.
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And this is one of the most frequently used lifecycle methods.
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But at the very end when I click on go back to screen 1, screen 2 is going to pop off,
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that means it's going to be destroyed.
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It will no longer exist.
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And that is the time when deactivate will get called.
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So there are other lifecycle methods but these are probably the most useful and the ones that you'll
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actually come across and need when you're creating Flutter apps.
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So we know that if we want something to happen the moment that our stateful widget is created and
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add into the tree, then we're going to put our code inside initState. If we want something to happen
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every single time
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our stateful widget gets rebuilt, then we'll put it into the build method.
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And finally if we want something to happen when our stateful widget gets destroyed, then we would put
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the code inside the deactivate method.
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So now using what we've learned on lifecycle methods, here's a challenge for you.
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I want you to change our code so that the position gets printed into the console without tapping on
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any buttons.
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In fact you can go ahead and delete the entire contents of the scaffold.
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What should happen is as soon as our app runs
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and as soon as this loading screen appears inside the phone then we should see our position being printed
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in the console.
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And this will rely on what you've learned from this lesson.
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Pause the video and try to complete that challenge all right.
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So currently when we run our app all we get is just the blank screen.
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There's nothing inside build, just a blank scaffold.
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So we're not gonna use any buttons to trigger getting the location.
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Instead we're going to add a initState to our loading screen. And because we know that this is going
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to be triggered as soon as our stateful widget gets created,
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so it's the moment that this appears on screen.
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Now it's important to know that initState only gets called once and it's only the moment when that
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state gets initialized and gets created.
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But something like build even though it does get called when our widget gets built onto the screen, but
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it gets caught every single time that our widgets rebuild.
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So every time a piece of text changes or an image changes or this animation, then build will be called again
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and again and again. And very often you don't want to put code in there that will get called repeatedly
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because it's very expensive.
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Instead we're going to put it inside initState.
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And so inside initState, we're going to call get location.
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And now if I go ahead and hot restart my app to make it reinitialize this particular stateful widget,
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then you can see as soon as it goes on the screen, I get my position printed in the console.
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I didn't have to press anything at all. So our lifecycle method are really useful if we want to tap in
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to a particular moment in the life of our stateful widgets. If we wanted to save a piece of data just
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before the stateful widget gets destroyed or if we want to deallocate something from memory or if we
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want to create a new object as soon as the stateful widget gets initialized, these are the methods that
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we can tap into to make our code run at a particular time. And in our case we're making our get location
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method run in the moment as soon as our loading screen state gets initialized which is going to be at
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the very start and we only make it run once. Now
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at the moment, we're getting our location and we're importing our geolocator all inside our loading
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screen which is kind of not its job right?
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So in the next lesson we're going to learn more about checking to see if the user has given permission
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to get the location.
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And we're also going to refactor all of the location related work into our location.dart file.
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So for all of that and more, I'll see on the next lesson.