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In the previous lessons, we worked out the BMI value. In the finished app,
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this value should be displayed on a separate screen when the calculate button gets pressed.
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So now that we've got the BMI value worked out, how can we navigate to a new screen and pass this result
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over to be displayed?
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Notice how our screen with the sliders is linked to a ViewController.swift file.
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So our second screen also needs its own view controller file.
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So how do we create such a new file?
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Well, firstly we're going to right-click on the Controllers folder and we're going to create a New File inside
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this folder and the type of file we're going to choose is a Swift File, and then we're going to name
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this file
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SecondViewController.
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So notice how we've currently got a ViewController.swift which gets created every time we build
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a new iOS project and it comes with some template code already written for us.
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Well, in this case, we've created a second ViewController.swift file entirely from scratch. And the
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first thing that I'm gonna do is I'm going to try and create what we have over here which is a class.
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And the class that I'm going to create, instead of being a view controller which controls our first screen,
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this calculator screen, I'm going to create a class called SecondViewController.
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So as soon as I start typing the word "class," you'll notice that the code snippet comes in. And if I hit
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enter, it'll insert all of the place holders that I need to fill.
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So what's the name of my class gonna be? Now, in Swift,
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it's convention to name the file that contains the class the same name as the class itself.
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So my class is actually gonna be called SecondViewController and the SuperClass is going to be the
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same as we've got over here in our first screen that Apple created for us.
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It's going to be a UIViewController.
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Now,
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notice that as I'm typing this, I'm getting some errors here. And the error tells me that I'm using an
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undeclared type 'UIViewController." so it doesn't know what data type this is.
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But how come we got to use it here?
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Well, it's because we were importing something called UIKit. And here, we're simply importing Foundation.
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These are what we call frameworks.
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These are bunches of code that Apple has written to make our experience of developing iOS apps much
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easier than it would be if we had to build everything from scratch.
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Now, Foundation allows you to work with most the features of the Swift programming language. But if you
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want anything that is specific to iOS, say, a UILabel, or a UISlider, or, in our case, a UIViewController,
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everything basically that starts with a UI comes from the UIKit Framework.
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If you go in to Help and go to Developer Documentation, here you'll find on the left here a section on
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UIKit.
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So this is everything that the framework contains. And you can read more about it here as well as see
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all of the things that it includes.
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And it's really, really broad-ranging, everything from the UIController to other things that we've been
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working with such as the UILabel or the UIViewController.
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And this is what we need to be able to manage our second screen.
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So as soon as we go over here and we switch out foundation for UIKits, because UIKit actually already
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includes everything that's in Foundation. It's a level higher and it already has the foundational capabilities,
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but it has a lot more. So now that we've imported this UIKit Framework,
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well, we now have access to all of the components that Apple has already built inside UIKit, including
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for example, a UILabel or a UIColor, or maybe the UIImageView.
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So these are already things that we've come across and we've used. These are the components that Apple
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has pre-built, so that we can simply drop it into our code to start using it without having to define
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that.
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Labels have text properties. They also have a background color property or a text color property.
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All of these things have already been done for us. So if we take a look at this new class that we've
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created,
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notice how this code looks really similar to how we created struct.
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Well, similar to struct, we start off by using a keyword. Instead of struct,
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it's now a class.
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And then, we defined the name of our class which is also capitalized begin with.
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And these names which start off with a capital letter are differentiated from the ones which don't, like
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the variable names
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and the function names, so that we know that when we use these, we're tapping into a blueprint, a Struct
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blueprint, or in this case, a Class blueprint.
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The other thing that normally gets created for us when we create a new iOS project and we get our default
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ViewController.swift is the viewDidLoad method.
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But in this case, because we're creating everything ourselve, let's create the viewDidLoad method from
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scratch.
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So as soon as I start writing viewDidLoad,
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you can see that there is the suggested viewDidLoad method. aAd if I hit enter, it'll insert all the
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code that's required.
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Now, after I delete this code place holder, you can see that this looks much the same as what we've got
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in our template ViewController.
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The only thing that's missing is this super.viewDidLoad.
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So let's add that to our viewDidLoad as well.
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Now, that we've successfully created our own ViewController from scratch, given how important of a component
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the ViewController is in iOS,
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I really recommend taking a quick look at the documentation for the UIViewController to start to understand
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what it does.
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And in this lesson, as we create our own ViewController from scratch, it'll help you understand a lot
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of things that are covered. Feel free to go either via Developer Documentation in Xcode if you have
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it downloaded, or simply just go to developer.apple.com/documentation, and use the search
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bar as we do normally.