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Now, at this stage, your developer instincts should be tingling because we've created this entire app
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with just a single file, ViewController.swift. And all of our functionality, all of the components are
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living and sharing this tiny space of one file.
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So given that in the advanced section of our Swift Tutorials, we're talking a lot about architecture
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and design patterns,
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so that our app is well organized and extensible, then we should really be thinking about applying MVC
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or the model-view-controller pattern to our app.
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So let's go ahead and do that. As a challenge,
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can you create a new Swift File to create a separate class? And inside that class, we're going to put
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in all of the calculation logic that's currently inside the ViewController.
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So create a new class called Calculator Logic and put that into a new Swift File.
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All right.
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So this is pretty easy.
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We're just going to select the folder and I'm going to hit command Nto bring up the shortcut for
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creating a new file, and we're inside iOS Swift File.
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So if you're ever confused about whether if you should create a new Cocoa Touch Class or if you want
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to use a new Swift File, then remember that whenever you want to subclass something, such as a UIViewController,
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a UITableViewController, a UIView, whatever it may be, then you're going to choose this one.
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But when you create your own custom classes, then you're going to be choosing a boggled standard Swift File.
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So let's hit next and let's call this file Calculator Logic.
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And I'm going to move it right up close to my ViewController.
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Now, at this stage, I'm going to separate it out into MVC.
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So I'm going to right click on my CalculatorLogic file and I'm going to select New Group from selection,
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so that I can put it inside my Model file, and then I'm going to do the same with my ViewController,
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and that is going to be my Controller. And my Main.storyboard is going to be placed inside a View
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folder.
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So, now I have MVC and we're going to go into our Model and create a brand-new Swift class.
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Now, the class name is going to be the same as the name of the file, which is good programming practice,
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so that way when you look inside your file inspector over here, you can see at a glance all of the classes
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that you have included in your project.
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Now, my class is not going to inherit from any other superclass. So I'm just going to delete that placeholder.
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And there we have it.
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We have a brand-new class and we've separated all of our files into the MVC structure.
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Now, the next part you'll notice is that here I'm importing Foundation which is a library of code that
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includes all of the Swift code.
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But if you look inside View Controller.swift,
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you can see I'm importing UIKit.
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Now, this includes everything that's inside Foundation, but it also has a whole bunch of UI related things,
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such as UIViewController or a UILabel.
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Basically, every class that has UI to begin with it is inside UIKit and you'll need to import UIKit
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if you need to work with the user interface.
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Now, in our CalculatorLogic, we don't need to do any of that.
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So Foundation is perfectly fine.
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Now, the next step is we want to move all of this calculation-related code into our brand-new 
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CalculatorLogic model. So see if you can complete this as a challenge,
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see if you can move all of this into our CalculatorLogic and make sure that it still works
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when you test the app.
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All right.
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So first things first, let's create a new function over here and let's call it calculate. Now, this calculate
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function is going to take a single parameter which is called symbol, and it's going to be in the form of
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a String.
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Now, my calculate function is where I'm going to transplant all of this code over.
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Now, you might think that you can just transplant everything including the "if let," but let me show you
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what happens when we do that.
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So if you ever want to refresh the app when your errors are not occurring, then just try and hit command B
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and all the errors that are not meant to be there should be cleared, and the errors that are meant
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to be there will show up.
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So the first issue we have is use of unresolved identifiers slender.
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And the problem is that that sender variable comes from this input here and it refers to the button that
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triggered the IBAction.
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So there is no such thing as sender over here.
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So we have to make sure that we actually keep the "if let" over here inside our ViewController right
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here.
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So we can't actually transplant that part over because we don't have an IBAction with a button that's
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going to act as the sender giving us the information about which button got tapped by the user.
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However, if the calc button gets pressed, then we can check to see if sender.currentTitle is not nil,
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and in that case, we'll bind it to something called calcMethod, and that is what we're going to pass
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over to our CalculatorLogic.
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So we have one extra bracket down here
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because, previously, I deleted the "if let,"
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so make sure that we delete the bottom one as well.
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Now, the next step is you can see that there's quite a few of "Use of unresolved identifiers."
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And the thing that it doesn't know about is this thing called calcMethod.
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Well, inside this function, we've decided to call the calcMethod a symbol.
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So we need to replace all the instances where we've got calcMethod with symbol.
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So you can go ahead and hit command F to find this thing called calcMethod in this file.
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And if you click the dropdown, we can give it some replacement text, and we'll instead replace it with
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symbol.
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So let's replace that one, that one, and that one.
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And now, those errors will go away because we're going to pass in the calcMethod as a symbol parameter,
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and then we're going to check against that symbol inside this function.
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The next one that it doesn't recognize is this thing called displayValue.
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And if you remember, displayValue was our computed property up here,
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and we figured out what the value of it is by looking into the displayLabel and getting the current
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value.
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Now, we don't have a display label over here and we don't have a display value either.
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So we're going to have to create a new property inside this class and we'll just call it something simple
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like number.
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This is going to be of type Double and it's not going to have a value until we pass it over from the
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ViewController. So we can either set this to a optional Double, or as you can see, if I want to leave
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it as this, then it tells me that I have to initialize number when we create a new object from this class.
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So that means I have to add some init code. So I could say something like init(number) which is going
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to be a Double. And inside the init, I'm going to tap into self.number.
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Now, whenever you're using properties or methods associated with the current class,
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so in this case, CalculatorLogic, you can refer to the properties and methods with this keyword "self"
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in front of it.
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This means that we're looking inside the current class for something called number.
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And when you start typing number, you can see that in the left-hand side, we've got this "V" here,
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and this is telling us that this is a global variable.
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So if I hit enter, then I'm saying self.number, namely, this property should equal number, and that
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number is going to be passed in when CalculatorLogic gets initialized.
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And when I select it, you can see it's got a "L" symbol next to it which means that is a local variable
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which refers to this number.
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So while I can easily just write
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number = number, and it sort of makes sense, right?
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We're saying this left-hand side is this one. We're trying to set it to equal this one which is going
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to be the number that gets passed in through the init.
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Now, unfortunately, that does not make very much sense to either human or the computer, because even though
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they're named exactly the same, it's very, very confusing which one you're trying to set and which one
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you're trying to get. Now, one way of solving this is to simply make your global and local variables have
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different names. Because then you could say number = n and that's a lot clearer what it is that
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you're trying to change and what it is you're trying to grab the value for.
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Now, another way of addressing this is simply by adding the keyword "self" in front of the properties.
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So in this case, this is the property and we're saying "self," namely, in this current class, find a property
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called number, and there it is, and set it to equal the local version of number.
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So that is what the keyword "self" is for.
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It always refers to the current class, whatever it is that we're within.
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So, now we have a way of passing in a number to calculate.
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We have a function that performs the calculation and all we need now is just a way of giving the result
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back,
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and the result is probably going to be in the format of a Double.
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So we can fix these three lines of code that currently have errors to, instead, return the number multiplied
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by minus 1.
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So that means that if the user taps a 6, and then taps the "+/-" sign, then that 6 will
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get passed in as the number, and the "+/-" sign gets passed into this function.
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And if that is true, then we're going to return 6
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multiplied by minus 1 as the output of this function.
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So pause the video and go ahead and fix the last two lines.
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All right.
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So it's pretty easy. We just have to follow the format that we had before.
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So if they tapped on the AC button, then they simply want to change the display to zero.
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So we'll just return the number zero.
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And if they've tapped on the percentage button, then the result should be the number multiplied by 0.01.
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Now, once you get to the button here, it tells you that there's an error: Missing return in a function
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expected to return Double.
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Well, you might say that I've got loads of returns here, right?
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I've got return this, return that, return that.
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But notice how we have a if statement and two else if statements,
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there is still a possibility that we get a symbol passed in here that is not "+/-" not "AC,"
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or the "%" sign.
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So in that case, then this function doesn't actually return anything.
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And so what we need to do is right at the end, if none of these other cases catches it, then we're simply
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going to return nil.
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Now, the next problem is that this function expects us to return a Double,
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right?
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And that's not an optional Double, that's just a normal Double,
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so it can't ever be nil.
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So in order to be able to return nil, then we have to change this into an optional Double.
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So there might be certain cases where we pass in a number and we try to perform a calculation on it
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that doesn't actually match any of these distinct cases that we've addressed.
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So in that case, we're simply going to return nil and we'll end up with a optional, and we'll deal with
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the optional on the other side inside the ViewController.swift.
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So, now that we've got our number, as well as our calculate function done inside our CalculatorLogic class,
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we can head back to our ViewController and call that function.
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So inside this "if let," if we actually have a title inside the button that triggered the
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calcButtonPressed IBAction, then we need to have a new instance of the calculator.
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So let's create that here.
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I'm going to create a new let constant called calculator and it's going to be a new instance of our class
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CalculatorLogic.
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And when I tried to initialize it, you can see that it's expecting a number to be passed in, and that
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number is going to be our displayValue.
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And so when I create my brand-new calculator, then this CalculatorLogic object will be able to have
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its number property initialized to whatever it is that I pass in here, and then the next step is to call
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the function that we created inside that object which was called calculate.
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So calculator.calculate, and the symbol that we want to pass over is this calcMethod because that is
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equal to the currentTitle of the sender.
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So let's go ahead and add that in here.
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And this, remember, has a output and this is what this little warning is telling you,
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"Result of call to calculate is unused."
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So what should we do with the result of the calculation?
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Well, we're going to assign it to our displayValue and we're going to set our displayValue to the calculation
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result.
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And remember, we have a didSet in here,
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so whenever we set this displayValue, then the value will populate our displayLabel.
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Now, the final thing we need to address is that do you remember how we made the output of our calculate
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function, a optional Double?
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Well, we have to deal with the situation where that might be nil, right?
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So we can simply add in a guard statement and we can say guard let results = calculator.calculate
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with the symbol,
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and if this fails, namely, if the result was nil, then we're going to throw a fatalError,
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and we're going to say, "The results of the calculation is nil," so that we can debug it if necessary.
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And now we can set our displayValue to results which is now a unwrapped Double, instead of what it used
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to be which was a optional Double.
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So let's run our app now and see if that did what we expected it to.
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All right.
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So let's try this with some numbers.
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Let's say 1.2 +/- becomes -1.2, and percentage becomes 0.012,
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and then AC sets it back to zero.
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So our code seems to be working just fine, but as we're on the advanced Swift section, it's a good time
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to think about our app's architecture.
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Up till now in the course, we've been creating classes for all of our models in our MVC.
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And this works just fine
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and makes our code comply with the model-view-controller design pattern that Apple is so fond of, but we
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can actually make our code a lot safer by simply changing our class into a struct.
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Now, to create a struct, which is short for a Swift structure, all we have to do is change this keyword
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from class to struct and this will turn it into a structure.
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Now, for those you guys who have worked with other programming languages, for example, C or C++ or even
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Objective-C, you'll think of struct as simply just the way of organizing little bits of data, just simply
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a way of grouping into related small pieces of data together into a structure.
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But struct in Swift is a little bit different.
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And over the years, Swift structures have morphed and changed many, many times, and with every iteration
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of the language, they've actually added more capabilities, more functionality to these structures to the
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point where we are right now where they're essentially as capable as any class, and they're no longer
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these little structures that just groups a little bit of data together. They're very much first-class citizens.
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You can create properties inside structures. You can create functions inside structures and they can
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be used absolutely any way, especially for your Data Models.
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Now, if you look inside the Swift standard library and you look at their definition for a string, for example,
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you can see that it's actually a struct, and so is a Double, and so are many, many other things that we've
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grown to rely on in Swift.
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So let's take a deeper dive into Swift structures and compare them against the classes that we now know
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and love, and see when we should be using a struct versus a class,
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and what are the pros and cons of each.