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In the last lesson, we added the logic to progress to the next question into our Quiz Brin. However,
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Xcode gave us an error as soon as we tried to change the questionNumber, and the reason is we're trying
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to change a property of our struct from its methods.
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Here is our struct blueprint.
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And here when we actually use that struct, that quizBrain, well, this is the actual struct object.
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So each of these properties, say, the questionNumber or the quiz,
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well, they are associated with that object created from the struct, and they cannot be changed.
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The only way we can change them from inside one of our methods is to mark those methods with the keyword
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mutating.
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In other words, we have two kinds of methods in our structure.
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The first kind is a method that doesn't change anything and just returns a value like getProgress.
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And the second method is one that changes a property in the Quiz Brain.
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The only thing we have to do is to help identify which functions will change a property in the
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Quiz Brain.
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That's why we have to mark our next question method as mutable. So if we click on this suggestion, you
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can see that it's telling us one of the fixes is to mark this method as mutating to make 'self' mutable.
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So if we click Fix, you can see it as the mutating keyword before the func keyword.
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And what this tells the computer is that when this function is run, it will replace the old value of
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questionNumber with a new value.
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So every time you create a struct, and inside one of the functions or one of its capabilities, you need
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to be able to update one of the properties,
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so a var or changeable property,
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you have to do that inside a mutating function. So if we run our app right now, you can see there are
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app works perfectly just as it did before.
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But in this case, we've shifted all of the quiz logic into a separate structure called the QuizBrain
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and our controller or the ViewController now is only responsible for delegating these tasks to the
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quizBrains, say,"Hey, quizBrain,
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give me the next question," or "Hey, quizBrain, check this answer for me," or "Get me the questionText,"
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and the control is now talking to our model to be able to make the quiz work. And our controller is also
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talking to our user interface, namely changing the button background colors or updating the UI every
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time we go to the next question. And the code is now much easier to understand, more reusable and more
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modular.
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So we've now implemented the model-view-controller design pattern to our Quizzler App. To practice coding
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in this MVC design pattern,
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we're going to set you a challenge. And the challenge is to make our app be able to keep track of the
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user's score.
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So what you're gonna do is you're gonna create a new label and you're going to drag it to the very top
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of our vertical stack view, so you get to place it here. And then, we're going to modify some of the attributes
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or properties of this label.
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And I'm going to change the text color to white to be able to see it against the dark background.
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Now, the next thing I want to do is I'm going to create an IBOutlet that links from this label to my
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view controller.
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So I'm going to create it just about maybe here and I'll call it a scoreLabel.
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Now, finally, we're gonna go to our view controller. And under updateUI, I'm going to set the scoreLabel's text
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property to equal to, say, "Score:"
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And then I'm going to use my string interpolation to insert a method to get from my quizBrain.
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So I'm gonna tap into my quizBrain and I'm gonna say getScore.
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So here comes the challenge for you. At the moment,
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there is no such thing as getScore in quizBrain.
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So pause the video and make sure that all of your score tracking and score managing and score keeping
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functionality happens inside the QuizBrain.
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And if you manage to successfully complete this challenge, this error should, of course, go away.
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But, also, we should be able to see the score start out with zero.
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And every time we get an answer right, the score will go up by 1.
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So if we get an answer wrong, nothing changes, but all the correct answers will increase our score.
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And once we get to the end of our quiz and it resets, well, our score should also reset to zero.
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So this is the challenge for you to practice using MVC and making sure that this quiz-related functionality
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lives inside the QuizBrain.
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Pause the video now and try to complete this challenge.
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All right.
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So here comes the solution.
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We know that we can't write any code in our view controller which is solely responsible for updating
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the UI and talking to our model.
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So we have to make our model be able to perform the functionality of keeping track of score and sending
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it back via this method called getScore.
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So if we go over to the QuizBrain, this is where all of the heavy lifting is going to happen.
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So the first thing I'm going to do is I'm going to create a variable that's going to keep track of the
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score in a similar way that I'm keeping track of the questionNumber that the user is on.
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The score is going to start out with a value of zero.
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But when is it going to get increased?
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Well, whenever we check the user's answer and it matches the correct answer,
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well, in that case, not only do we return true and tell our view controller that the user got it right,
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but we also increase the value of score by 1.
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Now, we're not going to do anything to punish them when they get it wrong.
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But we do have to address the issue of the mutating function because, again, we're modifying one of the
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properties of a struct
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so, therefore, we have to mark this function as mutating, so that we rebuild our structure with a new value
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for score.
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Now, the next thing we need to do is to be able to send this score back through the function that the
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view controller cause which is called getScore.
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So I'm going to copy that. And in my QuizBrain just above my getQuestionText, I'm going to create a
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new function called getScore and this is going to return the score which is, of course, a integer or
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a whole number.
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And it's simply going to return the value of our score property which lives inside our Quiz Brain.
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So now the very last thing we need to do with score is to remember to reset it down to zero when our
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quiz restarts.
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So not only do we reset our questionNumber, we also reset our score.
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And now if I run my app, firstly, my errors should go away because this method now exists, and also I'm
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now able to keep track of the user's score. when they get a question right,
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the score goes up, and when they get to the end of the quiz, the score resets.
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So did you manage to get that right?
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If you want more practice on using the MVC design pattern to be able to switch out or modify just the
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Quiz Brain component and leave everything else as is,
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we've got an optional challenge for you. And the challenge here is if you go into the README, you can
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see that in addition to the default quiz, we've also got a section called the Multiple Choice Quiz.
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So if you were to copy all of these multiple-choice questions and replace the questions that's currently
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inside our quiz array with the multiple-choice questions, can you figure out how to modify the code in
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the app so that you end up with a multiple choice quiz?
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And if you manage to successfully complete the challenge, then this is what your app will look like. And
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it's now modified to be able to serve up three possible answers, and then the user can select the answer
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as one from the three.
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So there's a number of things that you'll have to modify in the app for this to be able to work.
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And if you want to tackle this optional challenge, then go ahead and replace the quiz array now.
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And once you're done, you can compare your answer, against my answer by downloading the completed project
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for the Multiple Choice Quiz which we'll be able to find, of course, in the course resources.
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But either way, no matter what you choose, I'll see you on the next module.