All language subtitles for 11. Encapsulation in Action

Now in the last lesson, I showed you that in addition to being able to get the value of any of the properties
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in our quiz brain, so this other class. We're currently also able to change any of the properties in our
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other class. And in certain cases, that can make our app behave in a way that we don't expect.
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And so we get onto the second pillar of OOP which is encapsulation.
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And just as you might want to encapsulate your workers in different office cubes so that they work on
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their own thing,
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you might also want to encapsulate your classes and your objects.
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So recently I came across an interesting quiz on Inc. Magazine and it was about
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Are you a micromanager? And it's a quiz that checks to see whether if you're basically trying to micromanage
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everybody else's time and you're actually trying to do everybody else's job. When we're abstracting our
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classes and we're trying to separate out different jobs into different roles and into different classes,
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what we want is for each class to have a separate role and a separate job and to keep them simple.
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We want them to be just concerned with their own jobs and do their own jobs well.
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So if we have a waiter class and we have a chef class, we don't want the waiter to be meddling with the
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chef's job.
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We don't want the waiters to just come and, 'You're not cooking it right.
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You should be doing it like this.'
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That's not the waiter's job.
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And this will lead to a really sad chef who will be really annoyed that he's getting micromanaged by
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the waiter.
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But it also means that the waiters job gets more complex. And if the waiter was a class, then he would
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have more code and it would be more intermingled with the code of the chef.
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And this leads to actually more complexity and not less complexity, which defeats the purpose of abstracting
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all classes into separate modules.
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What do we do?
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Well we could build a wall between the waiter and the chef.
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And this is why the kitchen is usually separate from the restaurant
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right?
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So that the chefs can do their own job and the waiters can do their own job.
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So the chef is only concerned with cooking food and the waiters are only concerned with getting orders
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and delivering the food to people.
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And you only have small windows between the chef and the waiter where there are things that they strictly
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definitely need to interact with.
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So when the chef passes the food to the waiter for example.
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So how can we do this for our code?
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Now we saw in the last lesson that we probably shouldn't be able to change the value of the correct
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answers in our main.dart right?
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It's none of the main.dart's business to change the question answers. It's very much to do with the
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quiz brain. The quiz brain should control what is true and what is false.
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And ideally, the answers to the questions should never really change.
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So what can we do in this case to encapsulate this question bank?
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Well, we can make this property private. And we do that in Dart by simply adding a underscore in front
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of the name of the property.
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So now, if we went back to our main.dart and we tried to meddle with the value of question bank, it
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doesn't work anymore.
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In fact, we can't even tap in to that question bank anymore.
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We can't say 'quiz brain
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give me your question bank' because question bank is now private. So it means only the quiz brain has
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access to the contents of this property and nobody else can.
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So how do we make our app still work?
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Well let's delegate the task of serving up these questions and answers to the person who's responsible
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for all things quiz related, the quiz brain.
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So let's create a new function. Let's create something called a getQuestionText function. And remember
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inside a class it's now called a method but it's basically the same thing. And our get question text
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needs a integer passed in, which is the current question number.
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So what's the question number that you actually want to get hold of?
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And then it's going to return or output the actual question text for that question number.
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So the way we would express that is we would happen to the question bank, which is private, but because
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we're inside the same class, we're able to access it. And then we're going to tap into the question bank
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at the index of question number.
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So which question do we want? If that was equal to zero, then we're going to get hold of this question
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object and then we're going to tap into it's
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Question text, so .questionText. And now we can go back into our main.dart, and instead of trying
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to get hold of this private property which is none of our business to meddle with, we can simply ask
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our quiz brain to get me the question text, and get me the question text at this particular question number.
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So to start with that will be zero.
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And we're going to go into our quiz brain and call get question text on quiz brain, and we're going to
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get the first question, and get it's
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questionText. So now, you can see that we've actually not only just encapsulated our question bank and
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it's now safe from meddling from other parties, but we've also actually abstracted our quiz app a little
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bit more. So now as a challenge, I want you to be able to address the other two issues that we have.
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Don't worry about this one.
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But we need to get a hold of the correct answer when the user presses true, and when the user presses
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false. Pause the video and complete that challenge.
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All right.
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So first things first, let's go into quiz brain and we're going to get our quiz brain to serve up the
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correct answer.
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So I'm gonna create a method that returns a boolean, and it's going to be called getCorrectAnswer or
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whatever it may be that you called it, doesn't really matter. And we gonna pass in
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again, the question number that we are trying to get the correct answer for.
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So now we are going to return our question bank at the
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question number that they provided
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.questionAnswer. And now we can go back and do our main.dart and we can fix our code.
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So instead of trying to get hold of the entire question bank into our main.dart, we're instead gonna
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say correct answers equal to quiz brain get me the correct answer for the current question number. And
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then we can compare to see if the correct answer is equal to true or down here if it's equal to False.
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So let's fix this in the same way down here.
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Correct answer should now be equal to get correct answer from the quiz brain. So now our app works
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and we're able to get hold of the correct answer and the correct question, but we're no longer able to
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just meddle with that question bank anymore.
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We can't say quiz brain question bank,
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let's just set one of your answers to true. Because our question bank is now encapsulated inside its
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own class and only its own class.
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The quiz brain can decide what it should do with this question bank.
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So you can't accidentally in your main.dart, for example, completely mess up and change the answers
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or change the questions or even delete the question bank.
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So we're a little bit safer because of encapsulation. If we run our app at the moment,
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you can see that it still works exactly the same way. But it will still crash once you get to the end
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of the 13 questions.
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We've only got 13 questions and if we try to pull out the 13th item from our question bank, that's equivalent
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to the 14th question because lists of course start counting from zero.
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So what can we do about this?
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Well inside our quiz brain, we should be able to check to see if our question bank still has more questions
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to serve up.
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And if so, then we'll serve it up but otherwise we should protect the app by not trying to get hold of
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a question that doesn't actually exist.
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So how can we do this?
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Well instead of keeping track of which question we're on in our main.dart which is not really its
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job, let's remove this and let's put it into our quiz brain. So I'm going to put it inside my quiz brain
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class as a property and in fact I'm going to make it a private property.
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So only the quiz brain has access to this. So I can't accidentally say in main.dart or in question
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.dart, suddenly sets my question number to 12 or 13 which will crash my app. And instead of
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calling getQuestionText and providing the question number from our main.dart, I'm going to use our
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questionNumber inside our quiz brain. So we need to change it here to that private question number. And
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in order to go to the next question, I'm going to create a method that's just called nextQuestion and
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all it does is it checks to see if it's safe to actually go to the next question. And that means we have
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to check to see if the questionNumber is less than our questionBank.length.
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So we know that .length gives us the size of the questionBank.
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So this is going to be 13 because we have 13 items in our question bank.
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And when our question number is less than that, then theoretically, it means that it should be safe to
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go to the next question.
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So then we could say
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questionNumber++. And this should increment our questionNumber every time we need to go to
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the next question.
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So in our main.dart instead of updating our questionNumber, we're going to instead call upon our
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quiz brain to go to the next question instead. And we don't need to deal with our question numbers or
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keeping track of it inside our main.dart anymore.
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Let's go ahead and change that
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in both places.
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So it's gonna be quizBrain.nextQuestion. And we're gonna delete this print statement and also remember
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that get question answer no longer need an input and also get question text no longer needs an input.
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So now let's run up from the start by clicking either the run there or clicking our hot restart. And we
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should start off from the beginning of our quiz.
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So now if we go through our quiz and we get to the end you can see that our app still crashes. It's still
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saying that we're trying to get the 13th element of our list which only goes up to 12.
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So what's the reason here?
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Well let's add a print statement right here, so that we get to see what question number is.
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And let's also print our questionBank.length. And now let's run our app again from the start, so
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that the first time when we click true and we go to the next question, the question number goes to one.
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So we go to the next question and we have a total of 13 questions. So let's keep hitting true until we
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get close to crashing the app.
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So now currently, our question number is 12. And it means that we're trying to pull out the 12th item
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out of our questionBank which still exists.
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It's this last question right here,
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the question about West Virginia.
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The next time that I click the true button, it will trigger the next question method. And now it's going
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to check to see if the question number at this point, which is 12, is less than
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questionBank.length which is 13.
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This is true.
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So it's going to go to the next question and then it's going to update the question text. And it's going
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to try and pull out the 13th item out of our list, which doesn't exist.
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So right now, this is the point when our app crashes.
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So how can we solve this problem?
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Well, we can simply make our questionBank.length check it for one less than the size of the questionBank.
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Now we never actually get to 13.
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We get to 12 and the next time I click it, nothing else happens.
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We are at the end of the quiz and our app no longer crashes.
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Perfect. So now our app is being safety-checked inside our quiz brain, which is the logical place that
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should deal with all things quiz related.
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And our main.dart simply just is concerned with displaying things on screen or clicking a button
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to go to the next question, and it doesn't have to deal with the quiz functionality anymore.