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All right.
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So previously, we've seen how the UITextFieldDelegate is implemented.
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We've also talked through how protocols work and the delegation pattern.
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But because this is quite a complex topic and it's quite abstract to wrap your head around, it's probably
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a good idea to watch these videos
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couple of times before it'll actually start to sink in.
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In this lesson, I want to show you the full picture. So we're going to create our own protocol, our own
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delegates, and we're going to be able to see the entire process without any of Apple's code.
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And I want to make it easy to understand through the use of an analogy. In this analogy,
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we have a emergency call center staff and his responsibility is to respond to emergency calls and dispatch
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emergency staff as and when needed.
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So this is our class of emergency call handler. And the emergency call handler has access to a bleep.
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So I've never seen a bleep either until I started working in a hospital,
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and it's this very old and strangely very expensive piece of technology which is kind of like an
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old-school mobile phone where you can call the bleep, it has a phone number, and you can trigger it to report
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a message on the screen, as well as bleep,
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so make a sound,
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and sometimes you can even get it to play recorded messages.
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And in this case, our emergency call handler can activate this bleep remotely, and get it to trigger and
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play a sound, and notify the person that carries the bleep that there is a cardiac arrest, somebody's
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heart has stopped, and you need to go and perform CPR.
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So in this case, the bleep is going to be the delegate property, and whoever carries the bleep or whoever
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is the delegate is the one that's going to be notified when they need to perform some action.
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Now, the emergency call handler doesn't actually care who carries this bleep or who is the delegate.
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All it cares is that in order to be able to carry the cardiac arrest bleep, you must have first completed
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the Advanced Life Support Course.
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So this is a two-day course where you learn how to resuscitate somebody or bring them back to life through
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the use of various methods.
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And one of the methods you learn is CPR. The emergency call handler knows that whoever is the delegate,
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whoever carries the bleep must have completed this course,
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and so they must adopt this protocol.
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So this is the AdvancedLifeSupport protocol. And whoever has adopted this AdvancedLifeSupport protocol
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will, of course, surely know how to perform the CPR method.
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So now that everything is set up, a day goes by and a new day begins.
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So on this beautiful morning, Pete, the paramedic, comes in and he is the one who is on shift today to
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carry the bleep.
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So he is carrying around this bleep which can go off at any minute notifying him where he needs to be
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to perform CPR.
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And so the emergency call handler at some unknown time will get a call and he will trigger the bleep
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to go off.
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Often when you're working as a doctor, the bleep is the bane of your life.
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I love the classic thing that the doctor does. They're always checking the time and saying, "Oh, God. What am I working?"
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Now, that the emergency call handler has triggered the bleep,
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it basically tells whoever is the delegate or whoever is carrying the bleep that you must go here and
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perform CPR.
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Now, because today is the paramedic Pete who's on call,
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then he's the one who gets the message and he goes and does that action.
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And we know that he can do that action because he has the AdvancedLifeSupport certificate or protocol,
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so the emergency call handler can be safe in the knowledge that when they trigger this pager or this
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bleep, that whoever is carrying it, is able to perform CPR. And they're able to notify the delegates or
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the person on call to perform that action by calling delegate.performCPR.
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Now, the beauty of this is, of course, that it might be the Paramedic Pete who's on call, it might be the
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Dr. Dave
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who's on call, or it could be the Surgeon Sally who is carrying the pager or the bleep. But to the
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emergency call handler,
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they don't need to care. All that they care about is that as long as there is somebody carrying the pager
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or the bleep, they can trigger the method perform CPR and whoever it is, whichever class it is that is
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set as the delegate will be notified and will perform that functionality.
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So let's convert all of this logic into code. Inside Xcode, I'm going to create another new Project.
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And it's, again, going to be a macOS command line tool.
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And I'm going to call it Protocols and Delegates. So we're gonna head over to our main.swift and
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delete everything as usual.
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And the first thing I'm gonna do is I'm going to create that protocol.
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So I'm going to create that certificate effectively.
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I use the protocol keyword and I give my protocol a name which, in this case, is going to be called the
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AdvancedLifeSupport.
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Now, this AdvancedLifeSupport protocol has just one requirement. In order to adopt this protocol, in
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order to get this AdvancedLifeSupport certificate,
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you must be able to know how to performCPR. And there's no implementation in the protocol,
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it's up to the person who is adopting this protocol to say how they're going to perform CPR, but they
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must know how to do this,
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so they must define this function somewhere within their class or their struct.
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Now comes along our emergency call handler. And this class is going to have a important property which
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is the delegate property, and the delegate property has a data type.
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It has a advanced life support protocol as the data type.
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So this means that whoever sets themselves as the delegate, any class or struct must have the advanced
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life support protocol adopted, so they must be able to know how to performCPR.
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So the emergency call handler has a couple of functionalities or methods.
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For example, they might be able to take calls,
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they might be able to assess the situation,
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where are you at, and what happens. To keep this simple, let's just say that assessSituation involves
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asking the caller, "Can you tell me what happened?"
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But they also have the functionality of triggering a medicalEmergency.
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So in this case, what they're going to do is they're going to call upon the delegate, whoever it may be,
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and they have no knowledge or care of who the delegate is, but all they need to do is say, "Whoever is
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on call, whoever is carrying the pager, please perform CPR on this patient."
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And so now let's define who the delegate may be. Let's define our paramedic. Paramedic. And this paramedic
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is going to adopt the AdvancedLifeSupport protocol.
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So they've been on the course, they've got the certificate,
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and what this means is they must be able to know how to perform CPR.
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And you can see that this is the error that we're getting from Xcode at the moment:
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Type 'Paramedic' does not conform to protocol 'Advanced Life Support" because in order to conform, you must have
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some implementation of this method could performCPR.
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So if we go ahead and click fix, it'll add that function into our Paramedic struct, and we now have to
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define what happens when this method gets triggered. In our case, "The paramedic is simply going to do
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some chest compressions, 30 per second." And they know how to do this because they went on the course,
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and so they've got the certificate, and they know how to implement performCPR.
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And in addition, when the paramedic goes on shift when they are initialized, they get told who the handler
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is.
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So the handler is, of course, of class EmergencyCallHandler.
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So when the paramedic goes on shift, the first thing they do is they pick up the bleep or the pager and
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they set the handler's delegate property as themselves.
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This is the equivalent of picking up that pager, which is the emergency pager, and they're now on call.
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And through this line of code, they've said, "I am going to listen for notifications when I have to perform
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CPR from the emergency call handler.
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I know who the handler is. And when they tell me to perform CPR that's exactly what I'm gonna do."
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Now that we've defined all of our classes, structs, protocols, whatever it may be, it's time to create our objects
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from these blueprints.
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The first thing we've got is an EmergencyCallHandler,
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so we'll call him Emilio. Emilio,
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the EmergencyCallHandler.
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And then, we've also got Pete the paramedic who's on call.
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And when the paramedic gets started with his day, he knows who the handler is.
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He gets notified that today you're on call and Emilio is going to be the one who's going to give you
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notifications,
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tell you when you need to go and perform CPR.
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Now we've got Emilio and Pete, both working and on their shifts.
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So at some point, Emilio gets a call and he's going to, first, assess the situation, what's happening, what's
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going on, where are they based,
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and then he deems that this is an emergency.
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So he is going to trigger a medical emergency.
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And when that happens,
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so let's run our code,
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the first thing that gets printed is Emilio is assessing the situation, so he's asking the caller can
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you tell me what happened.
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The next thing that happens is the medical emergency and this triggers the delegate whoever is set as
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the delegate to perform CPR.
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And in this case, it is, of course, our paramedic who's on call because he's set himself as the delegate.
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So when Emilio triggers the medical emergency, the bleep goes off, and it's the paramedic who's on call,
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so he goes and does the CPR.
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Now, this is all possible because of the fact that we can use the protocol as a data type. So in this
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case, the EmergencyCallHandler has this delegate property which requires that whoever sets themselves
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as the delegate must have this data type. They must have the AdvancedLifeSupport protocol adopted.
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And the reason why they must have the protocol adopted is because the EmergencyCallHandler knows that.
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If you've adopted the AdvancedLifeSupport, then you must have this functionality of performing CPR.
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So when they need to call upon the delegate to performCPR, they don't actually care who is the delegate,
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what class it is, what Superclass it might have.
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All they care is that they must be of the type AdvancedLifeSupport.
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So this means that we could create some other classes. Let's say I've got a class, instead of struct here,
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and this is a Doctor class.
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And the Doctor class doesn't actually have a Superclass, it's just a class as it is.
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But the Doctor class has adopted the AdvancedLifeSupport protocols,
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so they've also been on the course.
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And that means that they know how to performCPR.
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So let's add that function performCPR in here.
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Now, when they performCPR, all that's going to happen is they're going to follow the AdvancedLifeSupport
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protocol, and they're just going to do some chest compressions, 30 per second.
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Now, the doctor also has some other functionality, and maybe they could--maybe they also like using their
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stethoscopes. And when they do that, all that we do is, we'll say, they print "Listening for heart sounds." 
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And the doctor similar to the paramedic,
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the first thing that they do when they get initialized or when they start their shift is they set themselves
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up with the EmergencyCallHandler, so they know that there is a EmergencyCallHandler and they set
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themselves as the delegate.
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They put on the pager and they are saying that when I go on shift, I'm gonna be the one that is gonna
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be the delegate.
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That's going to be notified when I need to performCPR.
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And if the EmergencyCallHandler triggers this method, then that's exactly what I'm going to do.
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Now, in addition, there could be a Surgeon class and the Surgeon class inherits from the Doctor class.
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So that means they immediately, by default, adopt the AdvancedLifeSupport because it's inherited from
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the Doctor class.
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And it also means that they already have access to the method performCPR. But they can, of course, override
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it.
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So they could say, "We'll do what the doctor would do when they need to perform CPR. But in addition, because
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we are surgeons, we also like to do a little bit of singing while we perform CPR.
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So we're going to sing 'Staying Alive' by the Bee Gees while we perform our chest compressions."
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Now in addition, maybe the surgeon also has some extra functionality, say, they are really good with electric
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drills which just make whirring sounds when you play with it,
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I mean, use it.
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So there we go.
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We've now defined a Doctor class, as well as a Surgeon class,
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and we're now able to put some other guys on shift. So, let's say that today it's not Pete who's on shift,
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let's say it's Angela the surgeon who's on shift.
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And the first thing I do, of course, is go ahead and set myself up with Emilio.
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And that means that I'm going to be the one carrying the pager and listening for when Emilio will trigger
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the performCPR method.
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And so now when Emilio triggers a medicalEmergency if we run the code, notice how we haven't changed
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any of the code that Emilio has performed,
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but as soon as the medicaEmergency goes off, it's actually the surgeon that does the chest compressions
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while singing "Staying Alive" by the Bee Gees.
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So it's, of course, this method that's getting called.
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So now we've seen how our Paramedic struct can adopt the AdvancedLifeSupport protocol, our Class doctor
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can adopt the AdvancedLifeSupport protocol, our class Surgeon which inherits from Doctor is automatically
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adopting the AdvancedLifeSupport protocol.
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And in this case, the EmergencyCallHandler, which is taking the role of our UITextField, can be completely,
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blissfully unaware of who is going to be performing the CPR, which class, or whatever they may be, as long
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as they adopt the AdvancedLifeSupport, we can trigger the performCPR method, and it could be the paramedic
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that's on call, it could be the doctor, or it could be the surgeon, and the honey badger,
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I mean, the EmergencyCallHandler doesn't care. So that's an overview of how we would create our own
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delegates and protocols.
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And so now that we're armed with this knowledge, we can be more dangerous. And in our project, we can start
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coding up the functionality for getting the weather data.