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Hey everybody has gone this is Caleb with slopestyle.



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And in this video we're going to be talking about extensions in swift now extensions are amazing because



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they allow you to add functionality to an existing class structure an enumeration or even a protocol.



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OK.



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There's lots of different things we can do to extend the capabilities of certain classes either or classes



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that have already been created by us or even classes we create ourselves.



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Now this video is going to be an introduction.



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We're going to be using a playground and I want to show you a few cool things you can do with extensions.



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And then in the next video we're actually going to build a very very simple app to show you how extensions



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could be useful to you as a developer.



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So let's go ahead and let's pull open X code and let's go ahead and get started with the new playground.



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So click that button.



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And of course it's going to pop open on the wrong window.



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There we go.



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All right click blank and let's call this extension is fun time.



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Yeah.



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No just yet.



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Let's do extension's funtime.



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OK click Create and we get ourselves a nice little playground to play in.



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So let's go ahead and let's do that.



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So first of all I'm going to show you the syntax for an extension and to do that I'm going to get rid



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of that and get rid of this silly comment at the top.



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We don't need.



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And so let's begin to create an extension what we need to do is we need to type extension.



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OK believe it or not.



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And an extension like I said allows us to extend an existing class a structure a protocol or an enumeration.



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So what we're going to do is to create an extension.



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We're going to extend it like you're going to extend to certain classes let's think of a class and swift



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How about integers k so extension int.



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We're going to extend the int subclass.



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OK.



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And you set it up just like you would a class or a function you put a set of curly brackets and that's



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where all of the important information goes inside.



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So that's how an extension is made.



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Now what if you had created your own class like class my class.



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OK.



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You can also extend your own custom classes and make your own extensions.



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And inside is where all the functionality is going to go.



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OK.



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Whoops.



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Functionality.



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OK so we're going to add some functions and lots of cool stuff to extend the capabilities of a certain



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class.



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OK.



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Now some of the things that extensions are able to do are they are able to add computed properties and



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computed type properties.



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They're able to define instance and type methods.



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They're actually able to provide new initializers So if you think about when you create something with



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an initializer like a C.G. wrecked car you have lots of different ways to initialize it.



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That's just something you can do with extensions.



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Pretty cool.



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You can also define and use subscripts as well as nested types which is pretty cool.



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And you're also able to make an existing type conform to a certain protocol.



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So lots of different things you can do with extensions.



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And so now let's go ahead and let's take a look at just one way we could extend an existing swift class.



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So let's go ahead let's make an extension of string type extension string.



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And what we're going to do is we're going to build our own function that's going to be able to reverse



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a string.



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OK.



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Now there already is one.



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I know that.



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But the interesting thing is I'll show you the difference later there is a difference.



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And we're going to basically write a function that will return the exact opposite of a string that we



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give it.



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So inside of the string extension go ahead and type phunk reverse.



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And you notice that pops up there reverse.



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There are the two functions that already exist within the string class reversed and it returns an array



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of character and reversed and it returns a reversed collection of type string.



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So those are the ones that already exist.



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But we're going to write our own simply called reverse.



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So what we're going to do is we're basically going to think of extension string here.



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Think of this as something that can be applied to any type of string.



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So inside of the extension let's say that we applied this function to a string that we've created inside



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the function.



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We can just call it self ok because this extension is referring to whatever instance of string this



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function is called upon.



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So we're going to be referring to self to refer to the string that this extension is is connecting to.



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So let's go ahead and let's write this function out.



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Oh you know what we need to actually return a string at the end right because we need the reverse string



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at the end.



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So let's go ahead and create an array of characters.



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So go ahead and type var character array.



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And that's going to be equal to an array of character.



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Like I said and let's just instantiate it for now.



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Now we're going to do is we're basically going to parse through and for every character in the string



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we've passed then we're going to go ahead and insert it into our character array at index 0 so that



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every single time a new character is added it's added to the front.



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And so therefore we will reverse the entire string as we pass through all the characters.



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So go ahead and type four character in.



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Remember we're referring to self because that's the instance of the string self dot characters we're



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going to parse through the array of characters that's already built into to a string for character self



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dumb characters.



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What we're going to do is we're going to go ahead and type character array and we're going to go ahead



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and call insert a new element at Int and that's like kind of like the index path of an array.



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So we're going to pass it in 0.



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Actually it is the index or because that's what we're dealing with we're going to insert whatever character



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we get through at index 0 so that every single new one is going to be passed in at 0 and it'll bump



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all the rest of the letters down.



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So the first character comes in and it's at index 0.



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The next one comes in and now it's at index 0.



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The next one comes and they all start to move down until the whole string is reversed.



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At the very end what we're going to do is we're going to return the character but remember it's an array



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of characters we need a string.



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So what we can do.



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Check this out.



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We can call string and when we instantiate a string.



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If you scroll down you can make a string from a sequence of characters.



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So select characters and we can pass in the character array because that's a sequence of characters.



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Pretty cool right.



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So this is one function.



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Now that is in our string extension.



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I want to show you how to use it now though it's really really cool.



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So what I'm going to do to show you this is I'm going to create a string.



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Let's just do var name and I'm going to say Marty McFly.



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OK and of course you can tell that that's a string.



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Now what I can do is because the variable name is of type string.



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I can call name dot reverse.



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Now be careful don't use the top two but look.



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Reverse without the D.



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That's our function.



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Anything that is a string we can call reverse on and check out what happens when we call it.



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Look at the print out here.



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We get a reversal of Marty McFly.



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The string is reversed it's really really cool.



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Now just to show you that this is only applicable to strings.



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I'm going to create an integer like 12 and if I try to call reverse on this integer it's not going to



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work because we only extended the string class not the integer class so therefore it doesn't work.



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That is one way you can extend string with a cool function that reverses any string that you attach



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that function to really really cool.



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Now we're going to go ahead and create an extension of it.



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So let's do that down here below let's give you some space here so you can see OK this looks good.



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All right.



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So go ahead and type extension and just like we did with the string and we're going to write a function



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that's going to basically allow us to square any value that we give it.



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So go ahead and type phunk square and inside of this function we're going to just go ahead and type



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self times self because that's what something squared is the value of it multiplied by that value.



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Now of course in order for this to actually do something we need to return a value of int loops of end



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so that we can actually use this on an integer.



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And of course since we need to return we'll just return self time self.



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So there's our extension.



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That's cool but let's go use it.



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What we can do is we can say var value equals nine.



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And now what I can do just like with the string is I can call value dot square and watch what happens



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when we print out in the terminal here.



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You can see it prints out 81 it's squared R value.



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That's really cool.



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But one of the most interesting things about extensions is that you're able to use what are called mutating



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functions and a mutating function is basically a function that is going to actually modify the value



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of the variable that it's called on.



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So right now var value equals nine k value is equal to nine.



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We call value Square and what it does is it takes the value of our variable here multiplies it by itself.



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And this right here value Square is currently equal to 81.



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But watch what happens if I print out value even below the function you'll notice it still comes back



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as 9.



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That's because we have not mutated the original value.



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We're going to do that now with a mutating function and that's something really cool about extensions



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is it's going to allow you to mutate a certain function K and to do that we're going to make a function



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that's going to return the area of a circle.



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OK.



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So let's go ahead and let's extend double K because we need some decimal places here because if you



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know the the function to determine the area of a circle is pi are squared right.



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We're gonna take the radius of the circle square it and then multiply it by pi.



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So go ahead and call extension double and we're using double again because we need numbers after the



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decimal point.



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And we're going to go ahead and write our area function here.



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So go ahead and type phunk area or you know how to calculate area that's a little more descriptive and



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let's create a constant to hold the value of pi.



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So A-type let pi equals three point 1 4 1 5 that's close enough.



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You know it's not exact but it's pretty good.



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And what we can do is we can say self is equal to pi times self time self k.



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So we have our squared right the radius squared self-teach self and then we're multiplying that by pi.



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Now we're getting an error.



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And you can tell it says self is immutable.



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And if you think back to our string extension we basically called name and then we called name that



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reverse.



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Now what we did is we took our name.



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We printed it and then we took the value of the name and we reversed it.



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Kind of making this like a temporary variable.



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We did the same thing with values not square.



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But what we want to do is we actually want to modify the value of the variable so to do that we just



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need to add a keyword at the beginning of this function mutating.



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And if you look at the little description here of mutating I don't know if it'll let me.



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It won't.



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But basically a mutating function allows that function to modify the value of whatever variable it's



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called upon.



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So let's go ahead let's try this out.



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I want to show you how it works and that to do that I'm actually going to create a quick little class



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here for a circle that's going to have a radius property so just really quickly type class circle.



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Set a variable of radius and make that a double.



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Then let's create an initializer with a property of radius of type double set self-doubt radius to be



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equal to radius.



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Sorry that was really quick but it's not super duper important.



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I just want to make sure that you saw what I did there.



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So we have our mutating function for calculate area.



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We have a class to make a circle.



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Let's make a circle.



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So type var circle WIPs circle equals circle and we're going to give it a radius value so let's think



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about three point three.



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OK so there's our radius three point three.



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And what we're able to do is we can print out a circle dot radius and watch what we get printed out



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3.3.



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Just like you would expect right.



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And let me pull this up here so you can see it.



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There we go.



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So we get three point three which is the radius we've passed then.



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So if I want to calculate the area of my circle I can call Circle dot radius and watch what I can do.



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I can call my mutating function here calculate area and you'll see in a second it'll pop up



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go calculate area.



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Cool.



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So once this runs you'll see that it prints out the area of the circle thirty four point to 1 0 9 3



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5 whatever it is.



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Now you're probably thinking how is this different at all from what we just did.



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Let me show you up here.



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When we called a square on our value of nine what it did was it created a temporary variable holding



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81 which is the square value for nine.



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But if we print out value again you'll see that the original value of the variable value is still 9



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K like that it prints out.



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Now what we can do though is with our mutating function we're actually able to change the value of the



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variable upon which this is called.



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So watch what happens if I print out circle dot radius.



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Now check out what prints out thirty four point to 1 0 9 3 5.



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That's the variable that we just changed.



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We used our mutating function to allow us to modify the variable.



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It's really cool and there are plenty of use cases for this.



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But guys this is just kind of a brief introduction to extensions some of the things you can do now in



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the next video.



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I'm actually going to show you some cool things you can do in Interface Builder in Iowa with actual



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things you might use in a real app case super duper cool.



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Thanks for watching this video guys and let's head over to part two right now.