0 1 00:00:01,320 --> 00:00:12,600 Now, in quite a few places, you'll see these things. For example, UserDefaults, or, for example, 1 2 00:00:12,600 --> 00:00:15,140 sharedURLSession. 2 3 00:00:15,240 --> 00:00:22,260 These are all what are known as singletons. And what's special about singletons is that there is only 3 4 00:00:22,260 --> 00:00:28,080 one copy of it that can be shared across all of your classes and objects. 4 5 00:00:28,090 --> 00:00:34,620 Now, let's say, we create a new class that's called Car, and by default, I'm going to give it a property 5 6 00:00:34,650 --> 00:00:39,390 code colour that's going to be set to equal "Red." 6 7 00:00:39,600 --> 00:00:43,590 Now, say, later on we create some objects. We create 7 8 00:00:43,620 --> 00:00:47,640 myCar = Car. 8 9 00:00:48,250 --> 00:00:53,020 And we're going to set myCar's colour property to blue. 9 10 00:00:53,050 --> 00:00:55,850 So, say, my favorite color, right? 10 11 00:00:55,870 --> 00:01:01,220 Now, let's say we create another object of car and we call it yourCar. 11 12 00:01:01,300 --> 00:01:10,360 Now, if we tried to print yourCar's color property, can you guess what the value of it will be based 12 13 00:01:10,360 --> 00:01:11,370 on our code? 13 14 00:01:12,380 --> 00:01:21,230 So the value of it is "Red" and that's because it's a new object that has nothing to do with this object 14 15 00:01:21,230 --> 00:01:27,780 myCar and it's getting its color from the defaults which is the color "Red." 15 16 00:01:27,780 --> 00:01:33,410 Now, let's try and do the same thing, but this time, we're going to create a singleton. So let's create 16 17 00:01:33,410 --> 00:01:36,770 a new class and I'm going to call it exactly the same. 17 18 00:01:36,770 --> 00:01:45,770 I'm going to call it Car and I'm going to have a property called colour which equals "Red." Now, the special 18 19 00:01:45,770 --> 00:01:49,720 thing inside this class is that I'm going to create a singleton. 19 20 00:01:49,910 --> 00:01:57,620 I'm going to use the "static let" keywords and I'm going to call my singleton singletonCar and this 20 21 00:01:57,620 --> 00:02:04,440 is going to be set to equal a new object initialized from its own class, 21 22 00:02:04,490 --> 00:02:12,230 the Car class. Now, instead of creating a new object of car in this file, I'm going to create a new constant 22 23 00:02:12,630 --> 00:02:15,590 that holds a reference to our singleton. 23 24 00:02:15,890 --> 00:02:18,400 So let's use the same name, 24 25 00:02:18,410 --> 00:02:24,980 let myCar = Car.singletonCar. 25 26 00:02:25,190 --> 00:02:31,080 And I say myCar.colour = "Blue." 26 27 00:02:31,790 --> 00:02:36,890 Now, later on, let's create another reference to our singleton. 27 28 00:02:36,890 --> 00:02:41,380 Let's call that yourCar and we say 28 29 00:02:41,420 --> 00:02:53,070 Car.singletonCar. And we try to print the value of your car's colour. 29 30 00:02:53,120 --> 00:03:00,650 Now, in this case, we get blue because it doesn't matter how many constants or variables, or different 30 31 00:03:00,650 --> 00:03:04,680 ways that we point towards this singletonCar, 31 32 00:03:04,910 --> 00:03:08,730 it's always going to be the same copy. 32 33 00:03:08,780 --> 00:03:14,640 So I can use it in different objects and different classes, so let's say, class A. 33 34 00:03:14,660 --> 00:03:26,150 So where when we initialized class A, we say Car.singletonCar.colour to "Brown," and class B-- 34 35 00:03:29,340 --> 00:03:40,190 class B, we print Car.singletonCar.colour, and we initialize a 35 36 00:03:42,810 --> 00:03:52,110 and b as new objects. You can see that even though we're affecting the color of the singletonCar in 36 37 00:03:52,320 --> 00:03:59,580 completely different classes, it's still tapping into the same thing in the entirety of this file. Every 37 38 00:03:59,580 --> 00:04:05,090 single time, we're trying to change or rather get and set our singletonCar, 38 39 00:04:05,160 --> 00:04:08,010 it's always been the same object. 39 40 00:04:08,100 --> 00:04:12,500 And when we think back to our UserDefaults, it's exactly the same. 40 41 00:04:12,540 --> 00:04:20,220 We're using that UserDefaults.standard as a singleton to this UserDefault. 41 42 00:04:20,580 --> 00:04:28,530 So you can see that this line and this line look structurally the same. And that's because inside this 42 43 00:04:28,530 --> 00:04:30,110 class code UserDefaults, 43 44 00:04:30,120 --> 00:04:36,900 there is a singleton called standard, and this points towards the same list every single time we tap 44 45 00:04:36,900 --> 00:04:38,490 into the UserDefaults. 45 46 00:04:38,490 --> 00:04:46,200 So we're always retrieving and saving to the same file and it stays the same across all of our classes 46 47 00:04:46,350 --> 00:04:47,930 and all of our objects. 47 48 00:04:47,940 --> 00:04:52,800 So singletons exist in other programming languages aside from Swift. 48 49 00:04:52,800 --> 00:04:58,300 So if you've programmed in other languages, this concept might not be new to you at all. 49 50 00:04:58,290 --> 00:05:03,990 But if this is the first time you've ever come across the singleton or how it works or what it's all 50 51 00:05:03,990 --> 00:05:10,770 about, it might be worth having a read of the Swift Documentation which I'm going to link in the resources 51 52 00:05:10,890 --> 00:05:12,680 for this lesson. 52 53 00:05:12,930 --> 00:05:19,410 And if you want to learn more about UserDefaults in terms of all the different default values that 53 54 00:05:19,410 --> 00:05:25,560 you can store and you can retrieve, then have a look at the documentation which is really good on this.