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Now, through your journey as an iOS developer, inevitably, you will come across UserDefaults
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and you'll probably use it in quite a few of your projects because it's really simple to use, very flexible, and
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doesn't take very many lines of code.
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Now, let's have a look at some of the things that you can do with UserDefaults.
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Let's establish a default and we'll call it--
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Now, remember, when you create property names or variable names, you can't call it certain words like default
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or class, and you can tell because it highlights it in a different color
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in this sort of pinkish color that tells you that this is a keyword that's reserved for certain uses.
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So let's create a new UserDefault called defaults and we'll set it to equal UserDefaults.standard.
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And let's see what are some of the things that we can set into our UserDefaults. So we can write something
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like set URL forKey, set value, which is type optional any key,
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so we can use optionals. We can set Booleans, doubles, floats, integers.
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So one of the scenarios that you might find yourself using UserDefault is saving user preferences inside
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your app.
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So, for example, if you had an app where you allow the user to set the music volume, you might say something
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like set 0.24 forKey volume, and that means that at a later date, you can say,
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let volume = defaults.float forKey volume.
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Then you can use that persisted data for volume to always set your games or your app's volume to
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0.24 which is what the user prefers.
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Now, aside from floats, you can also use set on Booleans.
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So, for example, we'll set true for the key "MusicOn, so that every time they open up your game, you always
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have the music on, the music off depending on their preference.
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And there's whole bunch of other ones I'm just going to quickly run through.
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So you can, obviously, set strings, right?
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So, for example, you might have "PlayerName."
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You can also track user behavior by using defaults like set, use any, we use the current date by just
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tapping into a new object of date.
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And the key is going to be "AppLastOpenedByUser." And you can see that when you try to retrieve this,
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let appLastOpen = defaults.object.
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Now, we're using object because we're not trying to grab a float and "in" to a string,
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we're trying to grab something that was saved using optional "Any."
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So we should try and get back using the method that grabs us an optional of any data type.
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So let's hit object forKey and I'm just going to paste that incredibly long key into here.
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And now, you can see that appLastOpened is set to equal the time when this default was set.
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And, of course, every single time I run the app, it's going to change that time just by a little bit. 
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Cool. So we've seen some pretty standard data types being saved.
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Now, if you want to get really fancy with your defaults, you can also save a collections like
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let array = [1, 2, 3] and we can say defaults set value of optional Any to array, and forKey,
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let's call it "myArray."
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And there's a special method that allows you to get that array back by using defaults.array
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forKey string, and it's going to be "myArray."
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Now, you can either let Swift infer what's the data type of the object contained inside the array, because
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if you option-click on this array here, you can see that when we use defaults to retrieve an array
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out of UseDefaults. The return type is actually an optional array,
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and it doesn't know what is the data type that's contained inside the array.
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So if you want to be more explicit, you can downcast the array into an array of integers. And we can do
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that safely because we saved it here and we should know what the data type is that went into that default
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for the key "myArray."
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And similarly, you can do the same thing for dictionaries.
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And let's just say let dictionary = 
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[name": "Angela"] and we'll say default.set.
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And, of course, we're using "Any" again, and we're going to put dictionary in there and
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we'll call it "myDictionary."
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And then we can retrieve it over here by using defaults.dictionary
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and we'll put the key back in.
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Now the spelling of your keys matter a great deal.
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And if you get the case wrong or if you spell it even slightly wrong, it's probably going to crash your
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program.
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So it's usually a good idea to create a constant at the top when you're trying to access these things
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like, for example, dictionaryKey, and we'll set it to equal "myDictionary."
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So that way, instead of putting "myDictionary" as a string in all of these places, you can simply use your
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dictionary key in all the places where you need to access it.
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And this way if you do need to change your key, then you can do it in one place.
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And also if you do have any errors, it's much easier to trace and debug.
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All right.
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So that's a little bit of an overview of UserDefault
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and what you can do with them. As you can see, they're really flexible, really adaptable, and that's one
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of the reasons why they're prone to abuse.
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Now, you should only really be using UserDefaults to persist small bits of data,
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anything that's a few kilobytes, like a volume here and there, or a Boolean, or maybe the player names.
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The danger is once you start getting into saving arrays or dictionaries, that the data starts getting
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really, really big, really unwieldy and unmanageable.
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Now, the reason why we say that is because UserDefaults is not a database and it should not be used
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as a database. As we saw earlier on, all of these key-value pairs get saved into a plist and that entire
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plist of your UserDefault has to be loaded up synchronously. So, all of it gets loaded up before you can
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access any of the values contained inside the keys.
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So that means that if you're looking for something really small like the volume that was saved which
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is going to be just 0.24 which is a very small piece of data. Even to grab that small piece
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of data your iPhone or your simulator or whatever device you're running your app on has to load up the
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entire UserDefaults plist.
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So everything that you store in it has to be loaded up.
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And that means that it can be very time consuming if you start saving large bits of data in there. And certainly
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across the Internet, you'll come across various implementations on StackOverflow or GitHub where people
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are basically trying to use UserDefaults as a database. But just keep in mind, it's not a database and
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it shouldn't be used as one because it's going to have significant impact on the efficiency and the
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speed of your app.
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Now, one of the things that people always find really mysterious about UserDefaults is, how is it possible
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that I can simply use UserDefault across all classes and objects and I still get back the same item.
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And the reason for this is because it's quite special.
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It's what's known as a singleton. And in the next lesson, I want to show you what singletons are and we'll
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go to some of the way towards understanding how UserDefault actually works.
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So I'll see you there.