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Now, those of you guys who are looking closely, you might have noticed that in the last lesson, we've ran
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our app and tested whether if our checking and unchecking was persisted into our plist, as soon as we checked
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off the first item, the last two items in our item.plist got erased.
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And the reason for that is because we're calling save items when we toggle the check box and that will
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overwrite the current Item.plist file.
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So let's go ahead and add it back in.
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So why does our app do that?
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Well, the problem is that we're not yet loading up our data from our Items.plist,
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instead we're loading it up by hardcoding the items and appending them to this itemArray which we
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know does not get persisted.
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So whereas, previously, we loaded our items from the UserDefault by using these lines,
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now we've commented out, and so we have to replace it with some more code that loads up our 
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Item.plist.
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So instead of writing our code here, I'm just going to call a method called loadItems and it doesn't
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exist yet,
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so Xcode is going to complain. But not for long because I'm going to create the function here and
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I'm going to call it loadItems, just as I called it above.
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And inside here, we're going to tap into our data by creating a constant and we're going to set it to
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equal data created using the contents of a URL,
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so this method. Hit enter. And the URL is, of course, still going to be our dataFilePath.
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And this is, again, a method that can throw an error.
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So I'm going to mark it with a "try?" which will turn the result of this method into an
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optional.
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So then I'm going to use optional binding to unwrap that safely.
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So, now just as above we created an object that was an encoder using the PropertyListEncoder in order
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to decode our items.
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We have to create a decoder, and you guessed it, it's a PropertyListDecoder.
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So let's hit enter. And we're going to create a new object of that class. Once we've got our decoder,
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we're going to set our array up here,
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itemArray, which contains an array of items.
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So itemArray is now going to be equal to
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decoder.decode.
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So this is the method that's going to decode our data from the dataFilePath.
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But we have to specify what is the data type of the thing that's going to be decoded because Swift isn't
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able to reliably infer the data type.
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And this is one of those rare cases where you actually have to tell it what is the data type.
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So the data type is, of course, an array of items.
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And because we're not specifying object, in order to refer to the type that is array of items, we actually
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also have to write .self here.
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And the Data is, of course, that data that we safely unwrapped above. And this method, again, needs a "try"
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keyword in front of it.
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So we're going to wrap this inside a do block and we're going to catch the errors by printing out.
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Now, let's just make sure that we called loadItems correctly inside viewDidLoad and we can safely
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delete all of this hardcoded item initialization, because as we can see, we've already successfully saved
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all of those items into our Item.plist,
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so we don't need to initialize it all over again.
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Now, before we run our app, there's one last thing that we need to do with decoding that we did with
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encoding.
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We need to say that our Item class is not only encodedable, but it's also conforming to the decodable protocol.
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This is a type that only contains standard data types and it can be decoded from another representation
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which in our case is a plist.
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Now, it's a little bit silly having encodable and decodable. And since Swift 4, you can replace it with
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just the word "Codable." And this specifies that a particular class and all of their objects conform to
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both the encodable and the decodable protocols.
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So let's hit save and let's check that we've called loadItems in vieDidLoad, and let's run our app.
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Cool.
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So we've now managed to successfully load up all of the items that currently exist in our Items.plist,
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and you can see that if I check on one of these items, that the done property updates live.
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Cool, right?
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So we've now managed to successfully do what we weren't able to do before which is save an array of
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custom objects into persisted storage and retrieve it at a later date.
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So, now if I go ahead and terminate this app, if I open it again, all my checkmarks still exist in the
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right places and all my items still get loaded up.
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So in this lesson, we use the NSCoder in order to encode and decode our data to a prespecified file path,
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and our code converted our array of items into a plist file which we can then save and retrieve from.
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Now, does encoding and decoding makes sense to you?
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Not yet?
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All right, let's think about it from a different perspective.
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When you encode data, you're essentially converting one data type into another.
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It's almost the same as converting your music into a vinyl disc.
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And what this machine does is that it takes music and it etches into a vinyl disc all the grooves that
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correspond to the music.
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So we're using the encoder to turn music into a vinyl disc, and that is our encoder. Now, our decoder takes
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that same vinyl disc and plays it or turns it back into the music.
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And this is exactly the same as what we're doing here.
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We're encoding one type of data,
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our array of custom objects into data that can be written into a plist, and then when we need that data,
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we use a plist decoder to take out that data in the form of an array of items.
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So this gives us way more flexibility over our NSUserDefaults. And when we save data to our
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NSUserDefaults,
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they all went into the same plist. But in our case, we can actually initialize different plists.
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For example, if we had different categories to our to-do list, we had one to-do list that was for home,
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one to-do list that was for work,
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instead of loading up a large plist that contains all of our items, we could actually separate it out
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into individual plist for each and every category,
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reducing the loading time every time we want to read and write to that plist.
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So have a play around with the code. And in the next lesson, we're going to make our persistent storage
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even more flexible and even more scalable by using Core Data.
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So I'll see you there.