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So in the last lesson, we looked at how to perform create in CRUD using Realm,
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and we were able to successfully add new categories that we created inside
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our addButtonPressed IBAction and save it to our Realm persistent data store.
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Now, in this lesson, we're going to tackle the next letter which is read inside CRUD and we're going to
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look at how we can load up our categories from all Realm database, rather than from Core Data.
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And so the first thing I'm going to do is I'm going to delete this thing called context because we no
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longer need it.
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So the place where we read from our database is inside our loadCategories method.
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So if we scroll down to find it, you can see that we've commented all out.
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So instead of having this request which is a fetch request or try context.fetch, we can replace
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all of this code with just a single line.
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We can say let categoryies = realm.objects, and then we just specify the type which is all of the
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category type objects. And this will pull out all of the items inside our realm that are of Category
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objects.
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And that means that we can delete all of these lines of Core Data code replaced with just this one line.
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Now, at the moment, I'm creating it as a local variable.
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But if we wanted to use it elsewhere, then we can use our variable categories.
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Now, what you'll see is that I can't straight up just assign it to this variable called categories,
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it's going to give me an error. And the error it says is that it can't assign a value of type results
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to type array.
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So what it's saying is that the data type of the objects that we get back is results which is a container
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type that comes from Realm Swift.
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So this is equivalent to list or array, but it's a different data type and it's not very easily converted.
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But what we can do is we can change the data type of our Category's property to the results data type.
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So we can say that categories is of data type Results, and Results is basically just an auto-updating
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container type,
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so container types are things like arrays, dictionary, results, lists, and it comes from Realm and it gets
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returned from object queries.
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So that means that whenever you try to query your Realm database, the results you get back is in the form of
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a Results object.
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So we can specify our categories, which is what we're going to get back when we query our database, as
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data type of results, and it's going to contain a whole bunch of Category objects. And I'm going to first
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unwrapped that and we'll go with that for now.
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So now this line of code is perfectly valid because the data type that we get back here is Results containing
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code bunch of categories as the element inside the results, and then categories is also the same data
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type. So that matches and that's now allowing us to proceed. But we get this error down here where we say
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self.categories.append newCategory.
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Now, because this result data type, as we found out earlier on, is an auto-updating container.
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We don't actually need to append things to it anymore.
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It will simply auto-update and monitor for those changes.
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So, now with just one line, we've managed to place four or five lines of Core Data code that does exactly
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the same thing.
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So let's give it a go and run our app and see if it works.
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So there you go.
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Those are our two items that we saved earlier on into our Realm Browser as you can see here.
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And if I terminate the app and go back into it, you can see that it's still able to persist and load
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up or read from our Realm database just based on those two lines of code.
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Now, let's address the issue of force unwrapping this particular property.
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It's not ideal because what if, say, we forgot to load up our categories,
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and this was, in fact, nil when we force unwrap it and try to use it to populate our table view.
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Well, then we're going to have an issue, and you're going to see it when I run our app. There we go. "Thread 1:
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Fatal error: Unexpectedly found nil while unwrapping..." this particular optional.
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So let's stop our app and let's make our code safer.
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So instead of making it a forced unwrapped, I'm going to add a question mark to this data type and make
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categories an optional.
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So, now instead of saying categories. count, I'm going to say if categories is not nil, then return
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categories?.count.
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But if it is nil, then just return 1.
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So this is a new bit of syntax. And this in Swift is called the Nil Coalescing Operator.
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What it means is that this could be nil.
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Because categoreis is an optional and we're only saying get the count of categories
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if it is not nil, but if it is nil, then this is going to be nil, right?
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But if that happens, then the nil coalescing operator will say then use this value instead.
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So if categories is not nil, then we'll return the number of categories we have.
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But if it is nil, then we're going to just return 1,
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so our table view will have just one row. And down here if categories, the optional is not nil,
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then we're going to get the item at the indexPath.row and we're going to grab the name property.
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But if it is nil, then we're simply going to say the text is just going to be equal to "No Categories
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Added yet."
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And finally, I'm going to add a question mark here to say that the selected category of the destinationVC
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will be set to the category
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if it's not nil at the indexPath.row that we selected. And that gets passed into here as the
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selectedCategory which is an optional category. And only when it is set,
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i.e., it's not nil,
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do we actually call load items.
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So we're still okay over here.
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So, now even though my loadCategories is still not commented in because it's not active and this categories
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will be nil when we try to run our app, you'll see that our app no longer crashes and burns. And instead, it
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actually has a slightly different but better user experience.
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So, now you can see that the app has not crashed even though this categories property is now nil.
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And that's because it has some default values. And the default value when categories is nil is to produce
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a single table view cell and for that cell to have the text of "No Categories Added yet."
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And that makes our app so much safer and a much better user experience.
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Now, the Add button is still going to break
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when we try to add new categories, and it's not going to show up.
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And that's because we need this loadCategories method to be called so that we start auto-updating our
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categories using the objects inside our realm.
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So let's go back and uncomment this method call and run our app again, and you'll see that we still have
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all of our items showing up and persisting.
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So we've now managed to use our Realm database to add new items and save them to our Realm, and we've
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also managed to use it to load up our Realm objects using a single line of code.
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And now all we have to do is to fix the same things inside our TodoListViewController.
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Now, all we have to do is to fix the same things inside our TodoListViewController. So we are passing
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over the current category as a optional category and it's going to land over here as the selectedCategory
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And when that happens or when that does get set, then we're going to call this method called loadItems,
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and we no longer need our context,
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so we're going to delete it, and we also no longer need Core Data.
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So I'm going to instead import Realm Swift. And, of course, because we're using Realm, we have to create a
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new instance of Realm, let Realm = try! Realm.
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So, now when our selectedCategory does didSet from our CategoriesViewController coming over from
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the segue, then we're going to call loadItems. Now, loadItems is an unresolved identifier because we
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commented out before
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in order for our app to be able to run without any errors so that we can fix it now.
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So I'm going to uncomment this method. And inside here, I'm going to delete all of this and all of this,
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and instead of all of those lines of complicated Core Data code, I'm, instead, just going to say itemArray
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= selectedCategory.items.
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Remember that relationship that we're going to use now?
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So all of the items that belong to the current selectedCategory and we're going to sort it by KeyPath
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and ascending, and the KeyPath is just as we've done before,
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sorted by the title of each item and we're going to make it ascending: true.
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So KeyPath is called "title," so that's the title of each of the items that come back into the itemArray,
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and ascending is going to be true. And because itemArray at the moment is a array of item objects,
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we need to change the declaration of the tub to be this results container containing item objects data
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type.
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So let's do the same as we did before
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and delete that,
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instead declare its data type as Results, the auto-updating container containing a whole bunch of item
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objects.
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I'm going to make it an optional.
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So, now we can replace all of this code with just a single line like this. Now, it no longer really makes
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sense for this to be called iitemArray because it's now no longer an array,
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right?
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It's actually a Results container containing a whole bunch of items.
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So we probably should change the name of this variable.
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Now, I can either go around finding all the places where I used itemArray, itemArray, itemArray.
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In fact, the errors actually help to guide me all the places where I need to change its name. But a slightly
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simpler way of doing this is you can simply hit command F to find all the places where itemArray exists
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and you can replace it with a new name,
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for example, something like todoitems. But this is still a little bit clunky, it actually gives you a
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really, really nice way of being able to rename yourvariables and not have to worry about changing it
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everywhere it's used.
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Now, you might get some errors if you tried to do it just like this because, remember, we changed the data
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type of this variable and in all the places where we used it, we used it in the context of an array of
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items.
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So if you get any strange behavior trying to rename, then you can actually revert this back to an array
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of Item objects, and you can initialize it to an empty one,
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and we can simply hold down command and click on item array.
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And if you scroll down, there's this thing called Rename and it'll go into this accordion view where
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it'll only show you the lines of code where you use this variable called itemArray, and you can select or deselect
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the ones that you want to change.
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So, for example, the comments are by default not included in your renaming because, as you can see, our
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comments come from our old code, so we can leave those alone.
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But all the ones highlighted in blue will get renamed as we type in here.
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So I'm going to call that todoItems and I'm going to hit Rename and it's now updated it in all of
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these locations.
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So, now we can change this back to our previous results containing a whole bunch of items and it's an
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optional
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and we managed to rename all of those places which used to read itemArray.
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So, now because todoItems is an optional, we're going to have to either unwrap it or do something about
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it.
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So we're going to do the same as we did before.
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We're going to use optional chaining to say if todoItems is not nil, then return the count.
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But if it is nil, then return 1. And down here,
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we'll, first, going to use optional chaining
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to say that if todoItems is not nil, then grab the item at indexPath.row.
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And if all of this works and it's not nil, then, in that case, we're going to change all of our accessories
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based on the item.title and the item.done properties.
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But if it is nil or if it fails a "else," then we're going to say cell.textLabel?.text is going
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to be equal to just a simple string, and it's going to be no items added,
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and we're going to return the cell whichever state it is in depending on this optional binding check.
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Now, all we have left is this error.
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And for now I'm just going to comment it out because in order to persist this data using Realm, we actually
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have to do it slightly differently than this.
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And we'll tackle this in the next lesson. We will learn about update in CRUD using Realm.
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So let's leave that for now.
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But we do have to change our saveItem data.
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So as a challenge, I want you to be able to fix these lines of code and change it to use Realm, instead
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of using Core Data, as we are here.
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Now, when you complete this challenge, I want you to ignore this function called saveItem.
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In fact, just go ahead and delete it.
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And I want you to do all of your saving just inside this closure.
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So I want you to create a newItem from our data model item and I want you to save it here as well.
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So pause the video and give it a go.
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All right.
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So how did that go?
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Let's go ahead and uncomment these few lines and we're going to update it in order to use Realm. So, we
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no longer have a context,
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so let's delete that. We're just initializing a new object of the item class. And the title is going to
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be the current text that's inside the textField. Done doesn't need to be specified anymore because it
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is already here as a default.
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Now, because our selected category is of data type optional Category, we have to somehow unwrap it or
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use optional binding in order to make it work.
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So if we say if let current category = self.selectedCategory, and if that's not nil, then
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we're going to say let newItem = Item, and newItem.title is the textField.text, and instead of setting
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the newItem's parentCategory property here,
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instead, we're going to go the other direction.
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We're going to tap into the items that belong to the current category
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and we're going to append this newItem to that list. so we can say currentCategory.items, which is a
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list of item objects, .append this newItem.
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And that takes care of these two lines
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and replaces them. Now, as we said, instead of calling self.saveItems to do our saving down here,
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we're going to do all of it up here.
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And so we need to be able to say
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try realm.write,
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And we'll going to write is all of this.
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So create a newItem, give the newItem a title, and appended to the items in the currentCategory.
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And this, of course,, has to happen after we've unwrapped our self.selectCategory, so it needs to
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go in here.
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And because this can throw, then we need to wrap it inside a do-catch block.
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And because we're inside a closure, we'll use self.realm.write,
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and we can delete this call to save items.
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So did you manage to get that?
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It's a little bit convoluted in terms of the logic, but you can clearly see that this is far less code.
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And to me, it's way more human readable and the logic is easier to follow than using Core Data.
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And the last thing we need to do here is that once we have written this new data to all Realm, we need
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to call tableView.reload data, so that we call those data source methods again and update our table
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view with the newItem because we've gotten rid of our save method.
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So, now all that's left to do is to run our app and see if it works.
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So as you can see, we've got all of the four categories that we added on earlier.
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And if I go inside Shopping as a category, then you can have a look inside the Realm Browser as well.
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You can see that Shopping does in fact contain a list of item objects, but it's currently completely
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empty, i.e., there are no items associated with Shopping.
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So let's go ahead and add one, let's say, "Buy Apples," hit Add Item, and you can see that, firstly, the item
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model gets incremented with a single new object.
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But also inside category, inside that link, you can see that when you click on this item, it takes you
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to that particular item.
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And it's now equal to 1.
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So as we add more,
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you can see that, firstly, they're getting fetched back in alphabetical order because of that sort that
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we specified here,
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and they're also showing up over here, and you can hover over it in order to see it at a glance.
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So in the next lesson, we're going to look at the next letter in CRUD which is "U" and how you can update
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your data inside Realm.
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So for all of that and more, I'll see you on the next lesson.