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So, let's quickly refresh ourselves by going through how our app works when it's using Realm.
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So let's first go into our AppDelegate.Swift class and we can delete Core Data now and we can also
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delete all of these Core Data saving related code
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and I'm also going to get rid of my applicationWillTerminate.
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Now, I'm going to leave my realm initialization here just to catch to see if there's any errors when
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I initialize a new Realm.
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And because we're not really using it here, we can simply make this an underscore, and delete that let
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keyword as well,
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and this print statement is also giving us a warning.
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And in the future, we might need to find out where our Realm file is, so I'm not going to delete it, but instead,
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I'm just going to comment it out.
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So, now we have no warnings and we can safely save our AppDelegate.
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Now, nothing really happens inside our AppDelegate.
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So let's go and have a look at our Data Models.
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The Category data model is the first one that we created and it is a Realm object,
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so by subclassing this Object class, we're able to save our data using Realm, and we can specify for our
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classes what properties it should have. And for the Category, it has a name property. It's going to be the
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name of the Category.
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And this is a dynamic variable. So we can monitor for changes in this property while the app is running,
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i.e., during runtime.
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Even if you look at the error's pane, you can see that the Buildtime errors and there's Runtime errors.
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So this is before you compile and this is while your app is running.
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And we've also got this relationship. And it specifies that each category can have a number of items
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and that is a list of item objects. And Item objects are also subclassing the Realm Object and that allows
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them to be saved using Realm as well. And the item is a little bit more fancy, it has three properties:
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a title whether if it was done or not,
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and also now it has a dateCreated property,
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and finally, we specify the inverse relationship that links each item back to a parentCategory and we
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specify the type of the destination of the link,
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and we also specify the property name of the inverse relationship., and that relates to this property.
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So, now we can go inside our CategoryViewController, and we can see that the first thing we did is we initialize
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a new access point to our Realm database, and then we changed our categories from an array of category
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items to this new collection type which is a collection of results that our Category objects.
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And this is an optional so that we can be safe.
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And when our view first gets loaded up, we load up all of the categories that we currently own.
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So inside loadCategories, we set that property categories to look inside our realm and fetch all of
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the objects that belong to the Category data type.
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And then we reload our tableView with the new data.
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And that reloadData calls all of these data source methods again and it returns the number of categories
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as the number of rows.
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And if this is nil, i.e., if we have no categories, then we simply return one cell, and that cell is going
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to have a textLabel depending on whether,
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again, if we have any categories. If we do, then we look at the categories results collection and we pick
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out the one for the current indexPath.row,
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and we use the name property to fill up that textLabel.
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But if this is nil, i.e., if we didn't have any categories at all, then we simply fill that single cell
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with the words "No Categories Added yet," and we return the cell.
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Now, when we add a new item by clicking on the Add button, then we create an alert and the alert has a
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text field and also an Add button. When we click on that Add button,
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we create a newCategory that is a new Category object and we give the newCategory the name based on
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what the user typed into the textField.
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And then we call the save method to save this newCategory to our Realm database. And inside the save category,
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we pass in that newCategory that we created here and we call this function called realm.write to commit
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some changes to our Realm. And the changes we want to make is we want to add a newCategory to our Realm
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database and we log if there are any errors.
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Now, the next part is, what happens when we click on any of our cells?
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Well, when we click on the cells, we fire didSeletRowAt indexPath and it performs a segue the withIdentifier
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that takes us to the TodoListViewController.
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But before we do so, we create a new instance of our destinationVC and we set the destinationVC's
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selectedCategory property to the category at the indexPath.row that was selected which triggered
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this particular segue.
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Now, that takes us onto the TodoListViewController.
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And when we set this selectedCategory, i.e., during that didSet block, we call loadItems which loads
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up all of the to-do list items which, again, is a collection of results that are Item objects.
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And this gets set to be equal to the selectedCategory?.items.
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So remember that list of items inside Category?
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Well, in the selectedCategory object, it also has a bunch of items associated with it. And those items,
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then in turn get sorted by their title, and they are listed in alphabetical order. And then we reload
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the tableView to call our Datasource Methods which specifies that we should have as many cells as we
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have todoItems for our current selectedCategory.
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But if there are no items for our current selectedCategory, then we just return one cell, and that cell is
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going to have a textLabel based on the current item's title. And if the item.done property is true,
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then we give that cell a checkmark,
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otherwise, we give it nothing.
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But if there are todoItems, i.e., this is nil, then the else block triggers, and that single cell
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that we created up here gets populated with the text "No Items Added."
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Now, in the TodoListViewContoller, when we press the Add Item Button inside our UIAlert, then we
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look to see if our selectedCategory is nil or not, because it's an optional, remember?
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And if it's not nil, then we set it to have this name called currentCategory. And then we try to update
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our realm by creating a new item, setting the newItem's title, setting the dateCreated for the new item
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to be the current date and time,
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and then we set the currentCategory's item property to append this newItem to the end of that list.
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And we commit all of that to our REalm database, and then we reload our tableView with all of that new data
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that we added.
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And when we select any of the cells, then we grab a reference to the item by looking into that collection
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of results called todoItems, and we grab the item at the current selected indexPath.row, and then we try
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to update its value by using 
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realm.write,
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and we toggle the item.done property to be the opposite of whatever it used to be.
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And then we, again, reload the data and deselect our cell.
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Now, finally, when we select the search bar and we type something, and we click on the searchBarSearchButton,
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then we update our collection of results called todoItems to a filtered version based on a predicate
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which says that the title of the items must CONTAIN this argument, and the argument is whatever the user
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entered into the searchBar,
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and then we sort the items by the date that they were created in ascending order,
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and then we reload our data. And if we dismiss our searchBar, then we call loadItems all over again.
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And that basically resets todoItems to all of the items inside the current selectedCategory and it's
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now sorted by alphabetical order.
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So as you can see, by using Realm, instead of Core Data, we actually vastly simplified our code.
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And I don't know if you'll agree with me, but I think the logic is also simpler.
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There is no extra classes, there's no hidden classes.
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All the lines of code are more human-readable like .filter, .sort, and it makes a lot more sense
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when you're just glancing through the code.
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So have a play around with Realm and it's completely your choice how you want to persist your data. And
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if you prefer Core Data, then that's an equally valid solution.
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Don't let me put you off it. But personally, I really, really like using Realm, and a lot of my colleagues
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also feel the same.
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And Realm has other benefits other than just ease of use. And you can have a look through the blog for
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a lot of these reasons on why.
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But, you know, in addition to its simplicity, it's also a lot faster than Core Data or SQLite.
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So if you have a look at the number of queries you can make per second to your database, you can see
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that Realm is almost double SQLite and it's way more than Core Data. So it has a lot of other benefits
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as well and it's well worth looking through the documentation, seeing what other things can you do with
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Realm. And, also, look at their example projects and see how far you can go with it.
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So that's all from me for now. In the next few lessons, we'll be styling it up for our
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Todoey app.
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Now, those lessons are completely optional because we're only going to be dealing with front-end and UI stuff,
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but we will be coding up and adding some libraries in order to make our app look beautiful as well as
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work well.
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So if you're interested, I'll see you for that on the next lesson.