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Now, when you look at most to-do list apps out there, it's very rare that you'll come across a to-do list
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that only has a single list. But, unfortunately, that's currently all that we've got. Most to-do list apps
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will have categories.
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So, for example, cleaning, bills, miscellaneous, grocery, and you'll be able to tap on them to view those
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lists,
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and this allows us to classify and categorize so that we can organize our to-do list items in a more
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logical way.
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So that's also a good feature for us to add to our Todoey app. And in order to implement it,
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we'll get to learn all about relationships and how we set them up using Core Data.
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So the first thing I'm going to do is I'm going to make some UI changes. At the moment,
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we've got a single table view controller that is embedded inside a navigation controller.
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Now, in order for us to have a category screen, then we need to drag on another table view controller.
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So I'm going to place that between the navigation controller and our to-do list items table view controller.
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Then I'm going to delete this link between the navigation controller and the to-do List controller because
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this link specifies which is the
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root view controller, i.e., if you have a navigation stack like a stack of cards, what is the controller that's
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at the bottom of the stack? And when you say something like pop to root view controller, then that's equivalent to
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the first screen that's embedded in the navigation stack.
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So let's go ahead and click on this link and delete it. And then I'm going to replace this category view
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controller into a navigation controller and our to-do list view controller, and then I'm going to click
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on the navigation controller,
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so I get this little yellow button here, I'm going to control drag it to point it towards our brand-new
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table view controller. And instead of using show or creating a segue, instead, I'm going to create a relationship
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segue and specify that this is now the root view controller because we first have to see the category
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table view,
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and only then can we select one of the categories to take us to all the items that are classified under
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that category.
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So that means we also need to set up a segue between our category and our to-do list items by control
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dragging.
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And in this case, we're going to go for a show segue.
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And if you remember every single time we set up a segue, we also have to specify an identifier.
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So we'll just call this goToItems. And we currently now have this free-floating table view because
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it doesn't have a view controller class that's associated with it.
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So let's go ahead and create a new one. And instead of creating a new Swift File, we want a new Cocoa
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Touch Class.
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So hit next.
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And this is going to be a subclass of UITableViewController because we want all of that freebie
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code that comes with it.
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And then we're going to call this class the CategoryViewController, make sure language is selected to the Swift,
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and hit Next.
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And then we're going to save it inside the Controllers folder or group and making sure that it targets
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our Todoey app, and then we're going to get Create, and we now have a CategoryViewController. So, I'm actually
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going to drag that on top so that it makes more sense because we first go to Category, then we go to the
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to-do list for each category.
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So, now if we go back to our Main.storyboard, we can specify the Swift file that is going to be responsible
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for managing this particular view.
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So if you don't do this step and you head over to the assistant editor, when you select the to-do list
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item view control, you see the under automatic,
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we've got TodoListViewController.swift.
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But when you select this new table you control that currently is not associated with any Swift class,
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then you can see that under Automatic, you only get a UITableViewcontroller file, and this is also not
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a Swift file.
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This is actually an Objective-C header file.
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So that's not what we want because we can't link up our outlets and we can't do anything,
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so we have to specify the identity of this
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view controller. And inside the custom class, we're going to say
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CategoryViewController and hit enter.
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So, now automatically, our assistant editor has updated to the corresponding class with our view controller.
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Before we leave the Main.storyboard is we want to give this prototype cell a identifier. So select the
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prototype cell, either on the canvas or select in the document outline,
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but we're going to call it CategoryCell, and hit enter.
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So, now that we've got our view and our class linked up, we can now create a add button or Bar Button Item
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rather,
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and we can drag it to the top right corner of this new
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view controller.
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And, again, this is going to be a system item of Add and we're going to change the tint to white.
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This is going to allow us to add new categories to this table view so that we can organize our to-do list.
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And once you've done that, we're going to control drag over to our CategoryViewController.swift file
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and we're going to create a new IBAction, and this IBAction is going to be called add ButtonPressed
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and it's going to be of type UIBarButtonItem, and hit enter.
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And the last thing I'm going to do here is select the navigation item here and I'm going to give it
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a title called Todoey, and then I'm going to change the title here to Items, and that is all we need to
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do
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inside the Main.storyboard.
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So let's head over to the CategoryViewController. And the first thing I want to do is when you set up
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a new Cocoa Touch Class, that subclasses UITableView, you get a whole bunch of free code, but you also
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get a lot of comments.
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And you know we're now pro enough with table views to not need all of this help and guidance.
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So I'm going to delete all of this code up until viewDidLoad, and then I'm also going to delete all
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the comments inside viewDidLoad. That cleans up.
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view controller a little bit, and allows us to work on it without all of this distraction.
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So, now that we've set up our view and our controller, we're going to go and figure out what we're going
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to do with our data model.
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So up till now, we've been editing our entities and attributes under this table view format, but you can also
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switch to the graph view and this presents each entity and all of the attributes as a little table.
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And for those guys who've been working with databases, then this might seem quite familiar to you.
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So it's under this format that we're going to add a new entity, and this new entity is going to be called
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Category.
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And once you hit enter, then you'll see this new table show up
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that's called Category which has no attributes and no relationship.
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If you select the Category entity and click Add Attribute, then you're going to get a new attribute.
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And we can click on it to modify its properties inside the Data Model Inspectah in the right-hand pane
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here.
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Now, for some reason I have no idea why the icon for it is like a little welding mask, but some designer
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somewhere felt that this was a good idea to represent a data model. But, hey, let's not argue with Apple
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designers, right?
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Okay, so we're going to change the attribute name to be called name.
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And this is going to refer to the name of each category that we're going to add, and we're going to make
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the name nonoptional.
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So it has to have a value for it to go into our data model. And you'll see this little error here and it won't
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go away until you change the attribute type from Undefined.
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It cannot be undefined.
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It must be specified. And our name for the category is, of course, going to be a string.
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And once you do that, then Xcode is all happy with you, and all the errors go away.
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Now, in addition to having a single attribute for the category, we're also now going to set up a relationship
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between our category and our item entities.
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And to do that, all you need to do is control drag from Category to Item, and now you have a relationship.
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Now, it's not very useful having a relationship called new Relationship.
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So we need to rename that.
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So click on it and that will allow us to edit it in the Data Model Inspector. We're going to change the
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name to something that makes sense.
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So each category can be associated with many items, right?
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If you have a category called shopping list, then you might have 10 item objects that refer to all the
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things you need to buy.
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So we can call this relationship Items because that's essentially what it's referring to.
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And we should change the type from To One to To Many because each category can have many items associated
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with it.
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Now, the inverse relationship refers to what you call the Category that the Item is associated with, and
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that should probably be called the parentCategory, and each item can only have one parentCategory.
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So this type should remain as To One. It will be really confusing if each of your items have two categories,
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say, you know, your "buy milk" belongs in shopping list, as well as another category. That doesn't make
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any sense, logically.
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So we've now created a new entity called Category and we've also started creating our graph which specifies
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how categories relate to Items. And this might be a good time to remind yourself of when we first started
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talking about databases in this module, when we are talking about our bakery having a product's table,
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a buyer's table, and an order's table. And in order to manage all of the complex data, we need to set up
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these relationships.
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So, now when we create a new Category, we can link it with many Item and as managed objects. And every
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single item object, we can look up its parent category.
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So this now gives us access to more complex things that we can do with our data including setting up
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this new view controller and having a whole bunch of categories that each have their individual to-do lists.
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Now, all we need to do is head back to our CategoryViewController and we need to set up some code to
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use our data model.
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So in order to do that, we'll need to import Core Data and we'll also need to have some TableView 
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DataSource methods, as well as TableView Delegate Methods, and then we'll need some Data Manipulation Methods.
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So I think you're probably getting the hint.
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I'm not going to do this for you.
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I want you, as a challenge, to think back to what we did inside our TodoList View Controller, how we
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set up an array of items, how we tapped into the context that we're going to use, and then how we set
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up our tableView to load our data from the context for all of those items. And then we used our 
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IBAction addButtonPressed to add new items into our tableView.
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And then we used the saveItems function to persist the items inside our container, and then we loaded
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those items from our persistent container using our context.
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Now, I want you to complete this challenge and be able to set up our CategoryViewController in a similar
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way to what we did with items to use our Core Data data model and to be able to add new categories and
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to be able to show those categories inside our table.
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So all you need to do is set up the data source so that we can display all the categories that are inside
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our persistent container.
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Set up the Data Manipulation Methods, namely, save data and load data so that we can use CRUD.
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And finally, to set up the addButtonPressed IBAction in order to Add New Categories using our category
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entity. And the part that I want you to leave for now is the TableViewDelegate methods
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So that refers to what should happen when we click on one of the cells inside the category table.
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So I know that these challenges are getting more and more challenging.
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But if you've come this far and you followed along with everything that we've been talking about with
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Core Data, this is a good time to really test to see if you understood the concepts and use what you learned
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to set up your very own table view controller that has a persistent data store using Core Data.
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And if you have completed the challenge successfully, then you should end up with a blank table view
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with the title Todoey, and you should be able to click on the Add button to add new categories to your
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table view,
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and when you double click on the home button and terminate your app.
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Now, I really, really recommend you to give it a go yourself. And it might require quite a bit of struggling,
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but struggle is good.
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And once you struggle through it and get it to work by yourself, then you'll feel this immense sense
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of achievement.
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But if you do get stuck and you do have any issues, then you can always head over to the next lesson
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where I will walk you through how we can do this.
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So pause the video and give it a go now.
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I'll see you on the other side.