Finding Problems and Creating a ViewModel5:27 with Ben Deitch
In this video we'll find 3 issues and talk about how we can fix them!
Now that we've got a basic understanding of how the app works, 0:00 it's time to start solving problems. 0:03 Let's take a look at the app and figure out what needs to be done. 0:05 Let's start by going into the creator activity. 0:09 Let's add some toppings, Give our pizza 0:13 a name, And then rotate the screen. 0:18 And as we've seen before, this is an issue. 0:26 Let's call this problem number one. 0:29 Creator activity does not save and restore state after a rotation. 0:31 Typically, you'd solve this by storing the state and bundle and 0:36 overwriting the activities onSave InstanceState method. 0:40 But with the view model, things have gotten easier. 0:43 Though technically, you should still save state the old way as well, 0:46 more on that in the teacher's notes. 0:50 Getting back to the app, if we add some toppings and 0:52 try to save the pizza, nothing happens. 0:56 So problem number two is, pizzas aren't being saved. 1:00 In addition to the save button not working, 1:04 the delete button also doesn't work. 1:06 But it's kind of hard to show deleting a pizza without first 1:08 being able to save a pizza. 1:11 So really there's three problems we need to solve. 1:13 CreatorActivity doesn't save and restore state after a rotation. 1:16 Pizzas aren't being saved, and pizzas can't be deleted. 1:20 Let's dive into problem number one with the help of a new tool called ViewModel. 1:23 In Android, 1:28 a ViewModel is just a special object that can survive a rotating activity. 1:29 In fact, if you look in the docs, 1:34 you'll find this chart showing the life cycle of a ViewModel. 1:36 It gets created with the activity and 1:41 then stays alive until the activity is actually finished. 1:43 So all we need to do to save state, is store it in a ViewModel and 1:47 have the UI look to the ViewModel to determine what to draw. 1:52 Back in the code, let's create a new class called CreatorViewModel. 1:56 And let's make it extend from the ViewModel class. 2:13 However, there's an import we should add before we start using the ViewModel. 2:19 Let's go over to our build.gradle file. 2:24 Let me close off this. 2:27 And add in a line from the teacher's notes below. 2:31 Then let's sync the project. 2:39 And get back to extending from ViewModel. 2:51 Let's use Alt+Enter to add the constructor invocation, and we've got our ViewModel. 2:55 Now, we just need to add some variables to store the state of our creator activity. 3:01 The state of the activity is the name of the pizza and 3:07 the state of all the topping switches. 3:10 To store the name of the pizza, let's add a variable called pizzaName, 3:13 and start it off with a value of New Pizza. 3:19 Also, since the name of the pizza can change, notice that we're using var, 3:23 instead of val. 3:28 To store the state of the topping switches, 3:30 we'll use a mutable map of toppings to Booleans. 3:32 Let's type val and call it switch states, and 3:36 set it equal to an empty mutableMap. 3:40 And for the types, let's give it Topping and Boolean. 3:48 Look like we'll need to import Topping with Alt+Enter. 3:53 It may seem weird that we're able to use val here. 3:57 However, when we make changes to this map, we'll just be adding and 4:00 removing key value pairs. 4:05 We'll never need to set switchStates equal to anything else, so 4:07 we can get away with making it a val, awesome. 4:12 Now that we've got our ViewModel, let's see how to use it. 4:15 Over in creator activity, at the top, 4:18 let's add another lateinit var named viewModel. 4:21 Let's give it a type of CreatorViewModel. 4:27 Then, down in on create, 4:31 let's add a line after we get our PIZZA_ID and 4:34 set viewModel = ViewModelProviders.of, 4:40 then you pass in the fragment or activity. 4:45 So let's just go with this. 4:51 And now, we have a ViewModelProvider for our activity. 4:54 To get the viewModel from here, we just need to call the get function and 4:58 pass in the class of the viewModel, 5:04 CreatorViewModel::class.java. 5:07 And there we go, we've got our viewModel. 5:12 Now we need to do two things. 5:15 We need to use our ViewModel to update the activity's UI and 5:18 we need to update the ViewModel when any UI changes are made. 5:21 We'll tackle those issues in the next video. 5:25
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