From the course: Android Compose with Kotlin

Introduction to state in Compose - Kotlin Tutorial

From the course: Android Compose with Kotlin

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Introduction to state in Compose

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- [Instructor] Understanding state management is crucial for creating dynamic and responsive UIs with Compose. The Compose runtime is always on the lookout for updates that require the screen to be redrawn. But how does it know that something has changed? During composition, Compose keeps track of all of the composable functions that are created. Not only that, it takes a snapshot of the data that was used as input. I find it helpful to visualize it like this. Each composable is stored in a table along with its state. So when any event occurs that changes the state of your app, think of a user clicking a button, the phone rotating, anything, Compose can compare this new state to the previous and update the UI accordingly. The process of re-executing the needed composables is officially called recomposition. Let's move over to Android Studio to take a closer look. Here we have a composable screen with a switch and a checkbox. Then we've declared a Boolean variable down here on line number 29. It determines whether or not the switch and the checkbox should be checked. We'll switch our preview to interactive mode to test this out. So we'll come over here and we'll go to interactive mode and then we'll click on the switch and we should expect the state to change, but it stays the same. And it's the same thing for the checkbox. This is because Compose doesn't see any changes to the state of the composable. Well, how so? We are using a standard Kotlin Boolean. Instead, we need something that can be observed, and that's where we enter the state. So let's create a new variable and we'll name this one checkedState, and then we're going to make this of type MutableState, and it's going to hold a Boolean. Okay, so that's the first step. Next, we are going to use the remember function with mutableStateOf, and the default value will be set to true. This allows us to define the initial state of the switch for Compose, which means that it will be on. Now, let's update the code inside of our switch statement. So we can see these changes one by one. We'll come here to where we have checked, and instead we're going to change this to checkedState. And since this is a mutable state object, we're going to have to use the value. And once again, we'll do that same update for changing the state. So we'll come here, we'll copy checkState.value, and we're going to update the value and then do its opposite. Okay, perfect. So once our preview renders, we would expect the switch to update correctly. So we're still here in interactive mode. And I'm going to toggle the switch. And great, it goes off and on. But if we use our checkbox, it's just the same. Understanding and managing state in Compose is key to building responsive UIs. By mastering state management, you'll create apps that are both dynamic and robust.

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