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Jetpack Compose by Tutorials

Second Edition · Android 13 · Kotlin 1.7 · Android Studio Dolphin

Before You Begin

Section 0: 6 chapters
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Section I: Getting Started With Jetpack Compose

Section 1: 4 chapters
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Section II: Composing User Interfaces

Section 2: 4 chapters
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Section III: Building Complex Apps With Jetpack Compose

Section 3: 5 chapters
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Section IV: UI Testing & Accessibility

Section 4: 2 chapters
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Section V: Glances & Keeping Upto Date With Jetpack Compose

Section 5: 3 chapters
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Section VI: Appendices

Section 6: 1 chapter
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12. Animating Properties Using Compose
Written by Denis Buketa

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Great job on completing the previous chapter. So far, in the third section of this book, you’ve learned how to use ConstraintLayout, build complex UI and react to Compose lifecycles. Those things are certainly fun, but what’s even more fun? Playing with animations! And that’s what you’ll do now. :]

In this chapter, you’ll learn how to:

  • Animate composable properties using animate*AsState().
  • Use updateTransition() to animate multiple properties of your composables.
  • Animate composable content.
  • Implement an animated button to join a subreddit.
  • Implement an animated toast that displays when the user joins a subreddit.

Before diving straight into the animation world, you’ll create a composable representing a button that lets users join an imaginary subreddit.

You’ll start by implementing a simple button, like the one shown below:

Simple Join Button
Simple Join Button

If a user hasn’t joined the subreddit yet, they can do so by clicking the blue button with the plus icon. If the user is a member already, a white button with a blue check represents that state. Clicking the button again returns it to its previous state.

To follow along with the code examples, open this chapter’s starter project in Android Studio and select Open an existing project.

Next, navigate to 12-animating-properties-using-compose/projects and select the starter folder as the project root. Once the project opens, let it build and sync and you’re ready to go!

Note that if you skip ahead to the final project, you’ll find the completed button with all the animation logic implemented.

Now that you’re all set, it’s time to start coding.

Building JoinButton

In the components package, add a new file named JoinButton.kt, then open it and add the following code:

@Composable
fun JoinButton(onClick: (Boolean) -> Unit = {}) {

}

enum class JoinButtonState {
  IDLE,
  PRESSED
}

@Preview
@Composable
fun JoinButtonPreview() {
  JoinButton(onClick = {})
}

Not much to see here. You just created a root composable for your button and added a preview. Right now, there’s nothing to preview because you haven’t added any content yet.

You also added JoinButtonState, which represents the state of the button, The two options for the state are IDLE or PRESSED.

Next, add the following code to JoinButton():

var buttonState: JoinButtonState
  by remember { mutableStateOf(JoinButtonState.IDLE) }

// Button shape
val shape = RoundedCornerShape(corner = CornerSize(12.dp))

// Button background
val buttonBackgroundColor: Color =
  if (buttonState == JoinButtonState.PRESSED)
    Color.White
  else
    Color.Blue

// Button icon
val iconAsset: ImageVector =
  if (buttonState == JoinButtonState.PRESSED)
    Icons.Default.Check
  else
    Icons.Default.Add
val iconTintColor: Color =
  if (buttonState == JoinButtonState.PRESSED)
    Color.Blue
  else
    Color.White

Box(
  modifier = Modifier
    .clip(shape)
    .border(width = 1.dp, color = Color.Blue, shape = shape)
    .background(color = buttonBackgroundColor)
    .size(width = 40.dp, height = 24.dp)
    .clickable(onClick = {
      buttonState =
        if (buttonState == JoinButtonState.IDLE) {
          onClick.invoke(true)
          JoinButtonState.PRESSED
        } else {
          onClick.invoke(false)
          JoinButtonState.IDLE
        }
    }),
  contentAlignment = Alignment.Center
) {
  Icon(
    imageVector = iconAsset,
    contentDescription = "Plus Icon",
    tint = iconTintColor,
    modifier = Modifier.size(16.dp)
  )
}

This might look like a lot of code, but you’ll see that it’s pretty simple. Here’s a breakdown, starting from the top.

You first declared a buttonState with remember(). Ideally, you’d represent your state with PostModel, but this simplified approach is enough to demonstrate how animations work.

Next, you used RoundedCornerShape() to define the shape of the button.

You also defined the button’s background color, which will change depending on the buttonState. When the button has JoinButtonState.PRESSED, it will be white. When it’s JoinButtonState.IDLE, it will be blue.

Next, you defined the button’s icon and icon color. When the button’s state is JoinButtonState.PRESSED, you’ll represent the icon with a white plus sign. If it’s JoinButtonState.IDLE, you’ll represent it with a blue check mark.

The last thing you added is the code that emits the button’s UI. You used Box() to define the button shape and background and Icon() to define how the button’s icon will look.

For that code to work, you need to add a few imports as well:

import androidx.compose.foundation.shape.CornerSize
import androidx.compose.foundation.shape.RoundedCornerShape
import androidx.compose.foundation.background
import androidx.compose.foundation.border
import androidx.compose.foundation.clickable
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.size
import androidx.compose.material.Icon
import androidx.compose.material.icons.Icons
import androidx.compose.material.icons.filled.Add
import androidx.compose.material.icons.filled.Check
import androidx.compose.runtime.*
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.draw.clip
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.vector.ImageVector
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp

Great! Now, build the project and check the preview panel.

JoinButton — Idle State
JoinButton — Idle State

Note that you can change buttonState’s initial state to PRESSED, to preview the different settings for your button.

JoinButton — Pressed State
JoinButton — Pressed State

Awesome! Next, you’ll add this button to Post().

Adding JoinButton to Post

Before animating JoinButton(), you’ll add it to Post() so you can see it in the app.

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Animating the JoinButton Background

So far, you’ve made the button background change from one color to another when the state changes. In this section, you’ll animate that transition.

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Using Transitions to Animate JoinButton

In the previous section, you saw how to animate one property of your composables. Now, you’ll add more content to JoinButton(). This will give you the opportunity to animate several properties at once.

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Defining the Transition

To animate these properties, you’ll use Transition. Transition manages one or more animations as its children and runs them simultaneously between multiple states.

Connecting the Transition to the Composables

Properties buttonBackgroundColor and iconTintColor are already in place so you don’t have to change that.

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Animating Composable Content

So far, you’ve seen how to animate the properties of your composables. In this section, you’ll explore a different approach to creating animations by learning how to animate composable content.

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Adding JoinedToast

In components, create a new file named JoinedToast.kt. Then, add the following code to it:

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Animating JoinedToast

In JoinedToast.kt, replace the JoinedToast() code with the following:

Bringing the JoinedToast Home

Before you can see this animation in action, you need to add JoinedToast() to HomeScreen().

Adding onJoinButtonClick to the Posts

Open Post.kt and replace TextPost(), ImagePost() and Post() with the following code:

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Key Points

  • You use animate*AsState() for fire-and-forget animations targeting single properties of your composables. This is very useful for animating size, color, alpha and similar simple properties.
  • You use Transition and updateTransition() for state-based transitions.
  • Use Transitions when you have to animate multiple properties of your composables, or when you have multiple states between which you can animate.
  • Transitions are very good when showing content for the first time or leaving the screen, menu, option pickers and similar. They are also great when animating between multiple states when filling in forms, selecting options and pressing buttons!
  • You use AnimatedVisibility() when you want to animate the appearance and disappearance of composable content.
  • AnimatedVisibility() lets you combine different types of visibility animations and lets you define directions if you use predefined transition animations.
11. Reacting to Compose Lifecycle
13. Adding View Compatibility
Have a technical question? Want to report a bug? You can ask questions and report bugs to the book authors in our official book forum here.
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