Creating Magical Shared Element Transitions with Jetpack Compose
Hi Android enthusiasts!
Have you ever admired the beautiful animations when navigating between screens in apps like Instagram, Spotify, or Airbnb? Ever wondered how those smooth, delightful transitions happen? Well, the magic you're seeing is often the Shared Element Transition!
A few days ago, I experimented with Jetpack Compose to build a quick sample app demonstrating Shared Element Transitions without relying on external libraries. I shared a video snippet on LinkedIn, and many of you loved it! Today, let's dive deeper into how you can implement this magical experience yourself, purely with Compose, and understand what's happening under the hood.
The Idea
Imagine an e-commerce app displaying a list of stylish shoes. When your user selects a shoe, the shoe image elegantly enlarges and transitions smoothly into the detail view. Let's break down how this happens step-by-step:
Implementing Shared Element Transitions (Compose-Only!)
🏗 1. SharedTransitionLayout
SharedTransitionLayout acts as a container that manages the shared element transitions between composables. It creates a transition scope, ensuring all shared elements animate seamlessly between screens:
SharedTransitionLayout {
NavHost(navController = navController, startDestination = "list") {
composable("list") { ListScreen() }
composable("detail") { DetailScreen() }
}
}
Internal Mechanics:
- Captures the initial and final bounds (position and size) of UI elements.
- Coordinates these bounds using Compose’s powerful recomposition mechanism, ensuring smooth transitions.
🎭 2. with(sharedTransitionScope)
Inside SharedTransitionLayout, you gain access to a powerful scope: sharedTransitionScope. This scope allows you to precisely define how transitions between bounds (positions and sizes of elements) will animate:
with(sharedTransitionScope) {
val boundsTransform = BoundsTransform { initialBounds, targetBounds ->
keyframes {
durationMillis = 1000
initialBounds at 0 using ArcMode.ArcBelow using FastOutSlowInEasing
targetBounds at 1000
}
}
}
Detailed Explanation of BoundsTransform:
- BoundsTransform calculates and animates transitions between two sets of bounds (positions and sizes of UI elements).
- It uses interpolation to generate intermediate points smoothly transitioning from the initial state to the final state.
- With keyframes, you explicitly define animation states at specific timestamps:
- The arc calculation involves mathematical functions (trigonometry) to generate curved paths instead of straight lines, enhancing visual appeal.
🔗 3. .sharedElement Modifier
The .sharedElement() modifier is critical—it tells Compose exactly which UI element is shared across screens and should animate between them:
Image(
painter = painterResource(shoe.imageRes),
contentDescription = shoe.name,
modifier = Modifier
.sharedElement(
rememberSharedContentState(key = "$KEY_BACKGROUND-$currentPage"),
animatedVisibilityScope = animatedVisibilityScope,
boundsTransform = boundsTransform
)
)
Internal Mechanics:
- Links UI components between screens by matching their state keys.
- Tracks element position, size, and visibility, automatically managing recompositions and state updates.
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🧠 4. rememberSharedContentState
This helper function, rememberSharedContentState, helps Compose uniquely track each shared element's transition state:
rememberSharedContentState(key = "$KEY_BACKGROUND-$currentPage")
Internal Mechanics:
- Stores and retrieves transition states consistently, ensuring animations remain smooth even through recompositions or layout updates.
- Provides stable references for efficient transition management.
🎬 5. animatedVisibilityScope
The animatedVisibilityScope manages visibility animations, ensuring smooth appearance and disappearance of elements:
.sharedElement(
rememberSharedContentState(key = "${KEY_BACKGROUND}-$currentPage"),
animatedVisibilityScope = animatedVisibilityScope,
boundsTransform = boundsTransform
)
Internal Mechanics:
- Controls opacity and visibility transitions, employing linear interpolation for smooth fading.
- Ensures visual continuity and enhances user experience by reducing abrupt UI changes.
Why Does This Matter?
Shared Element Transitions significantly enhance user experience by providing visual continuity, making your app feel polished, professional, and delightful. It brings life to your interface, captivating your users and encouraging deeper engagement.
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Happy coding! 🎉
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