compose-backstack
Simple library for Jetpack Compose for rendering backstacks of screens and animated transitions when the stack changes. It is not a navigation library, although it is meant to be easy to plug into your navigation library of choice (e.g. compose-router), or even just use on its own.
This library is compatible with Compose dev07.
Usage
The entry point to the library is the Backstack
composable. It essentially looks like this:
@Composable fun <T : Any> Backstack(
backstack: List<T>,
drawScreen: @Composable() (T) -> Unit
)
The API is very similar to a lot of composables that draw lists: it takes a list of keys and a composable function that knows how to draw a key. In this case, a key represents a distinct screen in the backstack. When the top key in the stack changes between compose passes, the screens will be animated with a transition.
The actual API takes a few more parameters, e.g. to allow custom animations. See the source kdoc for details!
Example
sealed class Screen {
object ContactList: Screen()
data class ContactDetails(val id: String): Screen()
data class EditContact(val id: String): Screen()
}
data class Navigator(
val push: (Screen) -> Unit,
val pop: () -> Unit
)
@Composable fun App() {
var backstack by state { listOf(Screen.ContactList) }
val navigator = remember {
Navigator(
push = { backstack += it },
pop = { backstack = backstack.dropLast(1) }
)
}
Backstack(backstack) { screen ->
when(screen) {
Screen.ContactList -> ShowContactList(navigator)
is Screen.ContactDetails -> ShowContact(screen.id, navigator)
is Screen.EditContact -> ShowEditContact(screen.id, navigator)
}
}
}
Custom transitions
Transitions between screens are defined by implementing the BackstackTransition
interface and
passing the implementation to the Backstack
composable. This interface has a single method:
fun modifierForScreen(
visibility: Float,
isTop: Boolean
): Modifier
The modifierForScreen
method is called for every screen in the backstack (even ones that are
completely hidden), and must return a Modifier
that will be applied to the entire screen. Compose has many Modifier
s built-in, which can be used
to do a wide variety of visual transformations such as adjust position, size, transparency, etc.
When not currently transitioning, visibility
will be 0 for all screens except the top one, for
which visibility
will be 1. When animating between two screens, visibility
will be somewhere
between 0 and 1 for both the top screen and the screen immediately under the top one. The visibility
of all other screens in the stack will continue to be 0.
The isTop
flag indicates if the returned modifier will be applied to the top screen or not, and
can be used to display a different animation for the top vs under-top screens. For example, the
Slide
transition uses isTop
to determine whether to translate the screen to the end/right, or
the start/left.
Visibility will always transition between 0 and 1. If you need to map that range to a different
range of floats, or any other type, you can use one of the lerp
functions provided by Compose.
Testing custom transitions
You can use the BackstackViewerApp
composable in the backstack-viewer
artifact to test your
custom transitions interactively. This composable is used by the sample app, and in the screenshots
below.
Inspecting the backstack
The Backstack
composable takes an optional InspectionParams
parameter. When not null, the entire
backstack will be rendered as a translucent 3D stack. The top-most screen in the stack will still
be rendered in its regular position, but with a very low opacity, and will still be interactive. The
BackstackInspectorParams
controls how the stack is rendered, including rotation, scaling,
opacity, etc.
You can wrap your Backstack
with the InspectionGestureDetector
composable to automatically
control the inspector mode using touch gestures.
Samples
There is a sample app in the sample
module that demonstrates various transition animations and
the behavior with different backstacks.
Gradle
compose-backstack
is on Maven Central:
allprojects {
repositories {
mavenCentral()
}
}
dependencies {
implementation 'com.zachklipp:compose-backstack:0.3.0'
// For BackstackViewerApp.
debugImplementation 'com.zachklipp:compose-backstack-viewer:0.3.0'
}