Crossfade.kt
/*
* Copyright 2019 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.compose.animation
import androidx.compose.animation.core.AnimationSpec
import androidx.compose.animation.core.FiniteAnimationSpec
import androidx.compose.animation.core.MutableTransitionState
import androidx.compose.animation.core.animateFloat
import androidx.compose.animation.core.tween
import androidx.compose.animation.core.updateTransition
import androidx.compose.foundation.layout.Box
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.key
import androidx.compose.runtime.mutableStateListOf
import androidx.compose.runtime.remember
import androidx.compose.ui.Modifier
import androidx.compose.ui.draw.alpha
import androidx.compose.ui.util.fastForEach
/**
* [Crossfade] allows to switch between two layouts with a crossfade animation.
*
* @sample androidx.compose.animation.samples.CrossfadeSample
*
* @param targetState is a key representing your target layout state. Every time you change a key
* the animation will be triggered. The [content] called with the old key will be faded out while
* the [content] called with the new key will be faded in.
* @param modifier Modifier to be applied to the animation container.
* @param animationSpec the [AnimationSpec] to configure the animation.
*/
@Composable
fun <T> Crossfade(
targetState: T,
modifier: Modifier = Modifier,
animationSpec: FiniteAnimationSpec<Float> = tween(),
content: @Composable (T) -> Unit
) {
val items = remember { mutableStateListOf<CrossfadeAnimationItem<T>>() }
val transitionState = remember { MutableTransitionState(targetState) }
val targetChanged = (targetState != transitionState.targetState)
transitionState.targetState = targetState
val transition = updateTransition(transitionState)
if (targetChanged || items.isEmpty()) {
// Only manipulate the list when the state is changed, or in the first run.
val keys = items.map { it.key }.run {
if (!contains(targetState)) {
toMutableList().also { it.add(targetState) }
} else {
this
}
}
items.clear()
keys.mapTo(items) { key ->
CrossfadeAnimationItem(key) {
val alpha by transition.animateFloat(
transitionSpec = { animationSpec }
) { if (it == key) 1f else 0f }
Box(Modifier.alpha(alpha = alpha)) {
content(key)
}
}
}
} else if (transitionState.currentState == transitionState.targetState) {
// Remove all the intermediate items from the list once the animation is finished.
items.removeAll { it.key != transitionState.targetState }
}
Box(modifier) {
items.fastForEach {
key(it.key) {
it.content()
}
}
}
}
private data class CrossfadeAnimationItem<T>(
val key: T,
val content: @Composable () -> Unit
)