offset collapsing animation implementation

Author: RareScrap

lib/src/main/java/me/onebone/toolbar/CollapsingToolbar.kt

@@ -31,6 +31,7 @@ import androidx.compose.foundation.gestures.FlingBehavior
 import androidx.compose.foundation.gestures.ScrollScope
 import androidx.compose.foundation.gestures.ScrollableState
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.MutableState
 import androidx.compose.runtime.Stable
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
@@ -136,6 +137,7 @@ class CollapsingToolbarState(
 	)
 	fun feedScroll(value: Float): Float = dispatchRawDelta(value)
 
+	// TODO: A strange jump in snap speed is often observed
 	@ExperimentalToolbarApi
 	suspend fun expand(duration: Int = CollapsingToolbarDefaults.EXPAND_DURATION) {
 		val anim = AnimationState(height.toFloat())
@@ -149,6 +151,7 @@ class CollapsingToolbarState(
 		}
 	}
 
+	// TODO: A strange jump in snap speed is often observed
 	@ExperimentalToolbarApi
 	suspend fun collapse(duration: Int = CollapsingToolbarDefaults.COLLAPSE_DURATION) {
 		val anim = AnimationState(height.toFloat())
@@ -162,6 +165,46 @@ class CollapsingToolbarState(
 		}
 	}
 
+	@ExperimentalToolbarApi
+	suspend fun expandOffset(snapStrategy: SnapStrategy, offsetY: MutableState<Int>) {
+		val anim = AnimationState(offsetY.value.toFloat())
+
+		anim.animateTo(0f, tween(snapStrategy.expandDuration)) {
+			offsetY.value = value.toInt()
+		}
+	}
+
+	@ExperimentalToolbarApi
+	suspend fun collapseOffset(snapStrategy: SnapStrategy, offsetY: MutableState<Int>) {
+		val anim = AnimationState(offsetY.value.toFloat())
+
+		anim.animateTo(-minHeight.toFloat(), tween(snapStrategy.collapseDuration)) {
+			offsetY.value = value.toInt()
+		}
+	}
+
+	// TODO: Is there a better solution rather OptIn ExperimentalToolbarApi?
+	@OptIn(ExperimentalToolbarApi::class)
+	internal suspend fun processSnap(strategy: SnapStrategy) {
+		if (progress > strategy.edge) {
+			expand(strategy.expandDuration)
+		} else {
+			collapse(strategy.collapseDuration)
+		}
+	}
+
+	// TODO: Is there a better solution rather OptIn ExperimentalToolbarApi?
+	@OptIn(ExperimentalToolbarApi::class)
+	internal suspend fun processOffsetSnap(snapStrategy: SnapStrategy, offsetY: MutableState<Int>) {
+		val offsetProgress =
+			1f - ((offsetY.value / (minHeight / 100f)) / 100f).absoluteValue
+		if (offsetProgress > snapStrategy.edge) {
+			expandOffset(snapStrategy, offsetY)
+		} else {
+			collapseOffset(snapStrategy, offsetY)
+		}
+	}
+
 	/**
 	 * @return Remaining velocity after fling
 	 */

lib/src/main/java/me/onebone/toolbar/ScrollStrategy.kt

@@ -192,6 +192,19 @@ internal class EnterAlwaysCollapsedNestedScrollConnection(
 			dy
 		}
 
+		// TODO: Cancel expand/collapse animation inside onPreScroll
+		snapStrategy?.let {
+			val isToolbarChangingOffset = offsetY.value != 0//toolbarState.progress == 0f
+			if (isToolbarChangingOffset) {
+				// When the toolbar is hiding, it does it through changing the offset and does not
+				// change its height, so we must process not the snap of the toolbar, but the
+				// snap of its offset.
+				toolbarState.processOffsetSnap(it, offsetY)
+			} else {
+				toolbarState.processSnap(it)
+			}
+		}
+
 		return available.copy(y = available.y - left)
 	}
 }

lib/src/main/java/me/onebone/toolbar/SnapStrategy.kt

@@ -8,14 +8,4 @@ class SnapStrategy(
 	@FloatRange(from = 0.0, to = 1.0) val edge: Float = CollapsingToolbarDefaults.EDGE,
 	val expandDuration: Int = CollapsingToolbarDefaults.EXPAND_DURATION,
 	val collapseDuration: Int = CollapsingToolbarDefaults.COLLAPSE_DURATION
-)
-
-// TODO: Is there a better solution rather OptIn ExperimentalToolbarApi?
-@OptIn(ExperimentalToolbarApi::class)
-internal suspend fun CollapsingToolbarState.processSnap(strategy: SnapStrategy) {
-	if (progress > strategy.edge) {
-		expand(strategy.expandDuration)
-	} else {
-		collapse(strategy.collapseDuration)
-	}
-}
+)