[RNMobile] Add useScrollWhenDragging hook (#39705)

* Introduce useScrollWhenDragging hook

* Cancel animation timer on stop scrolling

* Add documentation to useScrollWhenDragging hook

* Replace Dimensions with useWindowDimensions hook

packages/block-editor/src/components/block-draggable/index.native.js

@@ -24,6 +24,7 @@ import { usePreferredColorSchemeStyle } from '@wordpress/compose';
 /**
  * Internal dependencies
  */
+import useScrollWhenDragging from './use-scroll-when-dragging';
 import DraggableChip from './draggable-chip';
 import { store as blockEditorStore } from '../../store';
 import { useBlockListContext } from '../block-list/block-list-context';
@@ -90,10 +91,19 @@ const BlockDraggableWrapper = ( { children } ) => {
 	const isDragging = useSharedValue( false );
 	const scrollAnimation = useSharedValue( 0 );
 
+	const [
+		startScrolling,
+		scrollOnDragOver,
+		stopScrolling,
+		draggingScrollHandler,
+	] = useScrollWhenDragging();
+
 	const scrollHandler = ( event ) => {
 		'worklet';
 		const { contentOffset } = event;
 		scroll.offsetY.value = contentOffset.y;
+
+		draggingScrollHandler( event );
 	};
 
 	// Stop dragging blocks if the block draggable is unmounted.
@@ -124,9 +134,13 @@ const BlockDraggableWrapper = ( { children } ) => {
 					0,
 					blockLayout.y - EXTRA_OFFSET_WHEN_CLOSE_TO_TOP_EDGE
 				);
-				scrollAnimation.value = withTiming( scrollOffsetTarget, {
-					duration: SCROLL_ANIMATION_DURATION,
-				} );
+				scrollAnimation.value = withTiming(
+					scrollOffsetTarget,
+					{ duration: SCROLL_ANIMATION_DURATION },
+					() => startScrolling( position.y )
+				);
+			} else {
+				runOnUI( startScrolling )( position.y );
 			}
 		} else {
 			// We stop dragging if no block is found.
@@ -166,6 +180,9 @@ const BlockDraggableWrapper = ( { children } ) => {
 		const dragPosition = { x, y };
 		chip.x.value = dragPosition.x;
 		chip.y.value = dragPosition.y;
+
+		// Update scrolling velocity
+		scrollOnDragOver( dragPosition.y );
 	};
 
 	const stopDragging = () => {
@@ -174,6 +191,7 @@ const BlockDraggableWrapper = ( { children } ) => {
 
 		chip.scale.value = withTiming( 0 );
 		runOnJS( stopDraggingBlocks )();
+		stopScrolling();
 	};
 
 	const chipDynamicStyles = useAnimatedStyle( () => {

packages/block-editor/src/components/block-draggable/use-scroll-when-dragging.native.js

@@ -0,0 +1,135 @@
+/**
+ * External dependencies
+ */
+import { useWindowDimensions } from 'react-native';
+import {
+	useSharedValue,
+	useAnimatedRef,
+	scrollTo,
+	useAnimatedReaction,
+	withTiming,
+	withRepeat,
+	cancelAnimation,
+	Easing,
+} from 'react-native-reanimated';
+
+/**
+ * Internal dependencies
+ */
+import { useBlockListContext } from '../block-list/block-list-context';
+
+const SCROLL_INACTIVE_DISTANCE_PX = 50;
+const SCROLL_INTERVAL_MS = 1000;
+const VELOCITY_MULTIPLIER = 5000;
+
+/**
+ * React hook that scrolls the scroll container when a block is being dragged.
+ *
+ * @return {Function[]} `startScrolling`, `scrollOnDragOver`, `stopScrolling`
+ *                      functions to be called in `onDragStart`, `onDragOver`
+ *                      and `onDragEnd` events respectively. Additionally,
+ * 						`scrollHandler` function is returned which should be
+ * 						called in the `onScroll` event of the block list.
+ */
+export default function useScrollWhenDragging() {
+	const { scrollRef } = useBlockListContext();
+	const animatedScrollRef = useAnimatedRef();
+	animatedScrollRef( scrollRef );
+
+	const { height: windowHeight } = useWindowDimensions();
+
+	const velocityY = useSharedValue( 0 );
+	const offsetY = useSharedValue( 0 );
+	const dragStartY = useSharedValue( 0 );
+	const animationTimer = useSharedValue( 0 );
+	const isAnimationTimerActive = useSharedValue( false );
+	const isScrollActive = useSharedValue( false );
+
+	const scroll = {
+		offsetY: useSharedValue( 0 ),
+		maxOffsetY: useSharedValue( 0 ),
+	};
+	const scrollHandler = ( event ) => {
+		'worklet';
+		const { contentSize, contentOffset, layoutMeasurement } = event;
+		scroll.offsetY.value = contentOffset.y;
+		scroll.maxOffsetY.value = contentSize.height - layoutMeasurement.height;
+	};
+
+	const stopScrolling = () => {
+		'worklet';
+		cancelAnimation( animationTimer );
+
+		isAnimationTimerActive.value = false;
+		isScrollActive.value = false;
+		velocityY.value = 0;
+	};
+
+	const startScrolling = ( y ) => {
+		'worklet';
+		stopScrolling();
+		offsetY.value = scroll.offsetY.value;
+		dragStartY.value = y;
+
+		animationTimer.value = 0;
+		animationTimer.value = withRepeat(
+			withTiming( 1, {
+				duration: SCROLL_INTERVAL_MS,
+				easing: Easing.linear,
+			} ),
+			-1,
+			true
+		);
+		isAnimationTimerActive.value = true;
+	};
+
+	const scrollOnDragOver = ( y ) => {
+		'worklet';
+		const dragDistance = Math.max(
+			Math.abs( y - dragStartY.value ) - SCROLL_INACTIVE_DISTANCE_PX,
+			0
+		);
+		const distancePercentage = dragDistance / windowHeight;
+
+		if ( ! isScrollActive.value ) {
+			isScrollActive.value = dragDistance > 0;
+		} else if ( y > dragStartY.value ) {
+			// User is dragging downwards.
+			velocityY.value = VELOCITY_MULTIPLIER * distancePercentage;
+		} else if ( y < dragStartY.value ) {
+			// User is dragging upwards.
+			velocityY.value = -VELOCITY_MULTIPLIER * distancePercentage;
+		} else {
+			velocityY.value = 0;
+		}
+	};
+
+	useAnimatedReaction(
+		() => animationTimer.value,
+		( value, previous ) => {
+			if ( velocityY.value === 0 ) {
+				return;
+			}
+
+			const delta = Math.abs( value - previous );
+			let newOffset = offsetY.value + delta * velocityY.value;
+
+			if ( scroll.maxOffsetY.value !== 0 ) {
+				newOffset = Math.max(
+					0,
+					Math.min( scroll.maxOffsetY.value, newOffset )
+				);
+			} else {
+				// Scroll values are empty until receiving the first scroll event.
+				// In that case, the max offset is unknown and we can't clamp the
+				// new offset value.
+				newOffset = Math.max( 0, newOffset );
+			}
+
+			offsetY.value = newOffset;
+			scrollTo( animatedScrollRef, 0, offsetY.value, false );
+		}
+	);
+
+	return [ startScrolling, scrollOnDragOver, stopScrolling, scrollHandler ];
+}