fear(mobile): drag to select assets

mobile/lib/modules/home/ui/asset_grid/asset_drag_region.dart

@@ -0,0 +1,223 @@
+// ignore_for_file: library_private_types_in_public_api
+// Based on https://stackoverflow.com/a/52625182
+
+import 'dart:async';
+
+import 'package:collection/collection.dart';
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+
+class AssetDragRegion extends StatefulWidget {
+  final Widget child;
+
+  final void Function(AssetIndex valueKey)? onStart;
+  final void Function(AssetIndex valueKey)? onAssetEnter;
+  final void Function()? onEnd;
+  final void Function()? onScrollStart;
+  final void Function(ScrollDirection direction)? onScroll;
+
+  const AssetDragRegion({
+    super.key,
+    required this.child,
+    this.onStart,
+    this.onAssetEnter,
+    this.onEnd,
+    this.onScrollStart,
+    this.onScroll,
+  });
+  @override
+  State createState() => _AssetDragRegionState();
+}
+
+class _AssetDragRegionState extends State<AssetDragRegion> {
+  late AssetIndex? assetUnderPointer;
+  late AssetIndex? anchorAsset;
+
+  // Scroll related state
+  static const double scrollOffset = 0.10;
+  double? topScrollOffset;
+  double? bottomScrollOffset;
+  Timer? scrollTimer;
+  late bool scrollNotified;
+
+  @override
+  void initState() {
+    super.initState();
+    assetUnderPointer = null;
+    anchorAsset = null;
+    scrollNotified = false;
+  }
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    topScrollOffset = null;
+    bottomScrollOffset = null;
+  }
+
+  @override
+  void dispose() {
+    scrollTimer?.cancel();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return RawGestureDetector(
+      gestures: {
+        _CustomLongPressGestureRecognizer: GestureRecognizerFactoryWithHandlers<
+            _CustomLongPressGestureRecognizer>(
+          () => _CustomLongPressGestureRecognizer(),
+          _registerCallbacks,
+        ),
+      },
+      child: widget.child,
+    );
+  }
+
+  void _registerCallbacks(_CustomLongPressGestureRecognizer recognizer) {
+    recognizer.onLongPressMoveUpdate = (details) => _onLongPressMove(details);
+    recognizer.onLongPressStart = (details) => _onLongPressStart(details);
+    recognizer.onLongPressUp = _onLongPressEnd;
+    recognizer.onLongPressCancel = _onLongPressEnd;
+  }
+
+  AssetIndex? _getValueKeyAtPositon(Offset position) {
+    final box = context.findAncestorRenderObjectOfType<RenderBox>();
+    if (box == null) return null;
+
+    final hitTestResult = BoxHitTestResult();
+    final local = box.globalToLocal(position);
+    if (!box.hitTest(hitTestResult, position: local)) return null;
+
+    return (hitTestResult.path
+            .firstWhereOrNull((hit) => hit.target is _AssetIndexProxy)
+            ?.target as _AssetIndexProxy?)
+        ?.index;
+  }
+
+  void _onLongPressStart(LongPressStartDetails event) {
+    /// Calculate widget height and scroll offset when long press starting instead of in [initState]
+    /// or [didChangeDependencies] as the grid might still be rendering into view to get the actual size
+    final height = context.size?.height;
+    if (height != null &&
+        (topScrollOffset == null || bottomScrollOffset == null)) {
+      topScrollOffset = height * scrollOffset;
+      bottomScrollOffset = height - topScrollOffset!;
+    }
+
+    final initialHit = _getValueKeyAtPositon(event.globalPosition);
+    anchorAsset = initialHit;
+    if (initialHit == null) return;
+
+    if (anchorAsset != null) {
+      widget.onStart?.call(anchorAsset!);
+    }
+  }
+
+  void _onLongPressEnd() {
+    scrollNotified = false;
+    scrollTimer?.cancel();
+    widget.onEnd?.call();
+  }
+
+  void _onLongPressMove(LongPressMoveUpdateDetails event) {
+    if (anchorAsset == null) return;
+    if (topScrollOffset == null || bottomScrollOffset == null) return;
+
+    final currentDy = event.localPosition.dy;
+
+    if (currentDy > bottomScrollOffset!) {
+      scrollTimer ??= Timer.periodic(
+        const Duration(milliseconds: 50),
+        (_) => widget.onScroll?.call(ScrollDirection.forward),
+      );
+    } else if (currentDy < topScrollOffset!) {
+      scrollTimer ??= Timer.periodic(
+        const Duration(milliseconds: 50),
+        (_) => widget.onScroll?.call(ScrollDirection.reverse),
+      );
+    } else {
+      scrollTimer?.cancel();
+      scrollTimer = null;
+    }
+
+    final currentlyTouchingAsset = _getValueKeyAtPositon(event.globalPosition);
+    if (currentlyTouchingAsset == null) return;
+
+    if (assetUnderPointer != currentlyTouchingAsset) {
+      if (!scrollNotified) {
+        scrollNotified = true;
+        widget.onScrollStart?.call();
+      }
+
+      widget.onAssetEnter?.call(currentlyTouchingAsset);
+      assetUnderPointer = currentlyTouchingAsset;
+    }
+  }
+}
+
+class _CustomLongPressGestureRecognizer extends LongPressGestureRecognizer {
+  @override
+  void rejectGesture(int pointer) {
+    acceptGesture(pointer);
+  }
+}
+
+// ignore: prefer-single-widget-per-file
+class AssetIndexWrapper extends SingleChildRenderObjectWidget {
+  final int rowIndex;
+  final int sectionIndex;
+
+  const AssetIndexWrapper({
+    required Widget super.child,
+    required this.rowIndex,
+    required this.sectionIndex,
+    super.key,
+  });
+
+  @override
+  _AssetIndexProxy createRenderObject(BuildContext context) {
+    return _AssetIndexProxy(
+      index: AssetIndex(rowIndex: rowIndex, sectionIndex: sectionIndex),
+    );
+  }
+
+  @override
+  void updateRenderObject(
+    BuildContext context,
+    _AssetIndexProxy renderObject,
+  ) {
+    renderObject.index =
+        AssetIndex(rowIndex: rowIndex, sectionIndex: sectionIndex);
+  }
+}
+
+class _AssetIndexProxy extends RenderProxyBox {
+  AssetIndex index;
+
+  _AssetIndexProxy({
+    required this.index,
+  });
+}
+
+class AssetIndex {
+  final int rowIndex;
+  final int sectionIndex;
+
+  const AssetIndex({
+    required this.rowIndex,
+    required this.sectionIndex,
+  });
+
+  @override
+  bool operator ==(covariant AssetIndex other) {
+    if (identical(this, other)) return true;
+
+    return other.rowIndex == rowIndex && other.sectionIndex == sectionIndex;
+  }
+
+  @override
+  int get hashCode => rowIndex.hashCode ^ sectionIndex.hashCode;
+}