Add a snapAnimationDuration param in DraggableScrollableSheet (#107396)

* Add a snapAnimationDuration param in DraggableScrollableSheet

* snapAnimationDuration.inMilliseconds > 0

* Update draggable_scrollable_sheet.dart

packages/flutter/lib/src/widgets/draggable_scrollable_sheet.dart

@@ -289,6 +289,7 @@ class DraggableScrollableSheet extends StatefulWidget {
     this.expand = true,
     this.snap = false,
     this.snapSizes,
+    this.snapAnimationDuration,
     this.controller,
     required this.builder,
   })  : assert(initialChildSize != null),
@@ -298,6 +299,7 @@ class DraggableScrollableSheet extends StatefulWidget {
         assert(maxChildSize <= 1.0),
         assert(minChildSize <= initialChildSize),
         assert(initialChildSize <= maxChildSize),
+        assert(snapAnimationDuration == null || snapAnimationDuration > Duration.zero),
         assert(expand != null),
         assert(builder != null);
 
@@ -365,6 +367,12 @@ class DraggableScrollableSheet extends StatefulWidget {
   /// being built or since the last call to [DraggableScrollableActuator.reset].
   final List<double>? snapSizes;
 
+  /// Defines a duration for the snap animations.
+  ///
+  /// If it's not set, then the animation duration is the distance to the snap
+  /// target divided by the velocity of the widget.
+  final Duration? snapAnimationDuration;
+
   /// A controller that can be used to programmatically control this sheet.
   final DraggableScrollableController? controller;
 
@@ -466,6 +474,7 @@ class _DraggableSheetExtent {
     required this.snapSizes,
     required this.initialSize,
     required this.onSizeChanged,
+    this.snapAnimationDuration,
     ValueNotifier<double>? currentSize,
     bool? hasDragged,
   })  : assert(minSize != null),
@@ -486,6 +495,7 @@ class _DraggableSheetExtent {
   final double maxSize;
   final bool snap;
   final List<double> snapSizes;
+  final Duration? snapAnimationDuration;
   final double initialSize;
   final ValueNotifier<double> _currentSize;
   final VoidCallback onSizeChanged;
@@ -570,12 +580,14 @@ class _DraggableSheetExtent {
     required List<double> snapSizes,
     required double initialSize,
     required VoidCallback onSizeChanged,
+    Duration? snapAnimationDuration,
   }) {
     return _DraggableSheetExtent(
       minSize: minSize,
       maxSize: maxSize,
       snap: snap,
       snapSizes: snapSizes,
+      snapAnimationDuration: snapAnimationDuration,
       initialSize: initialSize,
       onSizeChanged: onSizeChanged,
       // Use the possibly updated initialSize if the user hasn't dragged yet.
@@ -599,6 +611,7 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
       maxSize: widget.maxChildSize,
       snap: widget.snap,
       snapSizes: _impliedSnapSizes(),
+      snapAnimationDuration: widget.snapAnimationDuration,
       initialSize: widget.initialChildSize,
       onSizeChanged: _setExtent,
     );
@@ -679,6 +692,7 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
       maxSize: widget.maxChildSize,
       snap: widget.snap,
       snapSizes: _impliedSnapSizes(),
+      snapAnimationDuration: widget.snapAnimationDuration,
       initialSize: widget.initialChildSize,
       onSizeChanged: _setExtent,
     );
@@ -902,6 +916,7 @@ class _DraggableScrollableSheetScrollPosition extends ScrollPositionWithSingleCo
         position: extent.currentPixels,
         initialVelocity: velocity,
         pixelSnapSize: extent.pixelSnapSizes,
+        snapAnimationDuration: extent.snapAnimationDuration,
         tolerance: physics.tolerance,
       );
     } else {
@@ -1065,12 +1080,17 @@ class _SnappingSimulation extends Simulation {
     required this.position,
     required double initialVelocity,
     required List<double> pixelSnapSize,
+    Duration? snapAnimationDuration,
     super.tolerance,
   }) {
     _pixelSnapSize = _getSnapSize(initialVelocity, pixelSnapSize);
+
+    if (snapAnimationDuration != null && snapAnimationDuration.inMilliseconds > 0) {
+       velocity = (_pixelSnapSize - position) * 1000 / snapAnimationDuration.inMilliseconds;
+    }
     // Check the direction of the target instead of the sign of the velocity because
     // we may snap in the opposite direction of velocity if velocity is very low.
-    if (_pixelSnapSize < position) {
+    else if (_pixelSnapSize < position) {
       velocity = math.min(-minimumSpeed, initialVelocity);
     } else {
       velocity = math.max(minimumSpeed, initialVelocity);

packages/flutter/test/widgets/draggable_scrollable_sheet_test.dart

@@ -15,6 +15,7 @@ void main() {
     double minChildSize = .25,
     bool snap = false,
     List<double>? snapSizes,
+    Duration? snapAnimationDuration,
     double? itemExtent,
     Key? containerKey,
     Key? stackKey,
@@ -40,6 +41,7 @@ void main() {
                 initialChildSize: initialChildSize,
                 snap: snap,
                 snapSizes: snapSizes,
+                snapAnimationDuration: snapAnimationDuration,
                 builder: (BuildContext context, ScrollController scrollController) {
                   return NotificationListener<ScrollNotification>(
                     onNotification: onScrollNotification,
@@ -485,58 +487,64 @@ void main() {
     });
   }
 
-  testWidgets('Zero velocity drag snaps to nearest snap target', (WidgetTester tester) async {
-    const Key stackKey = ValueKey<String>('stack');
-    const Key containerKey = ValueKey<String>('container');
-    await tester.pumpWidget(boilerplateWidget(null,
-      snap: true,
-      stackKey: stackKey,
-      containerKey: containerKey,
-      snapSizes: <double>[.25, .5, .75, 1.0],
-    ));
-    await tester.pumpAndSettle();
-    final double screenHeight = tester.getSize(find.byKey(stackKey)).height;
+  for (final Duration? snapAnimationDuration in <Duration?>[null, const Duration(seconds: 2)]) {
+    testWidgets(
+      'Zero velocity drag snaps to nearest snap target with '
+      'snapAnimationDuration: $snapAnimationDuration',
+      (WidgetTester tester) async {
+      const Key stackKey = ValueKey<String>('stack');
+      const Key containerKey = ValueKey<String>('container');
+      await tester.pumpWidget(boilerplateWidget(null,
+        snap: true,
+        stackKey: stackKey,
+        containerKey: containerKey,
+        snapSizes: <double>[.25, .5, .75, 1.0],
+        snapAnimationDuration: snapAnimationDuration
+      ));
+      await tester.pumpAndSettle();
+      final double screenHeight = tester.getSize(find.byKey(stackKey)).height;
 
-    // We are dragging up, but we'll snap down because we're closer to .75 than 1.
-    await tester.drag(find.text('Item 1'), Offset(0, -.35 * screenHeight));
-    await tester.pumpAndSettle();
-    expect(
-      tester.getSize(find.byKey(containerKey)).height / screenHeight,
-      closeTo(.75, precisionErrorTolerance),
-    );
+      // We are dragging up, but we'll snap down because we're closer to .75 than 1.
+      await tester.drag(find.text('Item 1'), Offset(0, -.35 * screenHeight));
+      await tester.pumpAndSettle();
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(.75, precisionErrorTolerance),
+      );
 
-    // Drag up and snap up.
-    await tester.drag(find.text('Item 1'), Offset(0, -.2 * screenHeight));
-    await tester.pumpAndSettle();
-    expect(
-      tester.getSize(find.byKey(containerKey)).height / screenHeight,
-      closeTo(1.0, precisionErrorTolerance),
-    );
+      // Drag up and snap up.
+      await tester.drag(find.text('Item 1'), Offset(0, -.2 * screenHeight));
+      await tester.pumpAndSettle();
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(1.0, precisionErrorTolerance),
+      );
 
-    // Drag down and snap up.
-    await tester.drag(find.text('Item 1'), Offset(0, .1 * screenHeight));
-    await tester.pumpAndSettle();
-    expect(
-      tester.getSize(find.byKey(containerKey)).height / screenHeight,
-      closeTo(1.0, precisionErrorTolerance),
-    );
+      // Drag down and snap up.
+      await tester.drag(find.text('Item 1'), Offset(0, .1 * screenHeight));
+      await tester.pumpAndSettle();
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(1.0, precisionErrorTolerance),
+      );
 
-    // Drag down and snap down.
-    await tester.drag(find.text('Item 1'), Offset(0, .45 * screenHeight));
-    await tester.pumpAndSettle();
-    expect(
-      tester.getSize(find.byKey(containerKey)).height / screenHeight,
-      closeTo(.5, precisionErrorTolerance),
-    );
+      // Drag down and snap down.
+      await tester.drag(find.text('Item 1'), Offset(0, .45 * screenHeight));
+      await tester.pumpAndSettle();
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(.5, precisionErrorTolerance),
+      );
 
-    // Fling up with negligible velocity and snap down.
-    await tester.fling(find.text('Item 1'), Offset(0, .1 * screenHeight), 1);
-    await tester.pumpAndSettle();
-    expect(
-      tester.getSize(find.byKey(containerKey)).height / screenHeight,
-      closeTo(.5, precisionErrorTolerance),
-    );
-  }, variant: TargetPlatformVariant.all());
+      // Fling up with negligible velocity and snap down.
+      await tester.fling(find.text('Item 1'), Offset(0, .1 * screenHeight), 1);
+      await tester.pumpAndSettle();
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(.5, precisionErrorTolerance),
+      );
+    }, variant: TargetPlatformVariant.all());
+  }
 
   for (final List<double>? snapSizes in <List<double>?>[null, <double>[]]) {
     testWidgets('Setting snapSizes to $snapSizes resolves to min and max', (WidgetTester tester) async {