Add friction coefficient to InteractiveViewer (#109443)

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

@@ -79,6 +79,7 @@ class InteractiveViewer extends StatefulWidget {
     // use cases.
     this.maxScale = 2.5,
     this.minScale = 0.8,
+    this.interactionEndFrictionCoefficient = _kDrag,
     this.onInteractionEnd,
     this.onInteractionStart,
     this.onInteractionUpdate,
@@ -93,6 +94,7 @@ class InteractiveViewer extends StatefulWidget {
        assert(constrained != null),
        assert(minScale != null),
        assert(minScale > 0),
+       assert(interactionEndFrictionCoefficient > 0),
        assert(minScale.isFinite),
        assert(maxScale != null),
        assert(maxScale > 0),
@@ -131,6 +133,7 @@ class InteractiveViewer extends StatefulWidget {
     // use cases.
     this.maxScale = 2.5,
     this.minScale = 0.8,
+    this.interactionEndFrictionCoefficient = _kDrag,
     this.onInteractionEnd,
     this.onInteractionStart,
     this.onInteractionUpdate,
@@ -143,6 +146,7 @@ class InteractiveViewer extends StatefulWidget {
        assert(builder != null),
        assert(minScale != null),
        assert(minScale > 0),
+       assert(interactionEndFrictionCoefficient > 0),
        assert(minScale.isFinite),
        assert(maxScale != null),
        assert(maxScale > 0),
@@ -326,6 +330,13 @@ class InteractiveViewer extends StatefulWidget {
   /// than maxScale.
   final double minScale;
 
+  /// Changes the deceleration behavior after a gesture.
+  ///
+  /// Defaults to 0.0000135.
+  ///
+  /// Cannot be null, and must be a finite number greater than zero.
+  final double interactionEndFrictionCoefficient;
+
   /// Called when the user ends a pan or scale gesture on the widget.
   ///
   /// At the time this is called, the [TransformationController] will have
@@ -408,6 +419,10 @@ class InteractiveViewer extends StatefulWidget {
   ///  * [TextEditingController] for an example of another similar pattern.
   final TransformationController? transformationController;
 
+  // Used as the coefficient of friction in the inertial translation animation.
+  // This value was eyeballed to give a feel similar to Google Photos.
+  static const double _kDrag = 0.0000135;
+
   /// Returns the closest point to the given point on the given line segment.
   @visibleForTesting
   static Vector3 getNearestPointOnLine(Vector3 point, Vector3 l1, Vector3 l2) {
@@ -549,10 +564,6 @@ class _InteractiveViewerState extends State<InteractiveViewer> with TickerProvid
   // https://github.com/flutter/flutter/issues/57698
   final bool _rotateEnabled = false;
 
-  // Used as the coefficient of friction in the inertial translation animation.
-  // This value was eyeballed to give a feel similar to Google Photos.
-  static const double _kDrag = 0.0000135;
-
   // The _boundaryRect is calculated by adding the boundaryMargin to the size of
   // the child.
   Rect get _boundaryRect {
@@ -913,18 +924,18 @@ class _InteractiveViewerState extends State<InteractiveViewer> with TickerProvid
     final Vector3 translationVector = _transformationController!.value.getTranslation();
     final Offset translation = Offset(translationVector.x, translationVector.y);
     final FrictionSimulation frictionSimulationX = FrictionSimulation(
-      _kDrag,
+      widget.interactionEndFrictionCoefficient,
       translation.dx,
       details.velocity.pixelsPerSecond.dx,
     );
     final FrictionSimulation frictionSimulationY = FrictionSimulation(
-      _kDrag,
+      widget.interactionEndFrictionCoefficient,
       translation.dy,
       details.velocity.pixelsPerSecond.dy,
     );
     final double tFinal = _getFinalTime(
       details.velocity.pixelsPerSecond.distance,
-      _kDrag,
+      widget.interactionEndFrictionCoefficient,
     );
     _animation = Tween<Offset>(
       begin: translation,

packages/flutter/test/widgets/interactive_viewer_test.dart

@@ -1649,6 +1649,62 @@ void main() {
       expect(scaleHighZoomedIn, greaterThan(scaleHighZoomedOut));
       expect(scaleHighZoomedIn - scaleHighZoomedOut, lessThan(scaleZoomedIn - scaleZoomedOut));
     });
+
+    testWidgets('interactionEndFrictionCoefficient', (WidgetTester tester) async {
+      // Use the default interactionEndFrictionCoefficient.
+      final TransformationController transformationController1 = TransformationController();
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: SizedBox(
+              width: 200,
+              height: 200,
+              child: InteractiveViewer(
+                constrained: false,
+                transformationController: transformationController1,
+                child: const SizedBox(width: 2000.0, height: 2000.0),
+              ),
+            ),
+          ),
+        ),
+      );
+
+      expect(transformationController1.value, equals(Matrix4.identity()));
+
+      await tester.flingFrom(const Offset(100, 100), const Offset(0, -50), 100.0);
+      await tester.pumpAndSettle();
+      final Vector3 translation1 = transformationController1.value.getTranslation();
+      expect(translation1.y, lessThan(-58.0));
+
+      // Next try a custom interactionEndFrictionCoefficient.
+      final TransformationController transformationController2 = TransformationController();
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: SizedBox(
+              width: 200,
+              height: 200,
+              child: InteractiveViewer(
+                constrained: false,
+                interactionEndFrictionCoefficient: 0.01,
+                transformationController: transformationController2,
+                child: const SizedBox(width: 2000.0, height: 2000.0),
+              ),
+            ),
+          ),
+        ),
+      );
+
+      expect(transformationController2.value, equals(Matrix4.identity()));
+
+      await tester.flingFrom(const Offset(100, 100), const Offset(0, -50), 100.0);
+      await tester.pumpAndSettle();
+      final Vector3 translation2 = transformationController2.value.getTranslation();
+
+      // The coefficient 0.01 is greater than the default of 0.0000135,
+      // so the translation comes to a stop more quickly.
+      expect(translation2.y, lessThan(translation1.y));
+    });
   });
 
   group('getNearestPointOnLine', () {