Prevent Switch from looping on and off (#17821)

packages/flutter/lib/src/material/toggleable.dart

@@ -134,13 +134,20 @@ abstract class RenderToggleable extends RenderConstrainedBox {
     _position
       ..curve = Curves.easeIn
       ..reverseCurve = Curves.easeOut;
-    switch (_positionController.status) {
+    if (tristate) {
-      case AnimationStatus.forward:
+      switch (_positionController.status) {
-      case AnimationStatus.completed:
+        case AnimationStatus.forward:
-        _positionController.reverse();
+        case AnimationStatus.completed:
-        break;
+          _positionController.reverse();
-      default:
+          break;
+        default:
+          _positionController.forward();
+      }
+    } else {
+      if (value == true)
         _positionController.forward();
+      else
+        _positionController.reverse();
     }
   }
 
@@ -257,8 +264,7 @@ abstract class RenderToggleable extends RenderConstrainedBox {
     if (isInteractive && !tristate) {
       if (status == AnimationStatus.completed && _value == false) {
         onChanged(true);
-      }
+      } else if (status == AnimationStatus.dismissed && _value != false) {
-      else if (status == AnimationStatus.dismissed && _value != false) {
         onChanged(false);
       }
     }

packages/flutter/test/material/switch_test.dart

@@ -187,4 +187,45 @@ void main() {
         ..circle(color: Colors.red[500])
     );
   });
+
+  testWidgets('Drag ends after animation completes', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/17773
+
+    bool value = false;
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return new Material(
+              child: new Center(
+                child: new Switch(
+                  value: value,
+                  onChanged: (bool newValue) {
+                    setState(() {
+                      value = newValue;
+                    });
+                  },
+                ),
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    expect(value, isFalse);
+
+    final Rect switchRect = tester.getRect(find.byType(Switch));
+    final TestGesture gesture = await tester.startGesture(switchRect.centerLeft);
+    await tester.pump();
+    await gesture.moveBy(new Offset(switchRect.width, 0.0));
+    await tester.pump();
+    await gesture.up();
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 200));
+
+    expect(value, isTrue);
+    expect(tester.hasRunningAnimations, false);
+  });
 }