monodrag fix (#21138)

* monodrag fix

* added a test

* added test and changes

* fixed import

* removed comments

packages/flutter/lib/src/gestures/monodrag.dart

@@ -172,11 +172,12 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
         )));
       }
       if (delta != Offset.zero && onUpdate != null) {
+        final Offset deltaForDetails = _getDeltaForDetails(delta);
         invokeCallback<void>('onUpdate', () => onUpdate(new DragUpdateDetails(
           sourceTimeStamp: timestamp,
-          delta: _getDeltaForDetails(delta),
+          delta: deltaForDetails,
           primaryDelta: _getPrimaryValueFromOffset(delta),
-          globalPosition: _initialPosition,
+          globalPosition: _initialPosition + deltaForDetails,
         )));
       }
     }

packages/flutter/test/gestures/drag_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
 
 import 'gesture_tester.dart';
 
@@ -330,4 +331,76 @@ void main() {
     expect(new DragUpdateDetails(globalPosition: Offset.zero), hasOneLineDescription);
     expect(new DragEndDetails(), hasOneLineDescription);
   });
+
+  testGesture('Should recognize drag', (GestureTester tester) {
+    final HorizontalDragGestureRecognizer drag = new HorizontalDragGestureRecognizer();
+
+    bool didStartDrag = false;
+    drag.onStart = (_) {
+      didStartDrag = true;
+    };
+
+    double updatedDelta;
+    drag.onUpdate = (DragUpdateDetails details) {
+      updatedDelta = details.primaryDelta;
+    };
+
+    bool didEndDrag = false;
+    drag.onEnd = (DragEndDetails details) {
+      didEndDrag = true;
+    };
+
+    final TestPointer pointer = new TestPointer(5);
+    final PointerDownEvent down = pointer.down(const Offset(10.0, 10.0));
+    drag.addPointer(down);
+    tester.closeArena(5);
+    expect(didStartDrag, isFalse);
+    expect(updatedDelta, isNull);
+    expect(didEndDrag, isFalse);
+
+    tester.route(down);
+    expect(didStartDrag, isTrue);
+    expect(updatedDelta, isNull);
+    expect(didEndDrag, isFalse);
+
+    tester.route(pointer.move(const Offset(20.0, 25.0)));
+    expect(didStartDrag, isTrue);
+    didStartDrag = false;
+    expect(updatedDelta, 10.0);
+    updatedDelta = null;
+    expect(didEndDrag, isFalse);
+
+    tester.route(pointer.move(const Offset(20.0, 25.0)));
+    expect(didStartDrag, isFalse);
+    expect(updatedDelta, 0.0);
+    updatedDelta = null;
+    expect(didEndDrag, isFalse);
+
+    tester.route(pointer.up());
+    expect(didStartDrag, isFalse);
+    expect(updatedDelta, isNull);
+    expect(didEndDrag, isTrue);
+    didEndDrag = false;
+
+    drag.dispose();
+  });
+
+  testGesture('Should recognize drag', (GestureTester tester) {
+    final HorizontalDragGestureRecognizer drag = new HorizontalDragGestureRecognizer();
+
+    Offset newGlobalPosition;
+    drag.onUpdate = (DragUpdateDetails details) {
+      newGlobalPosition = details.globalPosition;
+    };
+
+    final TestPointer pointer = new TestPointer(5);
+    final PointerDownEvent down = pointer.down(const Offset(10.0, 10.0));
+    drag.addPointer(down);
+    tester.route(pointer.move(const Offset(20.0, 25.0)));
+    tester.closeArena(5);
+    tester.route(down);
+    expect(newGlobalPosition, const Offset(20.0, 10.0));
+    tester.route(pointer.up());
+    drag.dispose();
+  });
 }