Implement the Draggable interface for mobile driver

This is not 'mobile dragging' as widgets will need to adapt to the form factor.
Progresses #65

internal/driver/gomobile/canvas.go

@@ -23,9 +23,11 @@ type canvas struct {
 	typedKey  func(event *fyne.KeyEvent)
 	shortcut  fyne.ShortcutHandler
 
-	inited, dirty bool
-	lastTapDown   int64
-	refreshQueue  chan fyne.CanvasObject
+	inited, dirty  bool
+	lastTapDown    int64
+	lastTapDownPos fyne.Position
+	dragging       fyne.Draggable
+	refreshQueue   chan fyne.CanvasObject
 }
 
 func (c *canvas) Content() fyne.CanvasObject {
@@ -170,11 +172,51 @@ func (c *canvas) walkTree(
 
 func (c *canvas) tapDown(pos fyne.Position) {
 	c.lastTapDown = time.Now().UnixNano()
+	c.lastTapDownPos = pos
+	c.dragging = nil
+}
+
+func (c *canvas) tapMove(pos fyne.Position,
+	dragCallback func(fyne.Draggable, *fyne.DragEvent)) {
+
+	if c.dragging == nil {
+		co, _ := driver.FindObjectAtPositionMatching(c.lastTapDownPos, func(object fyne.CanvasObject) bool {
+			if _, ok := object.(fyne.Draggable); ok {
+				return true
+			}
+
+			return false
+		}, c.overlay, c.content)
+
+		if drag, ok := co.(fyne.Draggable); ok {
+			c.dragging = drag
+		} else {
+			return
+		}
+	}
+	deltaX := pos.X - c.lastTapDownPos.X
+	deltaY := pos.Y - c.lastTapDownPos.Y
+	objPos := pos.Subtract(c.dragging.(fyne.CanvasObject).Position())
+
+	ev := new(fyne.DragEvent)
+	ev.Position = objPos
+	ev.DraggedX = deltaX
+	ev.DraggedY = deltaY
+
+	dragCallback(c.dragging, ev)
+	c.lastTapDownPos = pos
 }
 
 func (c *canvas) tapUp(pos fyne.Position,
 	tapCallback func(fyne.Tappable, *fyne.PointEvent),
-	tapAltCallback func(fyne.Tappable, *fyne.PointEvent)) {
+	tapAltCallback func(fyne.Tappable, *fyne.PointEvent),
+	dragCallback func(fyne.Draggable, *fyne.DragEvent)) {
+	if c.dragging != nil {
+		c.dragging.DragEnd()
+
+		c.dragging = nil
+	}
+
 	duration := time.Now().UnixNano() - c.lastTapDown
 
 	co, objPos := driver.FindObjectAtPositionMatching(pos, func(object fyne.CanvasObject) bool {

internal/driver/gomobile/canvas_test.go

@@ -43,6 +43,7 @@ func TestCanvas_Tapped(t *testing.T) {
 		wid.Tapped(ev)
 	}, func(wid fyne.Tappable, ev *fyne.PointEvent) {
 		wid.TappedSecondary(ev)
+	}, func(wid fyne.Draggable, ev *fyne.DragEvent) {
 	})
 
 	assert.True(t, tapped)
@@ -70,9 +71,36 @@ func TestCanvas_TappedSecondary(t *testing.T) {
 		tapped = true
 		tappedObj = wid
 		wid.TappedSecondary(ev)
+	}, func(wid fyne.Draggable, ev *fyne.DragEvent) {
 	})
 
 	assert.True(t, tapped)
 	assert.False(t, buttonTap)
 	assert.Equal(t, button, tappedObj)
 }
+
+func TestCanvas_Dragged(t *testing.T) {
+	dragged := false
+	var draggedObj fyne.Draggable
+	scroll := widget.NewScrollContainer(widget.NewLabel("Hi\nHi\nHi"))
+	c := &canvas{content: scroll}
+	c.Resize(fyne.NewSize(36, 24))
+	assert.Equal(t, 0, scroll.Offset.Y)
+
+	c.tapDown(fyne.NewPos(35, 3))
+	c.tapMove(fyne.NewPos(35, 10), func(wid fyne.Draggable, ev *fyne.DragEvent) {
+		wid.Dragged(ev)
+		dragged = true
+		draggedObj = wid
+	})
+
+	offset := scroll.Offset.Y
+	assert.True(t, dragged)
+	assert.NotNil(t, draggedObj)
+	assert.Greater(t, offset, 0)
+
+	c.tapMove(fyne.NewPos(35, 5), func(wid fyne.Draggable, ev *fyne.DragEvent) {
+		wid.Dragged(ev)
+	})
+	assert.Less(t, scroll.Offset.Y, offset)
+}

internal/driver/gomobile/driver.go

@@ -152,6 +152,8 @@ func (d *mobileDriver) Run() {
 				switch e.Type {
 				case touch.TypeBegin:
 					d.tapDownCanvas(canvas, e.X, e.Y)
+				case touch.TypeMove:
+					d.tapMoveCanvas(canvas, e.X, e.Y)
 				case touch.TypeEnd:
 					d.tapUpCanvas(canvas, e.X, e.Y)
 				}
@@ -212,6 +214,16 @@ func (d *mobileDriver) tapDownCanvas(canvas *canvas, x, y float32) {
 	canvas.tapDown(pos)
 }
 
+func (d *mobileDriver) tapMoveCanvas(canvas *canvas, x, y float32) {
+	tapX := internal.UnscaleInt(canvas, int(x))
+	tapY := internal.UnscaleInt(canvas, int(y))
+	pos := fyne.NewPos(tapX, tapY)
+
+	canvas.tapMove(pos, func(wid fyne.Draggable, ev *fyne.DragEvent) {
+		go wid.Dragged(ev)
+	})
+}
+
 func (d *mobileDriver) tapUpCanvas(canvas *canvas, x, y float32) {
 	tapX := internal.UnscaleInt(canvas, int(x))
 	tapY := internal.UnscaleInt(canvas, int(y))
@@ -221,6 +233,8 @@ func (d *mobileDriver) tapUpCanvas(canvas *canvas, x, y float32) {
 		go wid.Tapped(ev)
 	}, func(wid fyne.Tappable, ev *fyne.PointEvent) {
 		go wid.TappedSecondary(ev)
+	}, func(wid fyne.Draggable, ev *fyne.DragEvent) {
+		go wid.DragEnd()
 	})
 }