Keeping state of a drag operation separately.

ui/controlbase.go

@@ -19,7 +19,7 @@ type ControlBase struct {
 	bounds  geom.RectangleF32
 	offset  geom.PointF32
 	parent  Control
-	dragStart *geom.PointF32
+	drag    Dragable
 	over    bool
 	pressed bool
 
@@ -62,18 +62,18 @@ func (c *ControlBase) Handle(ctx Context, e Event) {
 	case *MouseMoveEvent:
 		c.over = over(e.MouseEvent)
 		if c.pressed {
-			if c.dragStart == nil {
-				var start = c.ToControlPosition(e.Pos())
-				c.dragStart = &start
-				if c.onDragStart != nil {
-					c.onDragStart(start)
-				}
-			} else {
-				var start = *c.dragStart
+			if start, ok := c.drag.IsDragging(); ok {
 				var move = c.ToControlPosition(e.Pos())
+				c.drag.Move(move)
 				if c.onDragMove != nil {
 					c.onDragMove(start, move)
 				}
+			} else {
+				var start = c.ToControlPosition(e.Pos())
+				c.drag.Start(start)
+				if c.onDragStart != nil {
+					c.onDragStart(start)
+				}
 			}
 		}
 	case *MouseLeaveEvent:
@@ -85,10 +85,9 @@ func (c *ControlBase) Handle(ctx Context, e Event) {
 		}
 	case *MouseButtonUpEvent:
 		if e.Button == MouseButtonLeft {
-			if c.dragStart != nil {
-				var start = *c.dragStart
+			if start, ok := c.drag.IsDragging(); ok {
 				var end = c.ToControlPosition(e.Pos())
-				c.dragStart = nil
+				c.drag.Cancel()
 				if c.onDragEnd != nil {
 					c.onDragEnd(start, end)
 				}

ui/dragable.go

@@ -0,0 +1,44 @@
+package ui
+
+import "opslag.de/schobers/geom"
+
+// Dragable keeps track of the mouse position during a drag operation.
+type Dragable struct {
+	start   *geom.PointF32
+	current *geom.PointF32
+}
+
+// Cancel cancels the drag operation and returns the start and end position.
+func (d *Dragable) Cancel() (geom.PointF32, geom.PointF32) {
+	if d.start == nil {
+		return geom.ZeroPtF32, geom.ZeroPtF32
+	}
+	start, end := *d.start, *d.current
+	d.start = nil
+	d.current = nil
+	return start, end
+}
+
+// IsDragging returns if the drag operation is in progress and the start location if so.
+func (d *Dragable) IsDragging() (geom.PointF32, bool) {
+	if d.start != nil {
+		return *d.start, true
+	}
+	return geom.ZeroPtF32, false
+}
+
+// Move calculates the delta between the start point and the current position.
+func (d *Dragable) Move(p geom.PointF32) (geom.PointF32, bool) {
+	if d.start == nil {
+		return geom.ZeroPtF32, false
+	}
+	delta := p.Sub(*d.current)
+	d.current = &p
+	return delta, true
+}
+
+// Start set the start point of the drag operation.
+func (d *Dragable) Start(p geom.PointF32) {
+	d.start = &p
+	d.current = &p
+}