improve UIDraggableMixin and update doc strings (#2510)

Author: eruvanos

arcade/gui/mixins.py

@@ -5,7 +5,8 @@
 from pyglet.event import EVENT_HANDLED, EVENT_UNHANDLED
 from typing_extensions import override
 
-from arcade.gui.events import UIMouseDragEvent, UIMouseEvent
+import arcade
+from arcade.gui.events import UIMouseDragEvent, UIMouseEvent, UIMousePressEvent, UIMouseReleaseEvent
 from arcade.gui.widgets import UILayout, UIWidget
 
 
@@ -17,8 +18,7 @@ class UIDraggableMixin(UILayout):
  class DraggablePane(UITexturePane, UIDraggableMixin):
  ...
 
- This does overwrite :class:`UILayout` behavior which position themselves,
- like :class:`UIAnchorWidget`
+ This will not work when placed in a layout, as the layout will overwrite the position.
 
  warning:
 
@@ -27,33 +27,37 @@ class DraggablePane(UITexturePane, UIDraggableMixin):
  It does not respect the layout system and can break other widgets
  which rely on the layout system.
 
- Further the dragging is not smooth, as it uses a very simple approach.
-
- Will be fixed in future versions, but might break existing code within a minor update.
-
  """
 
+ _dragging = False
+
  @override
  def do_layout(self):
- # FIXME this breaks all rules, let us not do this
-
- # Preserve top left alignment, this overwrites self placing behavior like
- # from :class:`UIAnchorWidget`
+ # FIXME this breaks core UI rules "Widgets are placed by parents", let us not do this
  rect = self.rect
  super().do_layout()
  self.rect = self.rect.align_top(rect.top).align_left(rect.left)
 
  @override
  def on_event(self, event) -> Optional[bool]:
  """Handle dragging of the widget."""
- if isinstance(event, UIMouseDragEvent) and self.rect.point_in_rect(event.pos):
+ if isinstance(event, UIMousePressEvent):
+ if event.button == arcade.MOUSE_BUTTON_LEFT and self.rect.point_in_rect(event.pos):
+ self._dragging = True
+ return EVENT_HANDLED
+
+ if isinstance(event, UIMouseDragEvent) and self._dragging:
  self.rect = self.rect.move(event.dx, event.dy)
  self.trigger_full_render()
-
- if super().on_event(event):
  return EVENT_HANDLED
 
- return EVENT_UNHANDLED
+ if isinstance(event, UIMouseReleaseEvent):
+ if event.button == arcade.MOUSE_BUTTON_LEFT and self._dragging:
+ self._dragging = False
+ self.trigger_full_render()
+ return EVENT_HANDLED
+
+ return super().on_event(event)
 
 
 class UIMouseFilterMixin(UIWidget):