Drag & Drop

Demonstrates the Draggable widget with pointer events and the "state machine as condition lock" pattern. A colored box can be dragged around; a status label shows the current state (Idle/Hovering/Dragging) and offset.

Running

.\build\bin\drag_demo.exe

Key Concepts

Draggable Widget

The Draggable widget extends Container and handles pointer events to track drag state. The drag offset is applied at paint time via context.Translate(), so Taitank layout is unaffected.

State Machine

The widget transitions between three states:

• kIdle — no interaction
• kHovering — pointer over the widget
• kDragging — pointer pressed and moving

Long-Press Detection

A long-press coroutine is launched on pointer-down. It uses a weak_ptr to the widget and checks the drag state after 500ms. If the pointer was released before the timeout, the coroutine observes the state change and returns silently. If held for 500ms+, a "[Long Press!]" indicator appears.

Task<void> LongPressCoroutine(std::weak_ptr<DragBox> weak) {
  co_await Sleep(std::chrono::milliseconds(500));
  auto box = weak.lock();
  if (!box) co_return;          // widget destroyed
  if (!box->IsDragging()) co_return;  // released before timeout
  box->SetLongPressFired(true);
  box->MarkNeedsBuild();
}

This is the "state machine as condition lock" pattern: no explicit cancellation is needed — the state machine gates coroutine execution.

Hit-Test Cache

Pointer-move events use a hit-test cache to avoid traversing the entire widget tree on every move. The cache is invalidated whenever layout changes.

See Also

• Draggable API
• Input & Events Guide
• Coroutines Guide