DragSession class abstract

Abstract interface for a drag session.

A drag session manages the lifecycle of a drag operation, handling canvas locking/unlocking automatically. Elements only need to:

• Request a session from the controller via createSession(type)
• Call lifecycle methods: start, end, cancel

Elements manage their own business state (positions, sizes) internally. The session purely handles canvas lock coordination.

Usage

// In widget - get session from controller
DragSession? _session;

void _handleDragStart(DragStartDetails details) {
  _originalPosition = widget.position; // Element manages its own state
  _session = controller.createSession(DragSessionType.nodeDrag);
  _session!.start(); // Locks canvas
  // ...
}

void _handleDragEnd(DragEndDetails details) {
  _session?.end(); // Unlocks canvas
  _session = null;
  // Commit state (no restore needed)
}

void _handleDragCancel() {
  _session?.cancel(); // Unlocks canvas
  _session = null;
  // Restore state
  widget.position = _originalPosition!;
}

Design

The controller owns session creation, ensuring centralized control over canvas locking. Elements don't need direct access to InteractionState; they only interact with this abstract session API. The implementation is private within the controller.