[SPEC] Overhaul event system

[josxha]

Work in progress, feedback and additions are welcome, event names are not final

Motivation

• Some event sources have their dedicated event class. Having both is redundant.
• An overhaul of the event system simplifies integration in the gesture system.
• multi!: rewrite gesture handling #1733 (comment)
• [FEATURE] Add catch-all MapEventZoom #1732

Current state

• Own event class for every event type
• EventSource for the origin of the event
• boolean hasGesture, false if gesture has been caused programmatically by the MapController

Needed additions

• Possibility to distinguish between different kinds of rotate
  events (multi!: rewrite gesture handling #1733)
• A convenient method to group zoom/move/rotate events ([FEATURE] Add catch-all MapEventZoom #1732)
• Less additional code to use it in the new gesture system
• Option to merge MapController.move(), .zoom() and .rotate() into one
  function (multi!: rewrite gesture handling #1733 (comment))

Class hierarchy

• abstract MapEvent
  • MapEventTap, MapEventSecondaryTap, MapEventTertiaryTap, MapEventLongPress, MapEventSecondaryLongPress,
    MapEventTertiaryLongPress
  • MapEventMoveStart, MapEventMoveEnd
  • MapEventFlingAnimationNotStarted, MapEventFlingAnimationStart, MapEventFlingAnimationEnd
  • MapEventDoubleTapZoomStart, MapEventDoubleTapZoomEnd
  • MapEventDoubleTapDragZoomStart, MapEventDoubleTapDragZoomEnd
  • MapEventRotateStart, MapEventRotateEnd
  • abstract MapEventWithMove
    • MapEventMove
    • MapEventFlingAnimation
    • MapEventDoubleTapZoom
    • MapEventScrollWheelZoom
    • MapEventNonRotatedSizeChange

Usage in onMapEvent callback

build() {
  return FlutterMap(
    options: MapOptions(
      onMapEvent: (event) {
        switch (event) {
          case final MapEventDoubleTapZoom event:
            print('double tap zoom');
            break;
          case final MapEventFlingAnimationNotStarted event:
            print('fling animation not started');
            break;
        }
      },
    ),
  );
}

Proposals

Proposal 1: Minimal class approach

Class hierarchy

• sealed MapEvent
  • MapCameraEvent(enum CameraEventType)
  • MapInteractionEvent(enum InteractionType)
  • MapStartEvent(enum CameraEventType)
  • MapEndEvent(enum CameraEventType)
  • MapEventFlingAnimationNotStarted()

Usage in onMapEvent callback

build() {
  return FlutterMap(
    options: MapOptions(
      onMapEvent: (event) {
        switch (event) {
          case MapCameraEvent():
            switch (event) {
              case CameraEventType.doubleTapZoom:
                print('double tap zoom');
                break;
            }
            break;
          case MapEventFlingAnimationNotStarted():
            print('fling animation not started');
            break;
        }
      },
    ),
  );
}

Proposal 2: Further embrace class subtyping

Class hierarchy

• sealed MapEvent
  • sealed MapInteractionEvent
    • MapEventTap, MapEventSecondaryTap, MapEventTertiaryTap, MapEventLongPress, MapEventSecondaryLongPress,
      MapEventTertiaryLongPress
  • MapEventFlingAnimationNotStarted
  • sealed MapStartEvent
    • MapMoveEventStart, MapFlingAnimationEventStart, MapDoubleTapZoomEventStart, MapDoubleTapDragZoomEventStart,
      MapEventRotateStart
  • sealed MapEndEvent
    • MapMoveEventEnd, MapEventFlingAnimationEnd, MapDoubleTapZoomEventEnd, MapDoubleTapDragZoomEventEnd,
      MapRotateEventEnd
  • sealed MapCameraEvent
    • MapMoveEvent
    • MapFlingEvent
    • MapDoubleTapZoomEvent
    • MapScrollWheelZoomEvent
    • MapWidgetSizeChangedEvent

Usage in onMapEvent callback

build() {
  return FlutterMap(
    options: MapOptions(
      onMapEvent: (event) {
        switch (event) {
          case MapDoubleTapZoomEvent():
            print('fling animation not started');
            break;
          case MapEventFlingAnimationNotStarted():
            print('fling animation not started');
            break;
          default:
            break;
        }
      },
    ),
  );
}

TODO: add more proposals

Challenges

• It could be that there is a feature conflict
  between multi!: rewrite gesture handling #1733 (comment)
  and [FEATURE] Add catch-all MapEventZoom #1732