[SPEC] Overhaul gesture system

[josxha]

Request for comments

Motivation

As mentioned by @JaffaKetchup in #1529 (comment), "much of the [gesture handling] code that does this was written in the first few releases, and has not been significantly changed since. [...] Gesture handling at this low-level often differs slightly between platforms, which means that manual testing succeeds on one platform, but a bug occurs on another."

Rewrite Proposals

• Break down and structure the FlutterMapInteractiveViewer class to improve readablity and reduce its complexity.
• Remove the usage of PositionedTapDetector2 and merge its GestureDetector with the one in FlutterMapInteractiveViewer.
• Use the high level GestureDetector as much as possible. This ensures the use of flutters GestureArena.
• Get rid of RawGestureDetector to avoid platform specific bugs (see issue tracker here [BUG] Tracker: Gesture Handling #1529).
• Use boolean fields in InteractiveFlag and MultiFingerGesture. This simplifies internal usage, is more beginner friendly in our public API and has no performance penalty. I'll write some code examples in the next comment.
• [FEATURE] Add catch-all MapEventZoom #1732
• open for extendibility like [FEATURE] Two fingers tap to zoom out #1432

If feasible do the following changes:

• [FEATURE] Revert to using single onPanUpdate to detect pan events #1729

More resources

• Flutter Deep Dive: Gestures. Flutter provides some really amazing… | by Nash | Flutter Community | Medium
• Taps, drags, and other gestures | Flutter
• GestureArena | Decoding Flutter - YouTube

[josxha]

Example implementation of the new `InteractiveFlags` class

@immutable
class InteractiveFlags {
  const InteractiveFlags._({
    required this.drag,
    required this.flingAnimation,
    required this.pinchMove,
    required this.pinchZoom,
    required this.doubleTapZoom,
    required this.doubleTapDragZoom,
    required this.scrollWheelZoom,
    required this.rotate,
  });

  const InteractiveFlags.all({
    this.drag = true,
    this.flingAnimation = true,
    this.pinchMove = true,
    this.pinchZoom = true,
    this.doubleTapZoom = true,
    this.doubleTapDragZoom = true,
    this.scrollWheelZoom = true,
    this.rotate = true,
  });

  const InteractiveFlags.none({
    this.drag = false,
    this.flingAnimation = false,
    this.pinchMove = false,
    this.pinchZoom = false,
    this.doubleTapZoom = false,
    this.doubleTapDragZoom = false,
    this.scrollWheelZoom = false,
    this.rotate = false,
  });

  /// Enable panning with a single finger or cursor
  final bool drag;

  /// Enable fling animation after panning if velocity is great enough.
  final bool flingAnimation;

  /// Enable panning with multiple fingers
  final bool pinchMove;

  /// Enable zooming with a multi-finger pinch gesture
  final bool pinchZoom;

  /// Enable zooming with a single-finger double tap gesture
  final bool doubleTapZoom;

  /// Enable zooming with a single-finger double-tap-drag gesture
  ///
  /// The associated [MapEventSource] is [MapEventSource.doubleTapHold].
  final bool doubleTapDragZoom;

  /// Enable zooming with a mouse scroll wheel
  final bool scrollWheelZoom;

  /// Enable rotation with two-finger twist gesture
  ///
  /// For controlling cursor/keyboard rotation, see
  /// [InteractionOptions.cursorKeyboardRotationOptions].
  final bool rotate;

  bool hasMultiFinger() => pinchMove || pinchZoom || rotate;

  /// This constructor gives wither functionality to the model
  InteractiveFlags withFlag({
    bool? pinchZoom,
    bool? drag,
    bool? flingAnimation,
    bool? pinchMove,
    bool? doubleTapZoom,
    bool? doubleTapDragZoom,
    bool? scrollWheelZoom,
    bool? rotate,
  }) =>
      InteractiveFlags._(
        pinchZoom: pinchZoom ?? this.pinchZoom,
        drag: drag ?? this.drag,
        flingAnimation: flingAnimation ?? this.flingAnimation,
        pinchMove: pinchMove ?? this.pinchMove,
        doubleTapZoom: doubleTapZoom ?? this.doubleTapZoom,
        doubleTapDragZoom: doubleTapDragZoom ?? this.doubleTapDragZoom,
        scrollWheelZoom: scrollWheelZoom ?? this.scrollWheelZoom,
        rotate: rotate ?? this.rotate,
      );
}

This would make the following changes:

 final int flags;
// turns into
final InteractiveFlags flags;

flags: InteractiveFlags.all - InteractiveFlags.rotate,
// turns into
flags: InteractiveFlags.all(rotate: false), 

if (InteractiveFlag.hasDoubleTapZoom(newOptions.flags)) {
// turns into
if (newOptions.flags.doubleTapZoom) {

[JaffaKetchup]

I think the rough rule on scope of animations is I think the rough rule is we provide basic animations in response to gestures only. It's for plugins to provide animations for programmatic controls.