Support trackpad gestures in framework (flutter#89944)

* Implement trackpad gestures in framework

* Touch and Pan/Zoom pointers have separate IDs now

* Handle trackpad pointer device type

* Respect supportedDevices for pan/zoom events

* Update after rebase

* Fix check failures

* Avoid error with very short drags

* Address feedback

* Refactor drag event handler

* Address more feedback

* Add some missing punctuation

Author: moffatman

packages/flutter/lib/src/gestures/binding.dart

@@ -346,21 +346,21 @@ mixin GestureBinding on BindingBase implements HitTestable, HitTestDispatcher, H
 
   void _handlePointerEventImmediately(PointerEvent event) {
     HitTestResult? hitTestResult;
-    if (event is PointerDownEvent || event is PointerSignalEvent || event is PointerHoverEvent) {
+    if (event is PointerDownEvent || event is PointerSignalEvent || event is PointerHoverEvent || event is PointerPanZoomStartEvent) {
       assert(!_hitTests.containsKey(event.pointer));
       hitTestResult = HitTestResult();
       hitTest(hitTestResult, event.position);
-      if (event is PointerDownEvent) {
+      if (event is PointerDownEvent || event is PointerPanZoomStartEvent) {
         _hitTests[event.pointer] = hitTestResult;
       }
       assert(() {
         if (debugPrintHitTestResults)
           debugPrint('$event: $hitTestResult');
         return true;
       }());
-    } else if (event is PointerUpEvent || event is PointerCancelEvent) {
+    } else if (event is PointerUpEvent || event is PointerCancelEvent || event is PointerPanZoomEndEvent) {
       hitTestResult = _hitTests.remove(event.pointer);
-    } else if (event.down) {
+    } else if (event.down || event is PointerPanZoomUpdateEvent) {
       // Because events that occur with the pointer down (like
       // [PointerMoveEvent]s) should be dispatched to the same place that their
       // initial PointerDownEvent was, we want to re-use the path we found when
@@ -443,9 +443,9 @@ mixin GestureBinding on BindingBase implements HitTestable, HitTestDispatcher, H
   @override // from HitTestTarget
   void handleEvent(PointerEvent event, HitTestEntry entry) {
     pointerRouter.route(event);
-    if (event is PointerDownEvent) {
+    if (event is PointerDownEvent || event is PointerPanZoomStartEvent) {
       gestureArena.close(event.pointer);
-    } else if (event is PointerUpEvent) {
+    } else if (event is PointerUpEvent || event is PointerPanZoomEndEvent) {
       gestureArena.sweep(event.pointer);
     } else if (event is PointerSignalEvent) {
       pointerSignalResolver.resolve(event);

packages/flutter/lib/src/gestures/converter.dart

@@ -15,14 +15,13 @@ import 'events.dart';
 int _synthesiseDownButtons(int buttons, PointerDeviceKind kind) {
   switch (kind) {
     case PointerDeviceKind.mouse:
+    case PointerDeviceKind.trackpad:
       return buttons;
     case PointerDeviceKind.touch:
     case PointerDeviceKind.stylus:
     case PointerDeviceKind.invertedStylus:
       return buttons == 0 ? kPrimaryButton : buttons;
     case PointerDeviceKind.unknown:
-    default: // ignore: no_default_cases, to allow adding new device types to [PointerDeviceKind]
-             // TODO(moffatman): Remove after landing https://github.com/flutter/flutter/issues/23604
       // We have no information about the device but we know we never want
       // buttons to be 0 when the pointer is down.
       return buttons == 0 ? kPrimaryButton : buttons;
@@ -209,9 +208,44 @@ class PointerEventConverter {
                     radiusMax: radiusMax,
                     embedderId: datum.embedderId,
                   );
-                default: // ignore: no_default_cases, to allow adding new pointer events to [ui.PointerChange]
-                         // TODO(moffatman): Remove after landing https://github.com/flutter/flutter/issues/23604
-                  throw StateError('Unreachable');
+                case ui.PointerChange.panZoomStart:
+                  return PointerPanZoomStartEvent(
+                    timeStamp: timeStamp,
+                    pointer: datum.pointerIdentifier,
+                    kind: kind,
+                    device: datum.device,
+                    position: position,
+                    embedderId: datum.embedderId,
+                    synthesized: datum.synthesized,
+                  );
+                case ui.PointerChange.panZoomUpdate:
+                  final Offset pan =
+                      Offset(datum.panX, datum.panY) / devicePixelRatio;
+                  final Offset panDelta =
+                      Offset(datum.panDeltaX, datum.panDeltaY) / devicePixelRatio;
+                  return PointerPanZoomUpdateEvent(
+                    timeStamp: timeStamp,
+                    pointer: datum.pointerIdentifier,
+                    kind: kind,
+                    device: datum.device,
+                    position: position,
+                    pan: pan,
+                    panDelta: panDelta,
+                    scale: datum.scale,
+                    rotation: datum.rotation,
+                    embedderId: datum.embedderId,
+                    synthesized: datum.synthesized,
+                  );
+                case ui.PointerChange.panZoomEnd:
+                  return PointerPanZoomEndEvent(
+                    timeStamp: timeStamp,
+                    pointer: datum.pointerIdentifier,
+                    kind: kind,
+                    device: datum.device,
+                    position: position,
+                    embedderId: datum.embedderId,
+                    synthesized: datum.synthesized,
+                  );
               }
             case ui.PointerSignalKind.scroll:
               final Offset scrollDelta =