Handle out-of-order add/remove pointer events

lib/ui/window/pointer_data_packet_converter.cc

@@ -74,7 +74,42 @@ void PointerDataPacketConverter::ConvertPointerData(
         break;
       }
       case PointerData::Change::kAdd: {
-        FML_DCHECK(states_.find(pointer_data.device) == states_.end());
+        auto iter = states_.find(pointer_data.device);
+        if (iter != states_.end()) {
+          // Synthesizes a remove event if the pointer was not previously
+          // removed.
+          PointerState state = iter->second;
+          PointerData synthesized_data = pointer_data;
+          synthesized_data.physical_x = state.physical_x;
+          synthesized_data.physical_y = state.physical_y;
+          synthesized_data.pan_x = state.pan_x;
+          synthesized_data.pan_y = state.pan_y;
+          synthesized_data.scale = state.scale;
+          synthesized_data.rotation = state.rotation;
+          synthesized_data.buttons = state.buttons;
+
+          if (delegate_.ViewExists(state.view_id)) {
+            synthesized_data.view_id = state.view_id;
+
+            if (state.is_down) {
+              // Synthesizes cancel event if the pointer is down.
+              PointerData synthesized_cancel_event = synthesized_data;
+              synthesized_cancel_event.change = PointerData::Change::kCancel;
+              synthesized_cancel_event.synthesized = 1;
+              UpdatePointerIdentifier(synthesized_cancel_event, state, false);
+
+              state.is_down = false;
+              converted_pointers.push_back(synthesized_cancel_event);
+            }
+          }
+
+          PointerData synthesized_remove_event = synthesized_data;
+          synthesized_remove_event.change = PointerData::Change::kRemove;
+          synthesized_remove_event.synthesized = 1;
+
+          converted_pointers.push_back(synthesized_remove_event);
+        }
+
         EnsurePointerState(pointer_data);
         converted_pointers.push_back(pointer_data);
         break;
@@ -84,6 +119,11 @@ void PointerDataPacketConverter::ConvertPointerData(
         auto iter = states_.find(pointer_data.device);
         FML_DCHECK(iter != states_.end());
         PointerState state = iter->second;
+        if (state.view_id != pointer_data.view_id) {
+          // Ignores remove event if the pointer was previously added to a
+          // different view.
+          break;
+        }
 
         if (state.is_down) {
           // Synthesizes cancel event if the pointer is down.
@@ -362,6 +402,7 @@ PointerState PointerDataPacketConverter::EnsurePointerState(
   state.physical_y = pointer_data.physical_y;
   state.pan_x = 0;
   state.pan_y = 0;
+  state.view_id = pointer_data.view_id;
   states_[pointer_data.device] = state;
   return state;
 }

lib/ui/window/pointer_data_packet_converter.h

@@ -38,6 +38,7 @@ struct PointerState {
   double scale;
   double rotation;
   int64_t buttons;
+  int64_t view_id;
 };
 
 //------------------------------------------------------------------------------

lib/ui/window/pointer_data_packet_converter_unittests.cc

@@ -510,6 +510,62 @@ TEST(PointerDataPacketConverterTest, CanSynthesizeAdd) {
   ASSERT_EQ(result[3].buttons, 0);
 }
 
+TEST(PointerDataPacketConverterTest, CanSynthesizeRemove) {
+  TestDelegate delegate;
+  delegate.AddView(100);
+  delegate.AddView(200);
+  PointerDataPacketConverter converter(delegate);
+  auto packet = std::make_unique<PointerDataPacket>(3);
+
+  PointerData data;
+  CreateSimulatedPointerData(data, PointerData::Change::kAdd, 0, 0.0, 0.0, 0);
+  data.view_id = 100;
+  packet->SetPointerData(0, data);
+  CreateSimulatedPointerData(data, PointerData::Change::kDown, 0, 3.0, 4.0, 1);
+  data.view_id = 100;
+  packet->SetPointerData(1, data);
+  CreateSimulatedPointerData(data, PointerData::Change::kAdd, 0, 0.0, 0.0, 0);
+  data.view_id = 200;
+  packet->SetPointerData(2, data);
+  auto converted_packet = converter.Convert(*packet);
+
+  std::vector<PointerData> result;
+  UnpackPointerPacket(result, std::move(converted_packet));
+
+  ASSERT_EQ(result.size(), (size_t)6);
+  ASSERT_EQ(result[0].synthesized, 0);
+  ASSERT_EQ(result[0].view_id, 100);
+
+  // A hover should be synthesized.
+  ASSERT_EQ(result[1].change, PointerData::Change::kHover);
+  ASSERT_EQ(result[1].synthesized, 1);
+  ASSERT_EQ(result[1].physical_delta_x, 3.0);
+  ASSERT_EQ(result[1].physical_delta_y, 4.0);
+  ASSERT_EQ(result[1].buttons, 0);
+
+  ASSERT_EQ(result[2].change, PointerData::Change::kDown);
+  ASSERT_EQ(result[2].pointer_identifier, 1);
+  ASSERT_EQ(result[2].synthesized, 0);
+  ASSERT_EQ(result[2].buttons, 1);
+
+  // A cancel should be synthesized.
+  ASSERT_EQ(result[3].change, PointerData::Change::kCancel);
+  ASSERT_EQ(result[3].pointer_identifier, 1);
+  ASSERT_EQ(result[3].synthesized, 1);
+  ASSERT_EQ(result[3].physical_x, 3.0);
+  ASSERT_EQ(result[3].physical_y, 4.0);
+  ASSERT_EQ(result[3].buttons, 1);
+
+  // A remove should be synthesized.
+  ASSERT_EQ(result[4].physical_x, 3.0);
+  ASSERT_EQ(result[4].physical_y, 4.0);
+  ASSERT_EQ(result[4].synthesized, 1);
+  ASSERT_EQ(result[4].view_id, 100);
+
+  ASSERT_EQ(result[5].synthesized, 0);
+  ASSERT_EQ(result[5].view_id, 200);
+}
+
 TEST(PointerDataPacketConverterTest, CanHandleThreeFingerGesture) {
   // Regression test https://github.com/flutter/flutter/issues/20517.
   TestDelegate delegate;