Implement HitTestAggregator

components/viz/hit_test/display_hit_test_data.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_VIZ_HIT_TEST_DISPLAY_HIT_TEST_DATA_H_
+#define COMPONENTS_VIZ_HIT_TEST_DISPLAY_HIT_TEST_DATA_H_
+
+#include <stdint.h>
+
+#include <map>

[rjkroege, 2017/06/15 21:58:43]

why?

[gklassen, 2017/06/16 21:49:38]

Done.

+#include <memory>

[rjkroege, 2017/06/15 21:58:43]

ditto?

[gklassen, 2017/06/16 21:49:38]

Done.

+
+#include "cc/surfaces/surface_id.h"
+#include "cc/surfaces/surface_observer.h"
+#include "components/viz/hit_test/hit_test_export.h"
+#include "components/viz/hit_test/public/interfaces/hit_test_data.mojom.h"
+
+namespace viz {
+
+// DiplayHitTestData contains the hit_test data for the Display.
+//
+// It is designed to be in shared memory so that the viz process can
+// write the hit_test data, and the browser / ws process can read without
+// process hops.
+//
+// One instance of this class contains a double buffer of entries -
+// the viz process will write into one half while client processes read from
+// the other.  read_offset_ is used to swap buffers atomically.

[rjkroege, 2017/06/15 21:58:43]

I think you need two buffers. I am not convinced that the read_offset can be
made secure.

[gklassen, 2017/06/16 21:49:38]

It needs to be made secure regardless of which approach.  I will investigate the
alternative of sending the synchronization with the BeginFrame message.  Any
pointers to where this message exists would be helpful. 

+
+// A DisplayHitTestRegion element with child_count of kEndOfList indicates
+// the last element ( end of the list ).
+constexpr int kEndOfList = -1;
+
+// A read_offset_ set to kOldPleaseReAcquire indicates that the buffer has been
+// resized and clients should re-acquire their reference.
+constexpr int kOldPleaseReAcquire = -1;
+
+struct DisplayHitTestRegion {
+  cc::FrameSinkId frame_sink_id;
+  uint32_t flags;
+  gfx::Rect rect;
+  gfx::Transform transform;
+  int child_count;
+};
+
+struct DisplayHitTestData {
+  int length;
+  base::subtle::Atomic32 read_offset;
+  DisplayHitTestRegion regions[];
+};
+
+}  // namespace viz
+
+#endif  //  COMPONENTS_VIZ_HIT_TEST_DISPLAY_HIT_TEST_DATA_H_