Add viz-host HitTestQuery.

components/viz/host/hit_test/hit_test_query.cc

+// 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.
+
+#include "components/viz/host/hit_test/hit_test_query.h"
+
+namespace viz {
+namespace hit_test {
+
+HitTestQuery::HitTestQuery() {}
+
+HitTestQuery::~HitTestQuery() {}
+
+Target HitTestQuery::FindTargetForLocation(const gfx::Point& location_in_root) {
+  // TODO(riajiang): Assumptions about the DisplayHitTestData received.
+  // 1. The list is in ascending (front to back) z-order.
+  // 2. Children count includes children of children.
+  // 3. After applying transform to the incoming point, point is in the same
+  // coordinate system as the bounds it is comparing against.
+  // For example,
+  //  +e-------------+
+  //  |   +c---------|
+  //  | 1 |+a--+     |
+  //  |   || 2 |     |
+  //  |   |+b--------|
+  //  |   ||         |
+  //  |   ||   3     |
+  //  +--------------+
+  // In this case, after applying identity transform, 1 is in the coordinate
+  // system of e; apply the transfrom-from-e-to-c and transform-from-c-to-a
+  // then we get 2 in the coordinate system of a; apply the
+  // transfrom-from-e-to-c and transform-from-c-to-b then we get 3 in the
+  // coordinate system of b.
+  // 4. Transform contains transform for the position.
+  // 5. HIT_TEST_MINE means the region can receive events, i.e., not just a
+  // bounding box. The opposite is HIT_TEST_IGNORE.
+  DCHECK(!display_hit_test_data_list_.empty());
+  Target target;
+  FindTargetForLocationInRegion(location_in_root,
+                                &display_hit_test_data_list_.front(), &target);
+  return target;
+}
+
+bool HitTestQuery::FindTargetForLocationInRegion(
+    const gfx::Point& location_in_region,
+    DisplayHitTestDataRegion* region,
+    Target* target) {
+  gfx::Point location_transformed(location_in_region);

[sadrul, 2017/06/27 03:37:54]

Can you document in the header file what the coordinate space of
|location_in_region| is?

The name |location_in_region| makes me think it is already in |region|'s
coordinate space. If it is, then why do we need to transform using region's
transform?

[riajiang, 2017/06/27 15:51:44]

Sry it is indeed confusing. Changed
FindTargetForLocationInRegion->FindTargetInRegionForLocation and
location_in_region->location_in_parent; documented in the header file.

+  if (!region->transform.IsIdentity())
+    region->transform.TransformPoint(&location_transformed);

[sadrul, 2017/06/27 03:37:54]

TransformPoint() already early-outs for identity transforms. No need to check it
first here.

[riajiang, 2017/06/27 15:51:44]

I see, removed.

+
+  if (!region->bounds.Contains(location_transformed))
+    return false;
+
+  DisplayHitTestDataRegion* child_region = region + 1;
+  DisplayHitTestDataRegion* child_region_end = region + region->child_count;

[sadrul, 2017/06/27 03:37:54]

This code makes me nervous. We are reading data generated by a potentially
compromised viz process. We need to check the bounds before we read.

[riajiang, 2017/06/27 15:51:44]

Added check for |child_region_end|.

+  while (child_region <= child_region_end) {
+    if (FindTargetForLocationInRegion(location_transformed, child_region,
+                                      target))
+      return true;
+
+    child_region = child_region + child_region->child_count + 1;
+  }
+
+  // TODO(riajiang): Use flag values defined in
+  // https://codereview.chromium.org/2938953002/.
+  if (region->flags) {
+    target->id = region->id;
+    target->location_in_target = location_transformed;
+    target->flags = region->flags;
+    return true;
+  }
+  return false;
+}
+
+}  // namespace hit_test
+}  // namespace viz