Initial checkin of the new Mozart compositor.

mojo/services/gfx/composition/interfaces/hit_tests.mojom

+// Copyright 2015 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.
+
+[DartPackage="mojo_services"]
+module mojo.gfx.composition;
+
+import "mojo/services/geometry/interfaces/geometry.mojom";
+import "mojo/services/gfx/composition/interfaces/scene_token.mojom";
+
+// Indicates that hit testing is not needed for a given node.
+const uint32 kHitIdNone = 0;
+
+// Provides information about the point of intersection of a hit test with
+// a node in a scene graph.
+struct Hit {
+  // The scene token of the scene which was hit.
+  SceneToken scene_token;
+
+  // The version of the scene which was consulted as part of evaluating the
+  // hit test.
+  uint32 scene_version;
+
+  // The node id of the node which was hit.
+  uint32 node_id;
+
+  // The hit test id of the node which was hit.
+  uint32 hit_id;
+
+  // The coordinates of the hit within the node's content space.
+  mojo.Point intersection;
+};
+
+// The result of a hit test operation.
+struct HitTestResult {
+  // A sorted list of hits tracing a path from the root of the scene graph
+  // to the innermost hit node, or null if none.  Omits nodes for which
+  // the hit_id was |kHitIdNone|.
+  array<Hit> hits;

[abarth, 2016/01/10 04:22:08]

I'm not sure it matters much, but this is backwards from what we do in Flutter. 
In Flutter, we give this list from the leaf up to the root.

There are some more subtle issues that we can discuss in more detail in person
if you like.  What's you've got here is fine, but less powerful that it could be
with some small tweaks.

[jeffbrown, 2016/01/16 03:28:31]

Good point.  I changed the description but I'll come back to this after Mozart
is landed.

+};
+
+// A hit testing service for scene graphs.
+interface HitTester {
+  // Performs a hit test on the specified point.
+  //
+  // TODO(jeffbrown): Specify a timestamp to allow for hit-tests of geometry
+  // as it appeared in the recent past.
+  HitTest(mojo.Point point) => (HitTestResult result);
+};