LatencyInfo mojo: Avoid extra copy of LatencyComponents

ui/events/mojo/latency_info_struct_traits.cc

Index: ui/events/mojo/latency_info_struct_traits.cc
diff --git a/ui/events/mojo/latency_info_struct_traits.cc b/ui/events/mojo/latency_info_struct_traits.cc
index 78705c2c16df602e03ef09aa395d44658c3a198a..63b01995049cc93792c5112249433985adad4c6d 100644
--- a/ui/events/mojo/latency_info_struct_traits.cc
+++ b/ui/events/mojo/latency_info_struct_traits.cc
@@ -276,12 +276,18 @@ bool StructTraits<ui::mojom::LatencyInfo, ui::LatencyInfo>::Read(
   if (!data.ReadTraceName(&out->trace_name_))
     return false;
 
-  // TODO(fsamuel): Figure out how to optimize deserialization.
-  mojo::Array<ui::mojom::LatencyComponentPairPtr> components;
-  if (!data.ReadLatencyComponents(&components))
-    return false;
-  for (uint32_t i = 0; i < components.size(); ++i)
-    out->latency_components_[components[i]->key] = components[i]->value;
+  mojo::ArrayDataView<ui::mojom::LatencyComponentPairDataView> components;
+  data.GetLatencyComponentsDataView(&components);
+  for (uint32_t i = 0; i < components.size(); ++i) {
+    ui::mojom::LatencyComponentPairDataView component_pair;
+    components.GetDataView(i, &component_pair);

[yzshen1, 2016/08/23 21:23:05]

This change itself LGTM.

Performance fyi: It would be more efficient to store keys and values in two
parallel arrays, rather than putting them in a single array of pair struct.
Because that would save the cost of serializing/validating pair structs.

(But Daniel discussed with me about his concern that checking size of the two
parallel arrays matches would be one more thing to consider. So if this is
really performance critical and you decide to change it to two parallel arrays,
you might want to discuss with him. There is a feature request filed to support
object keys for mojo map.)

+    ui::LatencyInfo::LatencyMap::key_type key;
+    if (!component_pair.ReadKey(&key))
+      return false;
+    auto& value = out->latency_components_[key];
+    if (!component_pair.ReadValue(&value))
+      return false;
+  }
 
   InputCoordinateArray input_coordinate_array = {
       0, ui::LatencyInfo::kMaxInputCoordinates, out->input_coordinates_};