cros: Don't send duplicate UserSession mojo messages to ash

ui/gfx/image/image_skia.h

 // Copyright (c) 2012 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 UI_GFX_IMAGE_IMAGE_SKIA_H_
 #define UI_GFX_IMAGE_IMAGE_SKIA_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 77 matching lines @@
   // Note that this does NOT generate ImageSkiaReps from its source.
   // If you want to create a deep copy with ImageSkiaReps for supported
   // scale factors, you need to explicitly call
   // |EnsureRepsForSupportedScales()| first.
   std::unique_ptr<ImageSkia> DeepCopy() const;
 
   // Returns true if this object is backed by the same ImageSkiaStorage as
   // |other|. Will also return true if both images are isNull().
   bool BackedBySameObjectAs(const gfx::ImageSkia& other) const;
 
+  // Alias for BackedBySameObjectAs(). Used by mojo EqualsTraits<> for
+  // comparing two ui::mojom::ImageSkia.
+  bool Equals(const gfx::ImageSkia& other) const;

[James Cook, 2017/04/25 00:58:28]

I think this is the right way to handle comparisons for mojo. 

Mojo has EqualsTraits<>, but they are not like StructTraits<>. I don't think
you're supposed to define your own EqualsTraits.

I could define my own mojo-only comparison like this:

namespace mojo {
namespace internals {  // uh oh, using internals

template <>
bool Equals(const ImageSkia& a, const ImageSkia& b) { ... }

}
}

However, then it's weird that ui::mojom::ImageSkia has an Equals() but the
underlying gfx::ImageSkia doesn't. That could also lead to a situation where
someone else adds gfx::ImageSkia::operator==() that doesn't match
ui::mojom::ImageSkia's concept of equals.

[xiyuan, 2017/04/25 16:04:53]

If gfx::ImageSkia needs an operation== in the future, it should probably be
BackedBySameObjectAs() as well. So I think this is fine.

[oshima, 2017/04/25 17:28:04]

Does this have to be named "Equals"? "equals" and "same" has subtle difference
in meaning so I want to use Same if possible.

[James Cook, 2017/04/25 17:32:47]

It must either be called Equals() or be operator==(). See
https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/lib/equals_trai...

I need this in order to compare two mojo structs, UserSessionPtrs.

Other options:

* Write my own comparison for the two UserSession objects, which means I have to
manually keep track of all the other fields to compare so I can do a special
comparison for the image field
* Do the hacky thing with mojo::internals mentioned above.

[James Cook, 2017/04/25 17:50:04]

Oshima, please wait on looking at this. Yuzhu had a different suggestion on how
to handles Equals(). I need to move some code in mojo.

+
   // Adds |image_rep| to the image reps contained by this object.
   void AddRepresentation(const gfx::ImageSkiaRep& image_rep);
 
   // Removes the image rep of |scale| if present.
   void RemoveRepresentation(float scale);
 
   // Returns true if the object owns an image rep whose density matches
   // |scale| exactly.
   bool HasRepresentation(float scale) const;
 
@@ skipping 62 matching lines @@
   // so that other thread can access the storage.
   void DetachStorageFromThread();
 
   // A refptr so that ImageRepSkia can be copied cheaply.
   scoped_refptr<internal::ImageSkiaStorage> storage_;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_IMAGE_IMAGE_SKIA_H_