Views: Support property getter func to return const ref

Make the return type of property metadata's getter function to be more
flexible so that both returning a value and returning a constant
reference of that value are allowed.

Also, convert ImageViewBase's accessible_name to a property as an
example of such usage.

BUG=938501,957272

Change-Id: I8d6adeb9392b06be948b9fcf727b408661791f1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615380Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660883}

ui/views/controls/image_view_base.cc

@@ -61,7 +61,11 @@ ImageViewBase::Alignment ImageViewBase::GetVerticalAlignment() const {
 }
 
 void ImageViewBase::SetAccessibleName(const base::string16& accessible_name) {
+  if (accessible_name_ == accessible_name)
+    return;
+
   accessible_name_ = accessible_name;
+  OnPropertyChanged(&accessible_name_, kPropertyEffectsNone);
 }
 
 const base::string16& ImageViewBase::GetAccessibleName() const {
@@ -153,6 +157,7 @@ BEGIN_METADATA(ImageViewBase)
 METADATA_PARENT_CLASS(View)
 ADD_PROPERTY_METADATA(ImageViewBase, Alignment, HorizontalAlignment)
 ADD_PROPERTY_METADATA(ImageViewBase, Alignment, VerticalAlignment)
+ADD_PROPERTY_METADATA(ImageViewBase, base::string16, AccessibleName)
 END_METADATA()
 
 }  // namespace views

ui/views/metadata/metadata_macros_internal.h

@@ -5,6 +5,8 @@
 #ifndef UI_VIEWS_METADATA_METADATA_MACROS_INTERNAL_H_
 #define UI_VIEWS_METADATA_METADATA_MACROS_INTERNAL_H_
 
+#include <utility>
+
 #include "base/compiler_specific.h"
 #include "ui/views/metadata/metadata_types.h"
 
@@ -33,15 +35,18 @@
     DISALLOW_COPY_AND_ASSIGN(METADATA_CLASS_NAME_INTERNAL(class_name));      \
   }
 
-#define METADATA_PROPERTY_TYPE_INTERNAL(class_name, property_type,        \
-                                        property_name)                    \
-  views::metadata::ClassPropertyMetaData<class_name, property_type,       \
-                                         &class_name::Set##property_name, \
-                                         &class_name::Get##property_name>
+#define METADATA_PROPERTY_TYPE_INTERNAL(class_name, property_type, \
+                                        property_name)             \
+  views::metadata::ClassPropertyMetaData<                          \
+      class_name, property_type, &class_name::Set##property_name,  \
+      decltype(std::declval<class_name>().Get##property_name()),   \
+      &class_name::Get##property_name>
 
 #define METADATA_READONLY_PROPERTY_TYPE_INTERNAL(class_name, property_type, \
                                                  property_name)             \
   views::metadata::ClassPropertyReadOnlyMetaData<                           \
-      class_name, property_type, &class_name::Get##property_name>
+      class_name, property_type,                                            \
+      decltype(std::declval<class_name>().Get##property_name()),            \
+      &class_name::Get##property_name>
 
 #endif  // UI_VIEWS_METADATA_METADATA_MACROS_INTERNAL_H_

ui/views/metadata/property_metadata.h

@@ -19,7 +19,12 @@ namespace metadata {
 
 // Represents meta data for a specific read-only property member of class
 // |TClass|, with underlying type |TValue|, as the type of the actual member.
-template <typename TClass, typename TValue, TValue (TClass::*Get)() const>
+// Using a separate |TRet| type for the getter function's return type to allow
+// it to return a type with qualifier and by reference.
+template <typename TClass,
+          typename TValue,
+          typename TRet,
+          TRet (TClass::*Get)() const>
 class ClassPropertyReadOnlyMetaData : public MemberMetaDataBase {
  public:
   ClassPropertyReadOnlyMetaData() {}
@@ -45,9 +50,10 @@ class ClassPropertyReadOnlyMetaData : public MemberMetaDataBase {
 template <typename TClass,
           typename TValue,
           void (TClass::*Set)(ArgType<TValue>),
-          TValue (TClass::*Get)() const>
+          typename TRet,
+          TRet (TClass::*Get)() const>
 class ClassPropertyMetaData
-    : public ClassPropertyReadOnlyMetaData<TClass, TValue, Get> {
+    : public ClassPropertyReadOnlyMetaData<TClass, TValue, TRet, Get> {
  public:
   ClassPropertyMetaData() {}
   ~ClassPropertyMetaData() override = default;