Gin: Add support for binding JS methods to C++ instance methods.

gin/object_template_builder.h

 // Copyright 2013 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 GIN_OBJECT_TEMPLATE_BUILDER_H_
 #define GIN_OBJECT_TEMPLATE_BUILDER_H_
 
 #include "base/bind.h"
+#include "base/callback.h"
 #include "base/strings/string_piece.h"
+#include "base/template_util.h"
 #include "gin/converter.h"
 #include "gin/function_template.h"
 #include "v8/include/v8.h"
 
 namespace gin {
 
+namespace {

[awong, 2013/12/10 02:37:24]

erp...I'm concerned about putting an anonymous namespace in a header.

I think that if you have a.cc and b.cc both of which cause an instantiation of
CreateTemplate<<MyType>, the final binary will end up with 2 blobs of code for
what is effectively the same function.

If I'm right, then the workaround is to use a nested named namespace like
internal.

[Jeffrey Yasskin, 2013/12/10 02:42:45]

Very good catch. It actually causes undefined behavior because
ObjectTemplateBuilder winds up with different definitions in different
translation units, although most of the time the compiler won't hurt you with
it.
+1.

+
+// Base template - used only for non-member function pointers. Other types
+// either go to one of the below specializations, or go here and fail to compile
+// because of base::Bind().
+template<typename T, typename Enable = void>
+struct CallbackTraits {
+  static v8::Handle<v8::FunctionTemplate> CreateTemplate(v8::Isolate* isolate,
+                                                         T callback) {
+    return CreateFunctionTemplate(isolate, base::Bind(callback));
+  }
+};
+
+// Specialization for base::Callback.
+template<typename T>
+struct CallbackTraits<base::Callback<T> > {
+  static v8::Handle<v8::FunctionTemplate> CreateTemplate(
+      v8::Isolate* isolate, const base::Callback<T>& callback) {
+    return CreateFunctionTemplate(isolate, callback);
+  }
+};
+
+// Specialization for member function pointers. We need to handle this case
+// specially because the first parameter for callbacks to MFP should typically
+// come from the the JavaScript "this" object the function was called on, not
+// from the first normal parameter.
+template<typename T>
+struct CallbackTraits<T, typename base::enable_if<
+                           base::is_member_function_pointer<T>::value >::type> {

[awong, 2013/12/10 02:37:24]

Why do we need this enable_if?

Also, can this be simplified to just be an enable_if around T on the first
template?

template<typename T>
struct CallbackTraits<
  typename base::enable_if<
     base::is_member_function_pointer<T>::value, T>::type> {
};

Then you don't need the "typename Enable = void" template parameter in line 23.

[Jeffrey Yasskin, 2013/12/10 02:42:45]

Probably not, because he's using the specialization to define different
definitions for different template arguments, and that sort of structure
matching won't deduce 'T' through a nested typedef.

+  static v8::Handle<v8::FunctionTemplate> CreateTemplate(v8::Isolate* isolate,
+                                                         T callback) {
+    return CreateFunctionTemplate(isolate, base::Bind(callback),
+                                  HolderIsFirstArgument);
+  }
+};
+
+// This specialization allows people to construct function templates directly if
+// they need to do fancier stuff.
+template<>
+struct CallbackTraits<v8::Handle<v8::FunctionTemplate> > {
+  static v8::Handle<v8::FunctionTemplate> CreateTemplate(
+      v8::Handle<v8::FunctionTemplate> templ) {
+    return templ;
+  }
+};
+
+}  // namespace
+
+
 // ObjectTemplateBuilder provides a handy interface to creating
 // v8::ObjectTemplate instances with various sorts of properties.
 class ObjectTemplateBuilder {
  public:
   explicit ObjectTemplateBuilder(v8::Isolate* isolate);
   ~ObjectTemplateBuilder();
 
   // It's against Google C++ style to return a non-const ref, but we take some
   // poetic license here in order that all calls to Set() can be via the '.'
   // operator and line up nicely.
   template<typename T>
   ObjectTemplateBuilder& SetValue(const base::StringPiece& name, T val) {
     return SetImpl(name, ConvertToV8(isolate_, val));
   }
 
-  template<typename T>
-  ObjectTemplateBuilder& SetMethod(const base::StringPiece& name, T val) {
-    return SetMethod(name, base::Bind(val));
-  }
-
+  // In the following methods, T and U can be function pointer, member function

[awong, 2013/12/10 02:37:24]

nit: can be -> can be a

+  // pointer, base::Callback, or v8::FunctionTemplate. Most clients will want to
+  // use one of the first two options. Also see gin::CreateFunctionTemplate()
+  // for creating raw function templates.
   template<typename T>
   ObjectTemplateBuilder& SetMethod(const base::StringPiece& name,
-                                   const base::Callback<T>& callback) {
-    return SetImpl(name, CreateFunctionTemplate(isolate_, callback));
+                                   const T& callback) {
+    return SetImpl(name, CallbackTraits<T>::CreateTemplate(isolate_, callback));
+  }
+  template<typename T>
+  ObjectTemplateBuilder& SetProperty(const base::StringPiece& name,
+                                     const T& getter) {
+    return SetPropertyImpl(name,
+                           CallbackTraits<T>::CreateTemplate(isolate_, getter),
+                           v8::Local<v8::FunctionTemplate>());
+  }
+  template<typename T, typename U>
+  ObjectTemplateBuilder& SetProperty(const base::StringPiece& name,
+                                     const T& getter, const U& setter) {
+    return SetPropertyImpl(name,
+                           CallbackTraits<T>::CreateTemplate(isolate_, getter),
+                           CallbackTraits<U>::CreateTemplate(isolate_, setter));
   }
 
   v8::Local<v8::ObjectTemplate> Build();
 
  private:
   ObjectTemplateBuilder& SetImpl(const base::StringPiece& name,
                                  v8::Handle<v8::Data> val);
+  ObjectTemplateBuilder& SetPropertyImpl(
+      const base::StringPiece& name, v8::Handle<v8::FunctionTemplate> getter,
+      v8::Handle<v8::FunctionTemplate> setter);
 
   v8::Isolate* isolate_;
 
   // ObjectTemplateBuilder should only be used on the stack.
   v8::Local<v8::ObjectTemplate> template_;
 };
 
 }  // namespace gin
 
 #endif  // GIN_OBJECT_TEMPLATE_BUILDER_H_