[wrapper-tracing] Bring remoteplayback up to date

third_party/WebKit/Source/bindings/templates/callback_function.cpp.tmpl

 {% from 'utilities.cpp.tmpl' import v8_value_to_local_cpp_value %}
 {% filter format_blink_cpp_source_code %}
 
 {% include 'copyright_block.txt' %}
 
 #include "{{cpp_class}}.h"
 
 {% for filename in cpp_includes %}
 #include "{{filename}}"
 {% endfor %}
 
 namespace blink {
 
 {{cpp_class}}::{{cpp_class}}(v8::Isolate* isolate, v8::Local<v8::Function> callback)
     : m_callback(isolate, callback)
 {
     DCHECK(!m_callback.isEmpty());
     m_callback.setPhantom();
 }
 
 DEFINE_TRACE({{cpp_class}})
 {
 }
 
+DEFINE_TRACE_WRAPPERS({{cpp_class}})
+{
+    visitor->traceWrappers(&reinterpret_cast<ScopedPersistent<v8::Object>&>(const_cast<ScopedPersistent<v8::Function>&>(m_callback)));

[Marcel Hlopko, 2016/10/19 15:08:17]

Kentaro, just to be sure, should really each blink callback object keep its
wrapper alive?

[Michael Lippautz, 2016/10/19 15:13:41]

This infra merely adds the capability of blink callbacks keeping the wrapper
alive. You still require the parent object to call traceWrappers on the
callback, which only RemotePlayback currently does.

[haraken, 2016/10/19 15:31:43]

In almost all cases, the spec requires to add a strong reference from the
wrapper to the callback functions.

In any case, making the parent object explicitly call traceWrappers() sounds
like a good programming pattern. We can represent any reachability with it.

[haraken, 2016/10/19 15:31:43]

Can we add an overloaded function to traceWrappers() so that we don't need to
write complicated reinterpret_cast on the caller side?

[Michael Lippautz, 2016/10/19 15:44:34]

traceWrappers is overloaded with the two basic v8 type v8::Object and v8::Value.


I'd rather add a 
  template<typename S>
  const ScopedPersistent<S>& ScopedPersistent<T>::cast() const;
and use that one here.

[Michael Lippautz, 2016/10/19 15:44:35]

Acknowledged.

+}
+
 bool {{cpp_class}}::call({{argument_declarations | join(', ')}})
 {
     if (!scriptState->contextIsValid())
         return false;
 
     ExecutionContext* context = scriptState->getExecutionContext();
     DCHECK(context);
     if (context->activeDOMObjectsAreSuspended() || context->activeDOMObjectsAreStopped())
         return false;
 
@@ skipping 29 matching lines @@
         returnValue = cppValue;
         {% endif %}
         return true;
     }
     return false;
 }
 
 } // namespace blink
 
 {% endfilter %}{# format_blink_cpp_source_code #}