[Extensions Bindings] Add Events support

extensions/renderer/api_event_handler.cc

+// Copyright 2016 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.
+
+#include "extensions/renderer/api_event_handler.h"
+
+#include <algorithm>
+#include <map>
+#include <memory>
+#include <vector>
+
+#include "base/bind.h"
+#include "base/logging.h"
+#include "base/memory/ptr_util.h"
+#include "base/supports_user_data.h"
+#include "base/values.h"
+#include "content/public/child/v8_value_converter.h"
+#include "gin/arguments.h"
+#include "gin/per_context_data.h"
+
+namespace extensions {
+
+namespace {
+
+const char kExtensionAPIEventPerContextKey[] = "extension_api_events";
+
+struct APIEventPerContextData : public base::SupportsUserData::Data {
+  APIEventPerContextData() = default;
+  ~APIEventPerContextData() override {
+    // We explicitly clear the event data map here to remove all references to
+    // v8 objects. This helps us avoid cycles in v8 where an event listener
+    // could hold a reference to the event, which in turn holds the reference
+    // to the listener.
+    event_data.clear();

[jbroman, 2016/11/03 19:28:04]

Apologies for not being clear here. event_data itself will be cleared
automatically in the map's destructor. But the listener vectors in each event
emitter won't be cleared implicitly (because that object isn't destroyed here).
It's the handles from EventEmitter to v8::Function that needs to be cleared.

for (const auto& pair : event_data) {
  EventEmitter* emitter = nullptr;
  if (gin::Converter<EventEmitter*>::FromV8(isolate, pair.second, &emitter))
    emitter->listeners()->clear();
}

This will clear any cycles that may exist between |emitter| and one of its
listeners, which may be necessary to make the emitter collectible.

[Devlin, 2016/11/04 17:59:30]

Whoops!  I understood what you meant, and somewhere along the line way over
simplified it.  Fixed now.

In your snippet, you guard the conversion failure in an if - when could this
reasonably fail?  Would a (D)CHECK be better?  (And if so, would it be better at
other places we are converting, like 101 and 124?)

[jbroman, 2016/11/04 18:33:04]

No, it shouldn't fail. CHECK or DCHECK sgtm.

+  }
+
+  // A map from event name -> event emitter.
+  std::map<std::string, v8::Global<v8::Object>> event_data;
+};
+
+}  // namespace
+
+APIEventHandler::APIEventHandler(const binding::RunJSFunction& call_js)
+    : call_js_(call_js) {}
+APIEventHandler::~APIEventHandler() {}
+
+v8::Local<v8::Object> APIEventHandler::CreateEventInstance(
+    const std::string& event_name,
+    v8::Local<v8::Context> context) {
+  gin::PerContextData* per_context_data = gin::PerContextData::From(context);
+  DCHECK(per_context_data);
+  APIEventPerContextData* data = static_cast<APIEventPerContextData*>(
+      per_context_data->GetUserData(kExtensionAPIEventPerContextKey));
+  if (!data) {
+    auto api_data = base::MakeUnique<APIEventPerContextData>();
+    data = api_data.get();
+    per_context_data->SetUserData(kExtensionAPIEventPerContextKey,
+                                  api_data.release());
+  }
+
+  DCHECK(data->event_data.find(event_name) == data->event_data.end());
+
+  gin::Handle<EventEmitter> emitter_handle =
+      gin::CreateHandle(context->GetIsolate(), new EventEmitter());
+  CHECK(!emitter_handle.IsEmpty());
+  v8::Local<v8::Value> emitter_value = emitter_handle.ToV8();
+  CHECK(emitter_value->IsObject());
+  v8::Local<v8::Object> emitter_object =
+      v8::Local<v8::Object>::Cast(emitter_value);
+  data->event_data[event_name] =
+      v8::Global<v8::Object>(context->GetIsolate(), emitter_object);
+
+  return emitter_object;
+}
+
+void APIEventHandler::FireEventInContext(const std::string& event_name,
+                                         v8::Local<v8::Context> context,
+                                         const base::ListValue& args) {
+  gin::PerContextData* per_context_data = gin::PerContextData::From(context);
+  DCHECK(per_context_data);
+  APIEventPerContextData* data = static_cast<APIEventPerContextData*>(
+      per_context_data->GetUserData(kExtensionAPIEventPerContextKey));
+  if (!data)
+    return;
+
+  auto iter = data->event_data.find(event_name);
+  if (iter == data->event_data.end())
+    return;
+
+  // Note: since we only convert the arguments once, if a listener modifies an
+  // object (including an array), other listeners will see that modification.
+  // TODO(devlin): This is how it's always been, but should it be?
+  std::vector<v8::Local<v8::Value>> v8_args;
+  v8_args.reserve(args.GetSize());
+  std::unique_ptr<content::V8ValueConverter> converter(
+      content::V8ValueConverter::create());
+  for (const auto& arg : args)
+    v8_args.push_back(converter->ToV8Value(arg.get(), context));
+
+  gin::Handle<EventEmitter> emitter;
+  gin::Converter<gin::Handle<EventEmitter>>::FromV8(
+      context->GetIsolate(), iter->second.Get(context->GetIsolate()), &emitter);
+  CHECK(!emitter.IsEmpty());
+
+  for (const auto& listener : *emitter->listeners()) {
+    call_js_.Run(listener.Get(context->GetIsolate()), context, v8_args.size(),
+                 v8_args.data());
+  }
+}
+
+size_t APIEventHandler::GetNumEventListenersForTesting(
+    const std::string& event_name,
+    v8::Local<v8::Context> context) {
+  gin::PerContextData* per_context_data = gin::PerContextData::From(context);
+  DCHECK(per_context_data);
+  APIEventPerContextData* data = static_cast<APIEventPerContextData*>(
+      per_context_data->GetUserData(kExtensionAPIEventPerContextKey));
+  DCHECK(data);
+
+  auto iter = data->event_data.find(event_name);
+  DCHECK(iter != data->event_data.end());
+  gin::Handle<EventEmitter> emitter;
+  gin::Converter<gin::Handle<EventEmitter>>::FromV8(
+      context->GetIsolate(), iter->second.Get(context->GetIsolate()), &emitter);
+  CHECK(!emitter.IsEmpty());
+  return emitter->listeners()->size();
+}
+
+}  // namespace extensions