Implement EventSubscriber<T> class.

This class simplifies the task of subscribing/handling javascript events from
C++ classes in the main process.

atom/browser/api/event_subscriber.h

@@ -0,0 +1,132 @@
+// Copyright (c) 2017 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#ifndef ATOM_BROWSER_API_EVENT_SUBSCRIBER_H_
+#define ATOM_BROWSER_API_EVENT_SUBSCRIBER_H_
+
+#include <map>
+#include <string>
+
+#include "atom/common/api/event_emitter_caller.h"
+#include "base/synchronization/lock.h"
+#include "content/public/browser/browser_thread.h"
+#include "native_mate/native_mate/arguments.h"
+
+namespace mate {
+
+namespace internal {
+
+class EventSubscriberBase {
+ public:
+  EventSubscriberBase(v8::Isolate* isolate, v8::Local<v8::Object> emitter);
+  virtual ~EventSubscriberBase();
+  virtual void EventEmitted(const std::string& event_name,
+                            mate::Arguments* args) = 0;
+
+ protected:
+  void On(const std::string& event_name);
+  void Off(const std::string& event_name);
+  void RemoveAllListeners();
+
+ private:
+  std::map<std::string, v8::Global<v8::Value>>::iterator RemoveListener(
+      std::map<std::string, v8::Global<v8::Value>>::iterator it);
+
+  v8::Isolate* isolate_;
+  v8::Global<v8::Object> emitter_;
+  std::map<std::string, v8::Global<v8::Value>> js_handlers_;
+
+  DISALLOW_COPY_AND_ASSIGN(EventSubscriberBase);
+};
+
+}  // namespace internal
+
+template <typename HandlerType>
+class EventSubscriber : internal::EventSubscriberBase {
+ public:
+  using EventCallback = void (HandlerType::*)(mate::Arguments* args);
+  // Alias to unique_ptr with deleter.
+  using unique_ptr = std::unique_ptr<EventSubscriber<HandlerType>,
+                                     void (*)(EventSubscriber<HandlerType>*)>;
+  // EventSubscriber should only be created/deleted in the main thread since it
+  // communicates with the V8 engine. This smart pointer makes it simpler to
+  // bind the lifetime of EventSubscriber with a class whose lifetime is managed
+  // by a non-UI thread.
+  class SafePtr : public unique_ptr {
+   public:
+    SafePtr() : SafePtr(nullptr) {}
+    explicit SafePtr(EventSubscriber<HandlerType>* ptr)
+        : unique_ptr(ptr, Deleter) {}
+
+   private:
+    // Custom deleter that schedules destructor invocation to the main thread.
+    static void Deleter(EventSubscriber<HandlerType>* ptr) {
+      DCHECK(
+          !::content::BrowserThread::CurrentlyOn(::content::BrowserThread::UI));
+      DCHECK(ptr);
+      // Acquire handler lock and reset handler_ to ensure that any new events
+      // emitted will be ignored after this function returns
+      base::AutoLock auto_lock(ptr->handler_lock_);
+      ptr->handler_ = nullptr;
+      content::BrowserThread::PostTask(
+          content::BrowserThread::UI, FROM_HERE,
+          base::Bind(
+              [](EventSubscriber<HandlerType>* subscriber) {
+                delete subscriber;
+              },
+              ptr));
+    }
+  };
+
+  EventSubscriber(HandlerType* handler,
+                  v8::Isolate* isolate,
+                  v8::Local<v8::Object> emitter)
+      : EventSubscriberBase(isolate, emitter), handler_(handler) {
+    DCHECK_CURRENTLY_ON(::content::BrowserThread::UI);
+  }
+
+  void On(const std::string& event, EventCallback callback) {
+    DCHECK_CURRENTLY_ON(::content::BrowserThread::UI);
+    EventSubscriberBase::On(event);
+    callbacks_.insert(std::make_pair(event, callback));
+  }
+
+  void Off(const std::string& event) {
+    DCHECK_CURRENTLY_ON(::content::BrowserThread::UI);
+    EventSubscriberBase::Off(event);
+    DCHECK(callbacks_.find(event) != callbacks_.end());
+    callbacks_.erase(callbacks_.find(event));
+  }
+
+  void RemoveAllListeners() {
+    DCHECK_CURRENTLY_ON(::content::BrowserThread::UI);
+    EventSubscriberBase::RemoveAllListeners();
+    callbacks_.clear();
+  }
+
+ private:
+  void EventEmitted(const std::string& event_name,
+                    mate::Arguments* args) override {
+    DCHECK_CURRENTLY_ON(::content::BrowserThread::UI);
+    base::AutoLock auto_lock(handler_lock_);
+    if (!handler_) {
+      // handler_ was probably destroyed by another thread and we should not
+      // access it.
+      return;
+    }
+    auto it = callbacks_.find(event_name);
+    if (it != callbacks_.end()) {
+      auto method = it->second;
+      (handler_->*method)(args);
+    }
+  }
+
+  HandlerType* handler_;
+  base::Lock handler_lock_;
+  std::map<std::string, EventCallback> callbacks_;
+};
+
+}  // namespace mate
+
+#endif  // ATOM_BROWSER_API_EVENT_SUBSCRIBER_H_