Plumb firing passive event listeners.

content/common/input/web_input_event_queue.h

+// 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.
+
+#ifndef CONTENT_COMMON_INPUT_WEB_INPUT_EVENT_QUEUE_H_
+#define CONTENT_COMMON_INPUT_WEB_INPUT_EVENT_QUEUE_H_
+
+#include <deque>
+
+#include "base/memory/scoped_ptr.h"
+
+namespace content {
+
+enum class WebInputEventQueueState { ITEM_PENDING, ITEM_NOT_PENDING };
+
+// WebInputEventQueue is a coalescing queue with the addition of a state
+// variable that represents whether an item is pending to be processed.
+// The desired usage sending with this queue is:
+//   if (queue.state() == WebInputEventQueueState::ITEM_PENDING) {
+//     queue.Queue(T);
+//   } else {
+//     send T
+//     queue.set_state(WebInputEventQueueState::ITEM_PENDING);
+//   }
+//
+// Processing the event response:
+//  if (!queue.empty()) {
+//    T = queue.Pop();
+//    send T now
+//  } else {
+//    queue.set_state(WebInputEventQueueState::ITEM_NOT_PENDING);
+//  }
+//
+template <typename T>
+class WebInputEventQueue {
+ public:
+  WebInputEventQueue() : state_(WebInputEventQueueState::ITEM_NOT_PENDING) {}
+
+  // Adds an event to the queue. The event may be coalesced with previously
+  // queued events.
+  void Queue(const T& event) {
+    if (!queue_.empty()) {
+      scoped_ptr<T>& last_event = queue_.back();
+      if (last_event->CanCoalesceWith(event)) {
+        last_event->CoalesceWith(event);
+        return;
+      }
+    }
+    queue_.emplace_back(scoped_ptr<T>(new T(event)));
+  }
+
+  scoped_ptr<T> Pop() {
+    scoped_ptr<T> result;
+    if (!queue_.empty()) {
+      result.reset(queue_.front().release());
+      queue_.pop_front();
+    }
+    return result;
+  }
+
+  bool empty() const { return queue_.empty(); }
+
+  size_t size() const { return queue_.size(); }
+
+  void set_state(WebInputEventQueueState state) { state_ = state; }
+
+  WebInputEventQueueState state() const WARN_UNUSED_RESULT { return state_; }
+
+ private:
+  typedef std::deque<scoped_ptr<T>> EventQueue;
+  EventQueue queue_;
+  WebInputEventQueueState state_;
+
+  DISALLOW_COPY_AND_ASSIGN(WebInputEventQueue);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_INPUT_WEB_INPUT_EVENT_QUEUE_H_