cc: Adding BeginFrameObserverMap.

cc/base/observer_map.h

+// Copyright 2014 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 CC_BASE_OBSERVER_MAP_H_
+#define CC_BASE_OBSERVER_MAP_H_
+
+#include "base/containers/hash_tables.h"
+#include "base/logging.h"
+
+namespace cc {
+
+template <class ObserverType, class ObserverDataType>

[danakj, 2015/09/11 17:56:09]

Do you need to use this with multiple ObserverTypes and ObserverDataTypes? Can
we not use templates and just put this beside the class that wants to use it,
specifically for it?

Let's not try to make something too general. Previous guidance from base/ owners
has been to avoid making general base-like things outside of base and keep it
specific to your use case.

[mithro-old, 2015/09/11 19:03:57]

For our use case we probably only have one ObserverType (BeginFrameObserver) but
definitely do want different ObserverDataTypes for different BeginFrameSource
objects.

Having it templated on the ObserverType (rather than specific to
BeginFrameObserver) does make the tests slightly simpler.

I can move this to cc/scheduler/begin_frame_observer_map.xx if preferred. I
don't think the implementation would actually change if I moved it however?
Okay.

+class ObserverMap {
+ private:
+  typedef typename base::hash_set<ObserverType*> SetType;
+  typedef typename base::hash_map<ObserverType*, ObserverDataType*> MapType;
+
+  MapType observers_;
+
+  volatile size_t const_iterators_;
+  size_t iterators_;
+  MapType observers_to_add_;
+  SetType observers_to_remove_;
+
+ public:
+  ObserverMap()
+      : observers_(),
+        const_iterators_(0),
+        iterators_(0),
+        observers_to_add_(),
+        observers_to_remove_() {}
+
+  class iterator : public MapType::iterator {
+   public:
+    explicit iterator(typename MapType::iterator it,
+                      ObserverMap<ObserverType, ObserverDataType>* list)
+        : MapType::iterator(it), list_(list) {
+      list_->iterators_++;
+    }
+    ~iterator() {
+      DCHECK_GT(list_->iterators_, 0U);
+      list_->iterators_--;
+      list_->CompactMaps();
+    }
+
+   private:
+    ObserverMap<ObserverType, ObserverDataType>* list_;
+  };
+
+  class const_iterator : public MapType::const_iterator {
+   public:
+    explicit const_iterator(
+        typename MapType::const_iterator it,
+        const ObserverMap<ObserverType, ObserverDataType>* list)
+        : MapType::const_iterator(it),
+          list_(
+              const_cast<ObserverMap<ObserverType, ObserverDataType>*>(list)) {
+      list_->const_iterators_++;
+    }
+    ~const_iterator() {
+      DCHECK_GT(list_->const_iterators_, 0U);
+      list_->const_iterators_--;
+    }
+
+   private:
+    ObserverMap<ObserverType, ObserverDataType>* list_;
+  };
+
+  void AddObserver(ObserverType* obs) {
+    DCHECK_EQ(const_iterators_, 0U);
+    if (observers_to_add_.count(obs) == 0) {
+      observers_to_add_[obs] = new ObserverDataType();
+    }
+    CompactMaps();
+  }
+
+  void RemoveObserver(ObserverType* obs) {
+    DCHECK_EQ(const_iterators_, 0U);
+    observers_to_remove_.insert(obs);
+    CompactMaps();
+  }
+
+  bool HasObservers() const { return !observers_.empty(); }
+  bool HasObserver(ObserverType* const obs) const {
+    return (*this)[obs] != nullptr;
+  }
+
+  iterator begin() { return iterator(observers_.begin(), this); }
+  iterator end() { return iterator(observers_.end(), this); }
+
+  const_iterator cbegin() const {
+    return const_iterator(const_cast<const MapType*>(&observers_)->begin(),
+                          this);
+  }
+
+  const_iterator cend() const {
+    return const_iterator(const_cast<const MapType*>(&observers_)->end(), this);
+  }
+
+  const_iterator begin() const { return cbegin(); }
+  const_iterator end() const { return cend(); }
+
+  ObserverDataType* operator[](ObserverType* const obs) {
+    {
+      auto it = observers_.find(obs);
+      if (it != observers_.end())
+        return it->second;
+    }
+    {
+      auto it = observers_to_add_.find(obs);
+      if (it != observers_to_add_.end())
+        return it->second;
+    }
+    return nullptr;
+  }
+  const ObserverDataType* operator[](ObserverType* const obs) const {
+    {
+      auto it = observers_.find(obs);
+      if (it != observers_.end())
+        return it->second;
+    }
+    {
+      auto it = observers_to_add_.find(obs);
+      if (it != observers_to_add_.end())
+        return it->second;
+    }
+    return nullptr;
+  }
+
+ private:
+  void CompactMaps() {
+    if (iterators_ != 0)
+      return;
+
+    for (auto it = observers_to_remove_.begin();
+         it != observers_to_remove_.end(); it++) {
+      delete observers_[*it];
+      observers_[*it] = nullptr;
+      observers_.erase(*it);
+    }
+    observers_to_remove_.clear();
+
+    for (auto it = observers_to_add_.begin(); it != observers_to_add_.end();
+         it++) {
+      observers_[it->first] = it->second;
+    }
+    observers_to_add_.clear();
+  }
+
+  friend class ObserverMapTest;
+};
+
+}  // namespace cc
+
+#endif  // CC_BASE_OBSERVER_MAP_H_