Add a CallbackRegistry class to base/ to manage callbacks

base/callback_list.h

+// Copyright 2013 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 BASE_CALLBACK_LIST_H_
+#define BASE_CALLBACK_LIST_H_
+
+#include <algorithm>
+#include <limits>
+#include <vector>
+
+#include "base/basictypes.h"
+#include "base/bind.h"
+#include "base/callback.h"
+#include "base/memory/weak_ptr.h"
+
+///////////////////////////////////////////////////////////////////////////////
+//
+// OVERVIEW:
+//
+//   A container for a list of callbacks.  Unlike a normal STL vector or list,
+//   this container can be modified during iteration without invalidating the
+//   iterator.  So, it safely handles the case of an callback removing itself
+//   or other callback from the list while callbacks are being run.
+//
+// TYPICAL USAGE:
+//
+//   class MyWidget {
+//    public:
+//     ...
+//
+//     typedef base::Callback<void(Foo*)> OnFooCallback;
+//
+//     base::Closure RegisterCallback(const OnFooCallback& cb) {
+//       return callback_list_.RegisterCallback(cb);
+//     }
+//
+//    private:
+//     void NotifyFoo(Foo* foo) {
+//       FOR_EACH_CALLBACK(OnFooCallback, callback_list_, foo);
+//     }
+//
+//     CallbackList<OnFooCallback> callback_list_;
+//   };
+//
+//
+//  class MyWidgetListener {
+//    public:
+//
+//     MyWidgetListener::MyWidgetListener() {
+//       remove_foo_callback_ = MyWidget::GetCurrent()->RegisterCallback(
+//           base::Bind(&MyWidgetListener::OnFoo, this));
+//     }
+//
+//     MyWidgetListener::~MyWidgetListener() {
+//       remove_foo_callback_.Run();
+//     }
+//
+//     void OnFoo(Foo* foo) {
+//       // Do something.
+//     }
+//
+//    private:
+//     base::Closure remove_foo_callback_;
+//   };
+//
+///////////////////////////////////////////////////////////////////////////////
+
+template <class SIG>
+class CallbackListBase {
+ public:
+  // An iterator class that can be used to access the list of callbacks.  See
+  // also the FOR_EACH_CALLBACK macro defined below.
+  class Iterator {
+   public:
+    Iterator(CallbackListBase<SIG>& list)
+        : list_(list.GetWeakPtr()),
+          index_(0) {
+      ++list_->active_iterator_count_;
+    }
+
+    ~Iterator() {
+      if (list_.get() && --list_->active_iterator_count_ == 0)
+        list_->Compact();
+    }
+
+    SIG GetNext() {
+      if (!list_.get())
+        return SIG();
+      ListType& callbacks = list_->callbacks_;
+      while (index_ < callbacks.size() && callbacks[index_].is_null())
+        ++index_;
+      return index_ < callbacks.size() ? callbacks[index_++] : SIG();
+    }
+
+   private:
+    base::WeakPtr<CallbackListBase<SIG> > list_;
+    size_t index_;
+  };
+
+  CallbackListBase() : active_iterator_count_(0),
+                       weak_factory_(this) {}
+
+  // Add a callback to the list. A callback should not be added to
+  // the same list more than once. The returned closure (used to remove the
+  // callback from the list) is guaranteed to be safe to run.
+  base::Closure RegisterCallback(const SIG& cb) {
+    if (cb.is_null())
+      return base::Bind(&base::DoNothing);

[Avi (use Gerrit), 2013/08/23 16:51:12]

Now that is defensive programming. I'm actually laughing as I read this.

OTOH, is that an idiotic enough behavior to warrant a DCHECK?

[Cait (Slow), 2013/08/23 18:21:10]

Done.

+
+    for (size_t i = 0; i < callbacks_.size(); i++) {
+      if (callbacks_[i].Equals(cb)) {
+        NOTREACHED() << "Callbacks can only be added once!";
+        return base::Bind(&base::DoNothing);
+      }
+    }
+    callbacks_.push_back(cb);
+    base::Closure removal_cb = base::Bind(
+        &CallbackListBase::RemoveCallback, GetWeakPtr(), cb);
+    return removal_cb;
+  }
+
+  // Remove an callback from the list if it is in the list.
+  void RemoveCallback(const SIG& cb) {
+    for (size_t i = 0; i < callbacks_.size(); i++) {
+      if (callbacks_[i].Equals(cb)) {
+        if (active_iterator_count_)
+          callbacks_[i].Reset();
+        else
+          callbacks_.erase(callbacks_.begin() + i);
+        return;
+      }
+    }
+  }
+
+  bool HasCallback(const SIG& cb) const {
+    for (size_t i = 0; i < callbacks_.size(); ++i) {
+      if (callbacks_[i].Equals(cb))
+        return true;
+    }
+    return false;
+  }
+
+  void Clear() {
+    if (active_iterator_count_) {
+      for (size_t i = 0; i < callbacks_.size(); ++i)
+        callbacks_[i].Reset();
+    } else {
+      callbacks_.clear();
+    }
+  }
+
+ protected:
+  base::WeakPtr<CallbackListBase<SIG> > GetWeakPtr() {
+    return weak_factory_.GetWeakPtr();
+  }
+
+  void Compact() {
+    for (size_t i = 0; i < callbacks_.size(); i++) {
+      if (callbacks_[i].is_null()) {
+        callbacks_.erase(callbacks_.begin() + i);
+      }
+    }
+  }
+
+  size_t size() const { return callbacks_.size(); }
+
+ private:
+  friend class CallbackListBase::Iterator;
+  typedef std::vector<SIG> ListType;
+  ListType callbacks_;
+  int active_iterator_count_;
+  base::WeakPtrFactory<CallbackListBase<SIG> > weak_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(CallbackListBase);
+};
+
+template <class SIG, bool check_empty = false>
+class CallbackList : public CallbackListBase<SIG> {
+ public:
+  CallbackList() {}
+
+  ~CallbackList() {
+    // When check_empty is true, assert that the list is empty on destruction.
+    if (check_empty) {
+      CallbackListBase<SIG>::Compact();
+      DCHECK_EQ(0U, CallbackListBase<SIG>::size());
+    }
+  }
+
+  bool might_have_callbacks() const {
+    return CallbackListBase<SIG>::size() != 0;
+  }
+};
+
+#define FOR_EACH_CALLBACK(CallbackType, callback_list, ...)          \
+  do {                                                               \
+    if ((callback_list).might_have_callbacks()) {                    \
+      CallbackListBase<CallbackType>::Iterator it(callback_list);    \
+      CallbackType cb;                                               \
+      while(!(cb = it.GetNext()).is_null())                          \
+        cb.Run(__VA_ARGS__);                                         \
+    }                                                                \
+  } while (0)
+
+#endif  // BASE_CALLBACK_LIST_H_