text: // 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 <list>
  #include <memory>
  
  #include "base/bind.h"
  #include "base/callback.h"
  #include "base/compiler_specific.h"
  #include "base/logging.h"
  #include "base/macros.h"
  #include "base/memory/weak_ptr.h"
  
  // OVERVIEW:
  //
  // A container for a list of (repeating) callbacks. Unlike a normal vector or
  // list, this container can be modified during iteration without invalidating
  // the iterator. It safely handles the case of a callback removing itself or
  // another callback from the list while callbacks are being run.
  //
  // TYPICAL USAGE:
  //
  // class MyWidget {
  //  public:
  //   ...
  //
  //   std::unique_ptr<base::CallbackList<void(const Foo&)>::Subscription>
  //   RegisterCallback(const base::RepeatingCallback<void(const Foo&)>& cb) {
  //     return callback_list_.Add(cb);
  //   }
  //
  //  private:
  //   void NotifyFoo(const Foo& foo) {
  //      callback_list_.Notify(foo);
  //   }
  //
  //   base::CallbackList<void(const Foo&)> callback_list_;
  //
  //   DISALLOW_COPY_AND_ASSIGN(MyWidget);
  // };
  //
  //
  // class MyWidgetListener {
  //  public:
  //   MyWidgetListener::MyWidgetListener() {
  //     foo_subscription_ = MyWidget::GetCurrent()->RegisterCallback(
  //             base::BindRepeating(&MyWidgetListener::OnFoo, this)));
  //   }
  //
  //   MyWidgetListener::~MyWidgetListener() {
  //      // Subscription gets deleted automatically and will deregister
  //      // the callback in the process.
  //   }
  //
  //  private:
  //   void OnFoo(const Foo& foo) {
  //     // Do something.
  //   }
  //
  //   std::unique_ptr<base::CallbackList<void(const Foo&)>::Subscription>
  //       foo_subscription_;
  //
  //   DISALLOW_COPY_AND_ASSIGN(MyWidgetListener);
  // };
  
  namespace base {
  
  namespace internal {
  
  template <typename CallbackType>
  class CallbackListBase {
   public:
    class Subscription {
     public:
      explicit Subscription(base::OnceClosure subscription_destroyed)
          : subscription_destroyed_(std::move(subscription_destroyed)) {}
  
      ~Subscription() { std::move(subscription_destroyed_).Run(); }
  
      // Returns true if the CallbackList associated with this subscription has
      // been deleted, which means that the associated callback will no longer be
      // invoked.
      bool IsCancelled() const { return subscription_destroyed_.IsCancelled(); }
  
     private:
      base::OnceClosure subscription_destroyed_;
  
      DISALLOW_COPY_AND_ASSIGN(Subscription);
    };
  
    // Add a callback to the list. The callback will remain registered until the
    // returned Subscription is destroyed. When the CallbackList is destroyed, any
    // outstanding subscriptions are safely invalidated.
    std::unique_ptr<Subscription> Add(const CallbackType& cb) WARN_UNUSED_RESULT {
      DCHECK(!cb.is_null());
      return std::make_unique<Subscription>(
          base::BindOnce(&CallbackListBase::OnSubscriptionDestroyed,
                         weak_ptr_factory_.GetWeakPtr(),
                         callbacks_.insert(callbacks_.end(), cb)));
    }
  
    // Sets a callback which will be run when a subscription list is changed.
    void set_removal_callback(const RepeatingClosure& callback) {
      removal_callback_ = callback;
    }
  
    // Returns true if there are no subscriptions. This is only valid to call when
    // not looping through the list.
    bool empty() {
      DCHECK_EQ(0u, active_iterator_count_);
      return callbacks_.empty();
    }
  
   protected:
    // An iterator class that can be used to access the list of callbacks.
    class Iterator {
     public:
      explicit Iterator(CallbackListBase<CallbackType>* list)
          : list_(list),
            list_iter_(list_->callbacks_.begin()) {
        ++list_->active_iterator_count_;
      }
  
      Iterator(const Iterator& iter)
          : list_(iter.list_),
            list_iter_(iter.list_iter_) {
        ++list_->active_iterator_count_;
      }
  
      ~Iterator() {
        if (list_ && --list_->active_iterator_count_ == 0) {
          list_->Compact();
        }
      }
  
      CallbackType* GetNext() {
        while ((list_iter_ != list_->callbacks_.end()) && list_iter_->is_null())
          ++list_iter_;
  
        CallbackType* cb = nullptr;
        if (list_iter_ != list_->callbacks_.end()) {
          cb = &(*list_iter_);
          ++list_iter_;
        }
        return cb;
      }
  
     private:
      CallbackListBase<CallbackType>* list_;
      typename std::list<CallbackType>::iterator list_iter_;
    };
  
    CallbackListBase() = default;
  
    ~CallbackListBase() { DCHECK_EQ(0u, active_iterator_count_); }
  
    // Returns an instance of a CallbackListBase::Iterator which can be used
    // to run callbacks.
    Iterator GetIterator() {
      return Iterator(this);
    }
  
    // Compact the list: remove any entries which were nulled out during
    // iteration.
    void Compact() {
      auto it = callbacks_.begin();
      bool updated = false;
      while (it != callbacks_.end()) {
        if ((*it).is_null()) {
          updated = true;
          it = callbacks_.erase(it);
        } else {
          ++it;
        }
      }
  
      if (updated && !removal_callback_.is_null())
        removal_callback_.Run();
    }
  
   private:
    void OnSubscriptionDestroyed(
        const typename std::list<CallbackType>::iterator& iter) {
      if (active_iterator_count_) {
        iter->Reset();
      } else {
        callbacks_.erase(iter);
        if (removal_callback_)
          removal_callback_.Run();
      }
      // Note that |removal_callback_| may destroy |this|.
    }
  
    std::list<CallbackType> callbacks_;
    size_t active_iterator_count_ = 0;
    RepeatingClosure removal_callback_;
    WeakPtrFactory<CallbackListBase> weak_ptr_factory_{this};
  
    DISALLOW_COPY_AND_ASSIGN(CallbackListBase);
  };
  
  }  // namespace internal
  
  template <typename Sig> class CallbackList;
  
  template <typename... Args>
  class CallbackList<void(Args...)>
      : public internal::CallbackListBase<RepeatingCallback<void(Args...)>> {
   public:
    using CallbackType = RepeatingCallback<void(Args...)>;
  
    CallbackList() = default;
  
    template <typename... RunArgs>
    void Notify(RunArgs&&... args) {
      auto it = this->GetIterator();
      CallbackType* cb;
      while ((cb = it.GetNext()) != nullptr) {
        cb->Run(args...);
      }
    }
  
   private:
    DISALLOW_COPY_AND_ASSIGN(CallbackList);
  };
  
  }  // namespace base
  
  #endif  // BASE_CALLBACK_LIST_H_
  