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// Copyright (c) 2011 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_OBSERVER_LIST_H_
#define BASE_OBSERVER_LIST_H_
#include <stddef.h>
#include <algorithm>
#include <iterator>
#include <limits>
#include <string>
#include <utility>
#include <vector>
#include "base/check_op.h"
#include "base/notreached.h"
#include "base/observer_list_internal.h"
#include "base/ranges/algorithm.h"
#include "base/sequence_checker.h"
///////////////////////////////////////////////////////////////////////////////
//
// OVERVIEW:
//
// A list of observers. Unlike a standard vector or list, this container can
// be modified during iteration without invalidating the iterator. So, it
// safely handles the case of an observer removing itself or other observers
// from the list while observers are being notified.
//
//
// WARNING:
//
// ObserverList is not thread-compatible. Iterating on the same ObserverList
// simultaneously in different threads is not safe, even when the ObserverList
// itself is not modified.
//
// For a thread-safe observer list, see ObserverListThreadSafe.
//
//
// TYPICAL USAGE:
//
// class MyWidget {
// public:
// ...
//
// class Observer : public base::CheckedObserver {
// public:
// virtual void OnFoo(MyWidget* w) = 0;
// virtual void OnBar(MyWidget* w, int x, int y) = 0;
// };
//
// void AddObserver(Observer* obs) {
// observers_.AddObserver(obs);
// }
//
// void RemoveObserver(Observer* obs) {
// observers_.RemoveObserver(obs);
// }
//
// void NotifyFoo() {
// for (Observer& obs : observers_)
// obs.OnFoo(this);
// }
//
// void NotifyBar(int x, int y) {
// for (Observer& obs : observers_)
// obs.OnBar(this, x, y);
// }
//
// private:
// base::ObserverList<Observer> observers_;
// };
//
//
///////////////////////////////////////////////////////////////////////////////
namespace base {
// Enumeration of which observers are notified by ObserverList.
enum class ObserverListPolicy {
// Specifies that any observers added during notification are notified.
// This is the default policy if no policy is provided to the constructor.
ALL,
// Specifies that observers added while sending out notification are not
// notified.
EXISTING_ONLY,
};
// When check_empty is true, assert that the list is empty on destruction.
// When allow_reentrancy is false, iterating throught the list while already in
// the iteration loop will result in DCHECK failure.
// TODO(oshima): Change the default to non reentrant. https://crbug.com/812109
template <class ObserverType,
bool check_empty = false,
bool allow_reentrancy = true,
class ObserverStorageType = internal::CheckedObserverAdapter>
class ObserverList {
public:
// Allow declaring an ObserverList<...>::Unchecked that replaces the default
// ObserverStorageType to use raw pointers. This is required to support legacy
// observers that do not inherit from CheckedObserver. The majority of new
// code should not use this, but it may be suited for performance-critical
// situations to avoid overheads of a CHECK(). Note the type can't be chosen
// based on ObserverType's definition because ObserverLists are often declared
// in headers using a forward-declare of ObserverType.
using Unchecked = ObserverList<ObserverType,
check_empty,
allow_reentrancy,
internal::UncheckedObserverAdapter>;
// An iterator class that can be used to access the list of observers.
class Iter {
public:
using iterator_category = std::forward_iterator_tag;
using value_type = ObserverType;
using difference_type = ptrdiff_t;
using pointer = ObserverType*;
using reference = ObserverType&;
Iter() : index_(0), max_index_(0) {}
explicit Iter(const ObserverList* list)
: list_(const_cast<ObserverList*>(list)),
index_(0),
max_index_(list->policy_ == ObserverListPolicy::ALL
? std::numeric_limits<size_t>::max()
: list->observers_.size()) {
DCHECK(list);
DCHECK(allow_reentrancy || list_.IsOnlyRemainingNode());
// Bind to this sequence when creating the first iterator.
DCHECK_CALLED_ON_VALID_SEQUENCE(list_->iteration_sequence_checker_);
EnsureValidIndex();
}
~Iter() {
if (list_.IsOnlyRemainingNode())
list_->Compact();
}
Iter(const Iter& other)
: index_(other.index_), max_index_(other.max_index_) {
if (other.list_)
list_.SetList(other.list_.get());
}
Iter& operator=(const Iter& other) {
if (&other == this)
return *this;
if (list_.IsOnlyRemainingNode())
list_->Compact();
list_.Invalidate();
if (other.list_)
list_.SetList(other.list_.get());
index_ = other.index_;
max_index_ = other.max_index_;
return *this;
}
bool operator==(const Iter& other) const {
return (is_end() && other.is_end()) ||
(list_.get() == other.list_.get() && index_ == other.index_);
}
bool operator!=(const Iter& other) const { return !(*this == other); }
Iter& operator++() {
if (list_) {
++index_;
EnsureValidIndex();
}
return *this;
}
Iter operator++(int) {
Iter it(*this);
++(*this);
return it;
}
ObserverType* operator->() const {
ObserverType* const current = GetCurrent();
DCHECK(current);
return current;
}
ObserverType& operator*() const {
ObserverType* const current = GetCurrent();
DCHECK(current);
return *current;
}
private:
friend class ObserverListTestBase;
ObserverType* GetCurrent() const {
DCHECK(list_);
DCHECK_LT(index_, clamped_max_index());
return ObserverStorageType::template Get<ObserverType>(
list_->observers_[index_]);
}
void EnsureValidIndex() {
DCHECK(list_);
const size_t max_index = clamped_max_index();
while (index_ < max_index &&
list_->observers_[index_].IsMarkedForRemoval()) {
++index_;
}
}
size_t clamped_max_index() const {
return std::min(max_index_, list_->observers_.size());
}
bool is_end() const { return !list_ || index_ == clamped_max_index(); }
// Lightweight weak pointer to the ObserverList.
internal::WeakLinkNode<ObserverList> list_;
// When initially constructed and each time the iterator is incremented,
// |index_| is guaranteed to point to a non-null index if the iterator
// has not reached the end of the ObserverList.
size_t index_;
size_t max_index_;
};
using iterator = Iter;
using const_iterator = Iter;
using value_type = ObserverType;
const_iterator begin() const {
// An optimization: do not involve weak pointers for empty list.
return observers_.empty() ? const_iterator() : const_iterator(this);
}
const_iterator end() const { return const_iterator(); }
explicit ObserverList(ObserverListPolicy policy = ObserverListPolicy::ALL)
: policy_(policy) {
// Sequence checks only apply when iterators are live.
DETACH_FROM_SEQUENCE(iteration_sequence_checker_);
}
ObserverList(const ObserverList&) = delete;
ObserverList& operator=(const ObserverList&) = delete;
~ObserverList() {
// If there are live iterators, ensure destruction is thread-safe.
if (!live_iterators_.empty())
DCHECK_CALLED_ON_VALID_SEQUENCE(iteration_sequence_checker_);
while (!live_iterators_.empty())
live_iterators_.head()->value()->Invalidate();
if (check_empty) {
Compact();
DCHECK(observers_.empty()) << GetObserversCreationStackString();
}
}
// Add an observer to this list. An observer should not be added to the same
// list more than once.
//
// Precondition: obs != nullptr
// Precondition: !HasObserver(obs)
void AddObserver(ObserverType* obs) {
DCHECK(obs);
if (HasObserver(obs)) {
NOTREACHED() << "Observers can only be added once!";
return;
}
observers_count_++;
observers_.emplace_back(ObserverStorageType(obs));
}
// Removes the given observer from this list. Does nothing if this observer is
// not in this list.
void RemoveObserver(const ObserverType* obs) {
DCHECK(obs);
const auto it = ranges::find_if(
observers_, [obs](const auto& o) { return o.IsEqual(obs); });
if (it == observers_.end())
return;
if (!it->IsMarkedForRemoval())
observers_count_--;
if (live_iterators_.empty()) {
observers_.erase(it);
} else {
DCHECK_CALLED_ON_VALID_SEQUENCE(iteration_sequence_checker_);
it->MarkForRemoval();
}
}
// Determine whether a particular observer is in the list.
bool HasObserver(const ObserverType* obs) const {
// Client code passing null could be confused by the treatment of observers
// removed mid-iteration. TODO(https://crbug.com/876588): This should
// probably DCHECK, but some client code currently does pass null.
if (obs == nullptr)
return false;
return ranges::find_if(observers_, [obs](const auto& o) {
return o.IsEqual(obs);
}) != observers_.end();
}
// Removes all the observers from this list.
void Clear() {
if (live_iterators_.empty()) {
observers_.clear();
} else {
DCHECK_CALLED_ON_VALID_SEQUENCE(iteration_sequence_checker_);
for (auto& observer : observers_)
observer.MarkForRemoval();
}
observers_count_ = 0;
}
bool empty() const { return !observers_count_; }
private:
friend class internal::WeakLinkNode<ObserverList>;
// Compacts list of observers by removing those marked for removal.
void Compact() {
// Detach whenever the last iterator is destroyed. Detaching is safe because
// Compact() is only ever called when the last iterator is destroyed.
DETACH_FROM_SEQUENCE(iteration_sequence_checker_);
observers_.erase(
std::remove_if(observers_.begin(), observers_.end(),
[](const auto& o) { return o.IsMarkedForRemoval(); }),
observers_.end());
}
std::string GetObserversCreationStackString() const {
#if EXPENSIVE_DCHECKS_ARE_ON()
std::string result;
for (const auto& observer : observers_) {
result += observer.GetCreationStackString();
result += "\n";
}
return result;
#else
return "For observer stack traces, build with "
"`enable_expensive_dchecks=true`.";
#endif // EXPENSIVE_DCHECKS_ARE_ON()
}
std::vector<ObserverStorageType> observers_;
base::LinkedList<internal::WeakLinkNode<ObserverList>> live_iterators_;
size_t observers_count_{0};
const ObserverListPolicy policy_;
SEQUENCE_CHECKER(iteration_sequence_checker_);
};
template <class ObserverType, bool check_empty = false>
using ReentrantObserverList = ObserverList<ObserverType, check_empty, true>;
} // namespace base
#endif // BASE_OBSERVER_LIST_H_