1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
base / observer_list_threadsafe.h [blame]
// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_OBSERVER_LIST_THREADSAFE_H_
#define BASE_OBSERVER_LIST_THREADSAFE_H_
#include <unordered_map>
#include <utility>
#include "base/auto_reset.h"
#include "base/base_export.h"
#include "base/check.h"
#include "base/check_op.h"
#include "base/containers/contains.h"
#include "base/dcheck_is_on.h"
#include "base/debug/stack_trace.h"
#include "base/functional/bind.h"
#include "base/location.h"
#include "base/memory/raw_ptr.h"
#include "base/memory/ref_counted.h"
#include "base/observer_list.h"
#include "base/strings/strcat.h"
#include "base/synchronization/lock.h"
#include "base/task/sequenced_task_runner.h"
#include "base/task/single_thread_task_runner.h"
#include "build/build_config.h"
///////////////////////////////////////////////////////////////////////////////
//
// OVERVIEW:
//
// A thread-safe container for a list of observers. This is similar to the
// observer_list (see observer_list.h), but it is more robust for multi-
// threaded situations.
//
// The following use cases are supported:
// * Observers can register for notifications from any sequence. They are
// always notified on the sequence from which they were registered.
// * Any sequence may trigger a notification via Notify().
// * Observers can remove themselves from the observer list inside of a
// callback.
// * If one sequence is notifying observers concurrently with an observer
// removing itself from the observer list, the notifications will be
// silently dropped. However if the observer is currently inside a
// notification callback, the callback will finish running.
//
// By default, observers can be removed from any sequence. However this can be
// error-prone since an observer may be running a callback when it's removed,
// in which case it isn't safe to delete until the callback is finished.
// Consider using the RemoveObserverPolicy::kAddingSequenceOnly template
// parameter, which will CHECK that observers are only removed from the
// sequence where they were added (which is also the sequence that runs
// callbacks).
//
// The drawback of the threadsafe observer list is that notifications are not
// as real-time as the non-threadsafe version of this class. Notifications
// will always be done via PostTask() to another sequence, whereas with the
// non-thread-safe ObserverList, notifications happen synchronously.
//
// Note: this class previously supported synchronous notifications for
// same-sequence observers, but it was error-prone and removed in
// crbug.com/1193750, think twice before re-considering this paradigm.
//
///////////////////////////////////////////////////////////////////////////////
namespace base {
namespace internal {
class BASE_EXPORT ObserverListThreadSafeBase
: public RefCountedThreadSafe<ObserverListThreadSafeBase> {
public:
struct NotificationDataBase {
NotificationDataBase(void* observer_list_in, const Location& from_here_in)
: observer_list(observer_list_in), from_here(from_here_in) {}
raw_ptr<void> observer_list;
Location from_here;
};
ObserverListThreadSafeBase() = default;
ObserverListThreadSafeBase(const ObserverListThreadSafeBase&) = delete;
ObserverListThreadSafeBase& operator=(const ObserverListThreadSafeBase&) =
delete;
protected:
template <typename ObserverType, typename Method>
struct Dispatcher;
template <typename ObserverType, typename ReceiverType, typename... Params>
struct Dispatcher<ObserverType, void (ReceiverType::*)(Params...)> {
static void Run(void (ReceiverType::*m)(Params...),
Params... params,
ObserverType* obj) {
(obj->*m)(std::forward<Params>(params)...);
}
};
static const NotificationDataBase*& GetCurrentNotification();
virtual ~ObserverListThreadSafeBase() = default;
private:
friend class RefCountedThreadSafe<ObserverListThreadSafeBase>;
};
} // namespace internal
enum class RemoveObserverPolicy {
// Observers can be removed from any sequence.
kAnySequence,
// Observers can only be removed from the sequence that added them.
kAddingSequenceOnly,
};
template <class ObserverType,
RemoveObserverPolicy RemovePolicy =
RemoveObserverPolicy::kAnySequence>
class ObserverListThreadSafe : public internal::ObserverListThreadSafeBase {
using Self = ObserverListThreadSafe<ObserverType, RemovePolicy>;
public:
enum class AddObserverResult {
kBecameNonEmpty,
kWasAlreadyNonEmpty,
};
enum class RemoveObserverResult {
kWasOrBecameEmpty,
kRemainsNonEmpty,
};
ObserverListThreadSafe() = default;
explicit ObserverListThreadSafe(ObserverListPolicy policy)
: policy_(policy) {}
ObserverListThreadSafe(const ObserverListThreadSafe&) = delete;
ObserverListThreadSafe& operator=(const ObserverListThreadSafe&) = delete;
// Adds |observer| to the list. |observer| must not already be in the list.
AddObserverResult AddObserver(ObserverType* observer) {
DCHECK(SequencedTaskRunner::HasCurrentDefault())
<< "An observer can only be registered when "
"SequencedTaskRunner::HasCurrentDefault. If this is in a unit test, "
"you're likely merely missing a "
"base::test::(SingleThread)TaskEnvironment in your fixture. "
"Otherwise, try running this code on a named thread (main/UI/IO) or "
"from a task posted to a base::SequencedTaskRunner or "
"base::SingleThreadTaskRunner.";
AutoLock auto_lock(lock_);
bool was_empty = observers_.empty();
// Add |observer| to the list of observers.
DCHECK(!Contains(observers_, observer));
const scoped_refptr<SequencedTaskRunner> task_runner =
SequencedTaskRunner::GetCurrentDefault();
// Each observer gets a unique identifier. These unique identifiers are used
// to avoid execution of pending posted-tasks over removed or released
// observers.
const size_t observer_id = ++observer_id_counter_;
#if DCHECK_IS_ON()
ObserverTaskRunnerInfo task_info = {task_runner, base::debug::StackTrace(),
observer_id};
#else
ObserverTaskRunnerInfo task_info = {task_runner, observer_id};
#endif
observers_[observer] = std::move(task_info);
// If this is called while a notification is being dispatched on this thread
// and |policy_| is ALL, |observer| must be notified (if a notification is
// being dispatched on another thread in parallel, the notification may or
// may not make it to |observer| depending on the outcome of the race to
// |lock_|).
if (policy_ == ObserverListPolicy::ALL) {
if (const NotificationDataBase* const current_notification =
GetCurrentNotification();
current_notification && current_notification->observer_list == this) {
const NotificationData* notification_data =
static_cast<const NotificationData*>(current_notification);
task_runner->PostTask(
current_notification->from_here,
BindOnce(&Self::NotifyWrapper, this,
// While `observer` may be dangling, we pass it and
// check it wasn't deallocated in NotifyWrapper() which can
// check `observers_` to verify presence (the owner of the
// observer is responsible for removing it from that list
// before deallocation).
UnsafeDangling(observer),
NotificationData(this, observer_id,
current_notification->from_here,
notification_data->method)));
}
}
return was_empty ? AddObserverResult::kBecameNonEmpty
: AddObserverResult::kWasAlreadyNonEmpty;
}
// Remove an observer from the list if it is in the list.
//
// If a notification was sent to the observer but hasn't started to run yet,
// it will be aborted. If a notification has started to run, removing the
// observer won't stop it.
RemoveObserverResult RemoveObserver(ObserverType* observer) {
AutoLock auto_lock(lock_);
if constexpr (RemovePolicy == RemoveObserverPolicy::kAddingSequenceOnly) {
const auto it = observers_.find(observer);
CHECK(it == observers_.end() ||
it->second.task_runner->RunsTasksInCurrentSequence());
}
observers_.erase(observer);
return observers_.empty() ? RemoveObserverResult::kWasOrBecameEmpty
: RemoveObserverResult::kRemainsNonEmpty;
}
// Verifies that the list is currently empty (i.e. there are no observers).
void AssertEmpty() const {
#if DCHECK_IS_ON()
AutoLock auto_lock(lock_);
bool observers_is_empty = observers_.empty();
DUMP_WILL_BE_CHECK(observers_is_empty)
<< "\n"
<< GetObserversCreationStackStringLocked();
#endif
}
// Asynchronously invokes a callback on all observers, on their registration
// sequence. You cannot assume that at the completion of the Notify call that
// all Observers have been Notified. The notification may still be pending
// delivery.
template <typename Method, typename... Params>
void Notify(const Location& from_here, Method m, Params&&... params) {
RepeatingCallback<void(ObserverType*)> method =
BindRepeating(&Dispatcher<ObserverType, Method>::Run, m,
std::forward<Params>(params)...);
AutoLock lock(lock_);
for (const auto& observer : observers_) {
observer.second.task_runner->PostTask(
from_here,
BindOnce(&Self::NotifyWrapper, this,
// While `observer.first` may be dangling, we pass it and
// check it wasn't deallocated in NotifyWrapper() which can
// check `observers_` to verify presence (the owner of the
// observer is responsible for removing it from that list
// before deallocation).
UnsafeDangling(observer.first),
NotificationData(this, observer.second.observer_id,
from_here, method)));
}
}
private:
friend class RefCountedThreadSafe<ObserverListThreadSafeBase>;
struct NotificationData : public NotificationDataBase {
NotificationData(ObserverListThreadSafe* observer_list_in,
size_t observer_id_in,
const Location& from_here_in,
const RepeatingCallback<void(ObserverType*)>& method_in)
: NotificationDataBase(observer_list_in, from_here_in),
method(method_in),
observer_id(observer_id_in) {}
RepeatingCallback<void(ObserverType*)> method;
size_t observer_id;
};
~ObserverListThreadSafe() override = default;
void NotifyWrapper(MayBeDangling<ObserverType> observer,
const NotificationData& notification) {
{
AutoLock auto_lock(lock_);
// Check whether the observer still needs a notification.
DCHECK_EQ(notification.observer_list, this);
auto it = observers_.find(observer);
if (it == observers_.end() ||
it->second.observer_id != notification.observer_id) {
return;
}
DCHECK(it->second.task_runner->RunsTasksInCurrentSequence());
}
// Keep track of the notification being dispatched on the current thread.
// This will be used if the callback below calls AddObserver().
//
// Note: GetCurrentNotification() may not return null if this runs in a
// nested loop started by a notification callback. In that case, it is
// important to save the previous value to restore it later.
const AutoReset<const NotificationDataBase*> resetter_(
&GetCurrentNotification(), ¬ification);
// Invoke the callback.
notification.method.Run(observer);
}
std::string GetObserversCreationStackStringLocked() const
EXCLUSIVE_LOCKS_REQUIRED(lock_) {
std::string result;
#if DCHECK_IS_ON()
for (const auto& observer : observers_) {
StrAppend(&result,
{observer.second.add_observer_stack_.ToString(), "\n"});
}
#endif
return result;
}
const ObserverListPolicy policy_ = ObserverListPolicy::ALL;
mutable Lock lock_;
size_t observer_id_counter_ GUARDED_BY(lock_) = 0;
struct ObserverTaskRunnerInfo {
scoped_refptr<SequencedTaskRunner> task_runner;
#if DCHECK_IS_ON()
base::debug::StackTrace add_observer_stack_;
#endif
size_t observer_id = 0;
};
// Keys are observers. Values are the SequencedTaskRunners on which they must
// be notified.
std::unordered_map<ObserverType*, ObserverTaskRunnerInfo> observers_
GUARDED_BY(lock_);
};
} // namespace base
#endif // BASE_OBSERVER_LIST_THREADSAFE_H_