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// Copyright 2020 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_SCOPED_OBSERVATION_H_
#define BASE_SCOPED_OBSERVATION_H_
#include <stddef.h>
#include "base/check.h"
#include "base/check_op.h"
#include "base/memory/raw_ptr.h"
namespace base {
// `ScopedObservation` is used to keep track of a singular observation, i.e.,
// where an observer observes a single source only. Use
// `base::ScopedMultiSourceObservation` for objects that observe multiple
// sources.
//
// When a `ScopedObservation` is destroyed, it unregisters the observer from the
// observable if it was currently observing something. Otherwise it does
// nothing.
//
// Using a `ScopedObservation` instead of manually observing and unobserving has
// the following benefits:
// - The observer cannot accidentally forget to stop observing when it is
// destroyed.
// - By calling `Reset`, an ongoing observation can be stopped before the
// `ScopedObservation` is destroyed. If nothing was currently observed, then
// calling `Reset` does nothing. This can be useful for when the observable is
// destroyed before the observer is destroyed, because it prevents the
// observer from accidentally unregistering itself from the destroyed
// observable a second time when it itself is destroyed. Without
// `ScopedObservation`, one might need to keep track of whether one has
// already stopped observing in a separate boolean.
//
// Basic example (as a member variable):
//
// class MyObserver : public Observable::Observer {
// public:
// MyObserver(Observable* observable) {
// observation_.Observe(observable);
// }
// // Note how there is no need to stop observing in the destructor.
// private:
// ScopedObservation<Observable, Observable::Observer> observation_{this};
// };
//
// For cases with methods not named AddObserver/RemoveObserver:
//
// class MyStateObserver : public Observable::StateObserver {
// ...
// private:
// ScopedObservation<Observable,
// Observable::StateObserver,
// &Observable::AddStateObserver,
// &Observable::RemoveStateObserver>
// observation_{this};
// };
template <class Source,
class Observer,
void (Source::*AddObsFn)(Observer*) = &Source::AddObserver,
void (Source::*RemoveObsFn)(Observer*) = &Source::RemoveObserver>
class ScopedObservation {
public:
explicit ScopedObservation(Observer* observer) : observer_(observer) {}
ScopedObservation(const ScopedObservation&) = delete;
ScopedObservation& operator=(const ScopedObservation&) = delete;
~ScopedObservation() { Reset(); }
// Adds the object passed to the constructor as an observer on |source|.
// IsObserving() must be false.
void Observe(Source* source) {
// TODO(https://crbug.com/1145565): Make this a DCHECK once ScopedObserver
// has been fully retired.
CHECK_EQ(source_, nullptr);
source_ = source;
(source_.get()->*AddObsFn)(observer_);
}
// Remove the object passed to the constructor as an observer from |source_|
// if currently observing. Does nothing otherwise.
void Reset() {
if (source_) {
(source_.get()->*RemoveObsFn)(observer_);
source_ = nullptr;
}
}
// Returns true if any source is being observed.
bool IsObserving() const { return source_ != nullptr; }
// Returns true if |source| is being observed.
bool IsObservingSource(Source* source) const {
DCHECK(source);
return source_ == source;
}
private:
const raw_ptr<Observer> observer_;
// The observed source, if any.
raw_ptr<Source> source_ = nullptr;
};
} // namespace base
#endif // BASE_SCOPED_OBSERVATION_H_
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