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94 lines
2.8 KiB
C++
94 lines
2.8 KiB
C++
// Copyright 2020 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_SCOPED_OBSERVATION_H_
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#define BASE_SCOPED_OBSERVATION_H_
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#include <stddef.h>
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#include "base/check_op.h"
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namespace base {
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// ScopedObservation is used to keep track of singular observation, e.g.
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// where an observer observes a single source only.
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//
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// Use base::ScopedMultiSourceObservation for objects that observe multiple
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// sources. This class and base::ScopedMultiSourceObservation replace
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// ScopedObserver.
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//
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// When ScopedObservation is destroyed, it removes the registered observation,
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// if any. Basic example (as a member variable):
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//
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// class MyFooObserver : public FooObserver {
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// ...
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// private:
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// ScopedObservation<Foo, FooObserver> foo_observation_{this};
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// };
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//
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// MyFooObserver::MyFooObserver(Foo* foo) {
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// foo_observation_.Observe(foo);
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// }
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//
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// For cases with methods not named AddObserver/RemoveObserver:
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//
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// class MyFooStateObserver : public FooStateObserver {
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// ...
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// private:
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// ScopedObservation<Foo,
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// FooStateObserver,
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// &Foo::AddStateObserver,
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// &Foo::RemoveStateObserver>
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// foo_observation_{this};
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// };
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template <class Source,
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class Observer,
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void (Source::*AddObsFn)(Observer*) = &Source::AddObserver,
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void (Source::*RemoveObsFn)(Observer*) = &Source::RemoveObserver>
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class ScopedObservation {
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public:
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explicit ScopedObservation(Observer* observer) : observer_(observer) {}
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ScopedObservation(const ScopedObservation&) = delete;
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ScopedObservation& operator=(const ScopedObservation&) = delete;
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~ScopedObservation() { Reset(); }
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// Adds the object passed to the constructor as an observer on |source|.
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// IsObserving() must be false.
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void Observe(Source* source) {
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// TODO(https://crbug.com/1145565): Make this a DCHECK once ScopedObserver
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// has been fully retired.
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CHECK_EQ(source_, nullptr);
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source_ = source;
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(source_->*AddObsFn)(observer_);
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}
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// Remove the object passed to the constructor as an observer from |source_|
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// if currently observing. Does nothing otherwise.
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void Reset() {
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if (source_) {
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(source_->*RemoveObsFn)(observer_);
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source_ = nullptr;
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}
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}
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// Returns true if any source is being observed.
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bool IsObserving() const { return source_ != nullptr; }
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// Returns true if |source| is being observed.
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bool IsObservingSource(Source* source) const {
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DCHECK(source);
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return source_ == source;
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}
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private:
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Observer* const observer_;
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// The observed source, if any.
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Source* source_ = nullptr;
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};
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} // namespace base
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#endif // BASE_SCOPED_OBSERVATION_H_
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