weak_ptr.h

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// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
// This file provides weak pointers and weak pointer factories that work like
// Chromium's |base::WeakPtr<T>| and |base::WeakPtrFactory<T>|.
 
#ifndef FLUTTER_FML_MEMORY_WEAK_PTR_H_
#define FLUTTER_FML_MEMORY_WEAK_PTR_H_
 
#include <utility>
 
#include "flutter/fml/logging.h"
#include "flutter/fml/memory/ref_counted.h"
#include "flutter/fml/memory/task_runner_checker.h"
#include "flutter/fml/memory/thread_checker.h"
#include "flutter/fml/memory/weak_ptr_internal.h"
 
namespace fml {
 
struct DebugThreadChecker {
  FML_DECLARE_THREAD_CHECKER(checker);
};
 
struct DebugTaskRunnerChecker {
  FML_DECLARE_TASK_RUNNER_CHECKER(checker);
};
 
// Forward declaration, so |WeakPtr<T>| can friend it.
template <typename T>
class WeakPtrFactory;
 
// Class for "weak pointers" that can be invalidated. Valid weak pointers
// can only originate from a |WeakPtrFactory| (see below), though weak
// pointers are copyable and movable.
//
// Weak pointers are not in general thread-safe. They may only be *used* on
// a single thread, namely the same thread as the "originating"
// |WeakPtrFactory| (which can invalidate the weak pointers that it
// generates).
//
// However, weak pointers may be passed to other threads, reset on other
// threads, or destroyed on other threads. They may also be reassigned on
// other threads (in which case they should then only be used on the thread
// corresponding to the new "originating" |WeakPtrFactory|).
template <typename T>
class WeakPtr {
 public:
  WeakPtr() : ptr_(nullptr) {}
 
  // Copy constructor.
  // NOLINTNEXTLINE(google-explicit-constructor)
  WeakPtr(const WeakPtr<T>& r) = default;
 
  template <typename U>
  WeakPtr(const WeakPtr<U>& r)  // NOLINT(google-explicit-constructor)
      : ptr_(static_cast<T*>(r.ptr_)), flag_(r.flag_), checker_(r.checker_) {}
 
  // Move constructor.
  WeakPtr(WeakPtr<T>&& r) = default;
 
  template <typename U>
  WeakPtr(WeakPtr<U>&& r)  // NOLINT(google-explicit-constructor)
      : ptr_(static_cast<T*>(r.ptr_)),
        flag_(std::move(r.flag_)),
        checker_(r.checker_) {}
 
  // The following methods are thread-friendly, in the sense that they may be
  // called subject to additional synchronization.
 
  // Copy assignment.
  WeakPtr<T>& operator=(const WeakPtr<T>& r) = default;
 
  // Move assignment.
  WeakPtr<T>& operator=(WeakPtr<T>&& r) = default;
 
  void reset() { flag_ = nullptr; }
 
  // The following methods should only be called on the same thread as the
  // "originating" |WeakPtrFactory|.
 
  explicit operator bool() const {
    CheckThreadSafety();
    return flag_ && flag_->is_valid();
  }
 
  T* get() const {
    CheckThreadSafety();
    return *this ? ptr_ : nullptr;
  }
 
  T& operator*() const {
    CheckThreadSafety();
    FML_DCHECK(*this);
    return *get();
  }
 
  T* operator->() const {
    CheckThreadSafety();
    FML_DCHECK(*this);
    return get();
  }
 
 protected:
  explicit WeakPtr(T* ptr, fml::RefPtr<fml::internal::WeakPtrFlag>&& flag)
      : ptr_(ptr), flag_(std::move(flag)) {}
 
  void CheckThreadSafety() const {
    FML_DCHECK_CREATION_THREAD_IS_CURRENT(checker_.checker);
  }
 
 private:
  template <typename U>
  friend class WeakPtr;
 
  friend class WeakPtrFactory<T>;
 
  explicit WeakPtr(T* ptr,
                   fml::RefPtr<fml::internal::WeakPtrFlag>&& flag,
                   const DebugThreadChecker& checker)
      : ptr_(ptr), flag_(std::move(flag)), checker_(checker) {}
  T* ptr_;
  fml::RefPtr<fml::internal::WeakPtrFlag> flag_;
  DebugThreadChecker checker_;
 
  // Copy/move construction/assignment supported.
};
 
// Forward declaration, so |TaskRunnerAffineWeakPtr<T>| can friend it.
template <typename T>
class TaskRunnerAffineWeakPtrFactory;
 
// A weak pointer that can be used in different threads as long as
// the threads are belong to the same |TaskRunner|.
//
// It is still not in general thread safe as |WeakPtr|.
template <typename T>
class TaskRunnerAffineWeakPtr {
 public:
  TaskRunnerAffineWeakPtr() : ptr_(nullptr) {}
 
  TaskRunnerAffineWeakPtr(const TaskRunnerAffineWeakPtr<T>& r) = default;
 
  template <typename U>
  // NOLINTNEXTLINE(google-explicit-constructor)
  TaskRunnerAffineWeakPtr(const TaskRunnerAffineWeakPtr<U>& r)
      : ptr_(static_cast<T*>(r.ptr_)), flag_(r.flag_), checker_(r.checker_) {}
 
  TaskRunnerAffineWeakPtr(TaskRunnerAffineWeakPtr<T>&& r) = default;
 
  template <typename U>
  // NOLINTNEXTLINE(google-explicit-constructor)
  TaskRunnerAffineWeakPtr(TaskRunnerAffineWeakPtr<U>&& r)
      : ptr_(static_cast<T*>(r.ptr_)),
        flag_(std::move(r.flag_)),
        checker_(r.checker_) {}
 
  ~TaskRunnerAffineWeakPtr() = default;
 
  TaskRunnerAffineWeakPtr<T>& operator=(const TaskRunnerAffineWeakPtr<T>& r) =
      default;
 
  TaskRunnerAffineWeakPtr<T>& operator=(TaskRunnerAffineWeakPtr<T>&& r) =
      default;
 
  void reset() { flag_ = nullptr; }
 
  // The following methods should only be called on the same thread as the
  // "originating" |TaskRunnerAffineWeakPtrFactory|.
 
  explicit operator bool() const {
    CheckThreadSafety();
    return flag_ && flag_->is_valid();
  }
 
  T* get() const {
    CheckThreadSafety();
    return *this ? ptr_ : nullptr;
  }
 
  T& operator*() const {
    CheckThreadSafety();
    FML_DCHECK(*this);
    return *get();
  }
 
  T* operator->() const {
    CheckThreadSafety();
    FML_DCHECK(*this);
    return get();
  }
 
 protected:
  void CheckThreadSafety() const {
    FML_DCHECK_TASK_RUNNER_IS_CURRENT(checker_.checker);
  }
 
 private:
  template <typename U>
  friend class TaskRunnerAffineWeakPtr;
  friend class TaskRunnerAffineWeakPtrFactory<T>;
 
  explicit TaskRunnerAffineWeakPtr(
      T* ptr,
      fml::RefPtr<fml::internal::WeakPtrFlag>&& flag,
      const DebugTaskRunnerChecker& checker)
      : ptr_(ptr), flag_(std::move(flag)), checker_(checker) {}
 
  T* ptr_;
  fml::RefPtr<fml::internal::WeakPtrFlag> flag_;
  DebugTaskRunnerChecker checker_;
};
 
// Class that produces (valid) |WeakPtr<T>|s. Typically, this is used as a
// member variable of |T| (preferably the last one -- see below), and |T|'s
// methods control how weak pointers to it are vended. This class is not
// thread-safe, and should only be created, destroyed and used on a single
// thread.
//
// Example:
//
//  class Controller {
//   public:
//    Controller() : ..., weak_factory_(this) {}
//    ...
//
//    void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
//    void WorkComplete(const Result& result) { ... }
//
//   private:
//    ...
//
//    // Member variables should appear before the |WeakPtrFactory|, to ensure
//    // that any |WeakPtr|s to |Controller| are invalidated before its member
//    // variables' destructors are executed.
//    WeakPtrFactory<Controller> weak_factory_;
//  };
//
//  class Worker {
//   public:
//    static void StartNew(const WeakPtr<Controller>& controller) {
//      Worker* worker = new Worker(controller);
//      // Kick off asynchronous processing....
//    }
//
//   private:
//    Worker(const WeakPtr<Controller>& controller) : controller_(controller) {}
//
//    void DidCompleteAsynchronousProcessing(const Result& result) {
//      if (controller_)
//        controller_->WorkComplete(result);
//    }
//
//    WeakPtr<Controller> controller_;
//  };
template <typename T>
class WeakPtrFactory {
 public:
  explicit WeakPtrFactory(T* ptr)
      : ptr_(ptr), flag_(fml::MakeRefCounted<fml::internal::WeakPtrFlag>()) {
    FML_DCHECK(ptr_);
  }
 
  ~WeakPtrFactory() {
    CheckThreadSafety();
    flag_->Invalidate();
  }
 
  // Gets a new weak pointer, which will be valid until this object is
  // destroyed.
  WeakPtr<T> GetWeakPtr() const {
    return WeakPtr<T>(ptr_, flag_.Clone(), checker_);
  }
 
 private:
  // Note: See weak_ptr_internal.h for an explanation of why we store the
  // pointer here, instead of in the "flag".
  T* const ptr_;
  fml::RefPtr<fml::internal::WeakPtrFlag> flag_;
 
  void CheckThreadSafety() const {
    FML_DCHECK_CREATION_THREAD_IS_CURRENT(checker_.checker);
  }
 
  DebugThreadChecker checker_;
 
  FML_DISALLOW_COPY_AND_ASSIGN(WeakPtrFactory);
};
 
// A type of |WeakPtrFactory| that produces |TaskRunnerAffineWeakPtr| instead of
// |WeakPtr|.
template <typename T>
class TaskRunnerAffineWeakPtrFactory {
 public:
  explicit TaskRunnerAffineWeakPtrFactory(T* ptr)
      : ptr_(ptr), flag_(fml::MakeRefCounted<fml::internal::WeakPtrFlag>()) {
    FML_DCHECK(ptr_);
  }
 
  ~TaskRunnerAffineWeakPtrFactory() {
    CheckThreadSafety();
    flag_->Invalidate();
  }
 
  // Gets a new weak pointer, which will be valid until this object is
  // destroyed.
  TaskRunnerAffineWeakPtr<T> GetWeakPtr() const {
    return TaskRunnerAffineWeakPtr<T>(ptr_, flag_.Clone(), checker_);
  }
 
 private:
  // Note: See weak_ptr_internal.h for an explanation of why we store the
  // pointer here, instead of in the "flag".
  T* const ptr_;
  fml::RefPtr<fml::internal::WeakPtrFlag> flag_;
 
  void CheckThreadSafety() const {
    FML_DCHECK_TASK_RUNNER_IS_CURRENT(checker_.checker);
  }
 
  DebugTaskRunnerChecker checker_;
 
  FML_DISALLOW_COPY_AND_ASSIGN(TaskRunnerAffineWeakPtrFactory);
};
 
}  // namespace fml
 
#endif  // FLUTTER_FML_MEMORY_WEAK_PTR_H_