Bring in a reduced/simplified version of Chromium's base::WeakPtr to //mojo/edk/util.

mojo/edk/util/weak_ptr.h

Index: mojo/edk/util/weak_ptr.h
diff --git a/mojo/edk/util/weak_ptr.h b/mojo/edk/util/weak_ptr.h
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+// Copyright 2016 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.
+
+// This file provides weak pointers and weak pointer factories that work like
+// Chromium's |base::WeakPtr<T>| and |base::WeakPtrFactory<T>|. Note that we do
+// not provide anything analogous to |base::SupportsWeakPtr<T>|.
+
+#ifndef MOJO_UTIL_WEAK_PTR_H_
+#define MOJO_UTIL_WEAK_PTR_H_
+
+#include <assert.h>
+
+#include <utility>
+
+#include "mojo/edk/util/ref_ptr.h"
+#include "mojo/edk/util/weak_ptr_internal.h"
+
+namespace mojo {
+namespace util {
+
+// 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.
+  WeakPtr(const WeakPtr<T>& r) = default;
+
+  template <typename U>
+  WeakPtr(const WeakPtr<U>& r) : ptr_(r.ptr_), flag_(r.flag_) {}
+
+  // Move constructor.
+  WeakPtr(WeakPtr<T>&& r) = default;
+
+  template <typename U>
+  WeakPtr(WeakPtr<U>&& r) : ptr_(r.ptr_), flag_(std::move(r.flag_)) {}
+
+  ~WeakPtr() = default;
+
+  // 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;
+
+  template <typename U>
+  WeakPtr<T>& operator=(const WeakPtr<T>& r) {
+    ptr_ = r.ptr_;
+    flag_ = r.flag_;
+  }
+
+  // Move assignment.
+  WeakPtr<T>& operator=(WeakPtr<T>&& r) = default;
+
+  template <typename U>
+  WeakPtr<T>& operator=(WeakPtr<T>&& r) {
+    ptr_ = r.ptr_;
+    flag_ = std::move(r.flag_);
+  }
+
+  void reset() { flag_ = nullptr; }
+
+  // The following methods should only be called on the same thread as the
+  // "originating" |WeakPtrFactory|.
+
+  explicit operator bool() const { return flag_ && flag_->is_valid(); }
+
+  T* get() const { return *this ? ptr_ : nullptr; }
+
+  T& operator*() const {
+    assert(*this);
+    return *get();
+  }
+
+  T* operator->() const {
+    assert(*this);
+    return get();
+  }
+
+ private:
+  template <typename U>
+  friend class WeakPtr;
+
+  friend class WeakPtrFactory<T>;
+
+  explicit WeakPtr(T* ptr, RefPtr<internal::WeakPtrFlag>&& flag)
+      : ptr_(ptr), flag_(std::move(flag)) {}
+
+  T* ptr_;
+  RefPtr<internal::WeakPtrFlag> flag_;
+
+  // Copy/move construction/assignment supported.
+};
+
+// 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 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) { assert(ptr_); }
+  ~WeakPtrFactory() { InvalidateWeakPtrs(); }
+
+  // Gets a new weak pointer, which will be valid until either
+  // |InvalidateWeakPtrs()| is called or this object is destroyed.
+  WeakPtr<T> GetWeakPtr() {
+    if (!flag_)
+      flag_ = MakeRefCounted<internal::WeakPtrFlag>();
+    return WeakPtr<T>(ptr_, flag_.Clone());
+  }
+
+  // Call this method to invalidate all existing weak pointers. (Note that
+  // additional weak pointers can be produced even after this is called.)
+  void InvalidateWeakPtrs() {
+    if (!flag_)
+      return;
+    flag_->Invalidate();
+    flag_ = nullptr;
+  }
+
+  // Call this method to determine if any weak pointers exist. (Note that a
+  // "false" result is definitive, but a "true" result may not be if weak
+  // pointers are held/reset/destroyed/reassigned on other threads.)
+  bool HasWeakPtrs() const { return flag_ && !flag_->HasOneRef(); }
+
+ 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_;
+  RefPtr<internal::WeakPtrFlag> flag_;
+
+  MOJO_DISALLOW_COPY_AND_ASSIGN(WeakPtrFactory);
+};
+
+}  // namespace util
+}  // namespace mojo
+
+#endif  // MOJO_UTIL_WEAK_PTR_H_