TaskScheduler: Add SequenceToken and ScopedSetSequenceTokenForCurrentThread

base/sequence_token.h

Index: base/sequence_token.h
diff --git a/base/sequence_token.h b/base/sequence_token.h
new file mode 100644
index 0000000000000000000000000000000000000000..d023ae68ca84f9489573b737e6ffb8333d1bc3ac
--- /dev/null
+++ b/base/sequence_token.h
@@ -0,0 +1,64 @@
+// 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.
+
+#ifndef BASE_SEQUENCE_TOKEN_H_
+#define BASE_SEQUENCE_TOKEN_H_
+
+#include "base/base_export.h"
+#include "base/macros.h"
+
+namespace base {
+namespace internal {

[gab, 2016/07/21 21:14:24]

I think internal:: should be reserved for implementation details referred by no
others than the impl and its tests.

This is intended to be used by other things (albeit very few) and as such
shouldn't be internal I think.

[fdoray, 2016/07/25 13:24:28]

Done.

+
+// A token that identifies a series of sequenced tasks (i.e. tasks that run one
+// at a time in posting order).
+class BASE_EXPORT SequenceToken {
+ public:
+  // Instantiates an invalid SequenceToken.
+  SequenceToken() = default;
+
+  // Explicitly allow copy.
+  SequenceToken(const SequenceToken& other) = default;
+  SequenceToken& operator=(const SequenceToken& other) = default;
+
+  bool operator==(const SequenceToken& other) const;
+  bool operator!=(const SequenceToken& other) const;
+
+  // Returns true if this is a valid SequenceToken.
+  bool IsValid() const;
+
+  // Returns a valid SequenceToken which isn't equal to any previously returned
+  // SequenceToken.
+  static SequenceToken Create();
+
+  // Returns the SequenceToken associated with the task running on the current
+  // thread, as determined by the active ScopedSetCurrentSequenceToken if any.
+  static SequenceToken GetCurrent();
+
+ private:
+  SequenceToken(int token) : token_(token) {}
+
+  static constexpr int kInvalidSequenceToken = -1;
+  int token_ = kInvalidSequenceToken;
+};
+
+// Throughout its lifetime, determines the value returned by
+// SequenceToken::GetCurrent().
+class BASE_EXPORT ScopedSetCurrentSequenceToken {
+ public:
+  ScopedSetCurrentSequenceToken(const SequenceToken& token);
+  ~ScopedSetCurrentSequenceToken();
+
+ private:
+  friend class SequenceToken;
+
+  const SequenceToken token_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedSetCurrentSequenceToken);
+};
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_SEQUENCE_TOKEN_H_