1 files changed, 70 insertions, 0 deletions
diff --git a/layout/generic/ScrollGeneration.h b/layout/generic/ScrollGeneration.h
new file mode 100644
index 0000000000..90eb243e9e
--- /dev/null
+++ b/layout/generic/ScrollGeneration.h
@@ -0,0 +1,70 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_ScrollGeneration_h_
+#define mozilla_ScrollGeneration_h_
+
+#include <cstdint>
+#include <iosfwd>
+
+namespace mozilla {
+
+struct ScrollGenerationCounter;
+
+class APZTag {};
+class MainThreadTag {};
+
+template <typename Tag>
+struct ScrollGeneration;
+
+template <typename Tag>
+std::ostream& operator<<(std::ostream& aStream,
+                         const ScrollGeneration<Tag>& aGen);
+
+template <typename Tag>
+struct ScrollGeneration {
+  friend struct ScrollGenerationCounter;
+
+ private:
+  // Private constructor; use ScrollGenerationCounter to get a new instance.
+  explicit ScrollGeneration(uint64_t aValue);
+
+ public:
+  // Dummy constructor, needed for IPDL purposes. Not intended for manual use.
+  ScrollGeneration();
+
+  uint64_t Raw() const { return mValue; }
+
+  bool operator<(const ScrollGeneration<Tag>& aOther) const;
+  bool operator==(const ScrollGeneration<Tag>& aOther) const;
+  bool operator!=(const ScrollGeneration<Tag>& aOther) const;
+
+  friend std::ostream& operator<< <>(std::ostream& aStream,
+                                     const ScrollGeneration<Tag>& aGen);
+
+ private:
+  uint64_t mValue;
+};
+
+using APZScrollGeneration = ScrollGeneration<APZTag>;
+using MainThreadScrollGeneration = ScrollGeneration<MainThreadTag>;
+
+struct ScrollGenerationCounter {
+  MainThreadScrollGeneration NewMainThreadGeneration() {
+    uint64_t value = ++mCounter;
+    return MainThreadScrollGeneration(value);
+  }
+
+  APZScrollGeneration NewAPZGeneration() {
+    uint64_t value = ++mCounter;
+    return APZScrollGeneration(value);
+  }
+
+ private:
+  uint64_t mCounter = 0;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ScrollGeneration_h_