1 files changed, 45 insertions, 0 deletions
diff --git a/layout/generic/ScrollSnap.h b/layout/generic/ScrollSnap.h
new file mode 100644
index 0000000000..4440a6c754
--- /dev/null
+++ b/layout/generic/ScrollSnap.h
@@ -0,0 +1,45 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_layout_ScrollSnap_h_
+#define mozilla_layout_ScrollSnap_h_
+
+#include "mozilla/ScrollTypes.h"
+#include "mozilla/Maybe.h"
+
+struct nsPoint;
+struct nsRect;
+
+namespace mozilla {
+
+namespace layers {
+struct ScrollSnapInfo;
+}
+
+struct ScrollSnapUtils {
+  /**
+   * GetSnapPointForDestination determines which point to snap to after
+   * scrolling. |aStartPos| gives the position before scrolling and
+   * |aDestination| gives the position after scrolling, with no snapping.
+   * Behaviour is dependent on the value of |aUnit|.
+   * |aSnapInfo| and |aScrollRange| are characteristics of the scroll frame for
+   * which snapping is being performed.
+   * If a suitable snap point could be found, it is returned. Otherwise, an
+   * empty Maybe is returned.
+   * IMPORTANT NOTE: This function is designed to be called both on and off
+   *                 the main thread. If modifying its implementation, be sure
+   *                 not to touch main-thread-only data structures without
+   *                 appropriate locking.
+   */
+  static mozilla::Maybe<nsPoint> GetSnapPointForDestination(
+      const layers::ScrollSnapInfo& aSnapInfo, ScrollUnit aUnit,
+      const nsRect& aScrollRange, const nsPoint& aStartPos,
+      const nsPoint& aDestination);
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_layout_ScrollSnap_h_