gecko-dev/layout/generic/ScrollSnap.h

/* -*- 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/Maybe.h"
#include "nsIScrollableFrame.h"

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|, |aScrollPortSize|, 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,
      nsIScrollableFrame::ScrollUnit aUnit,
      const nsSize& aScrollPortSize,
      const nsRect& aScrollRange,
      const nsPoint& aStartPos,
      const nsPoint& aDestination);
};

} // namespace mozilla

#endif // mozilla_layout_ScrollSnap_h_