chromium/cc/input/snap_selection_strategy.h

// Copyright 2018 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef CC_INPUT_SNAP_SELECTION_STRATEGY_H_
#define CC_INPUT_SNAP_SELECTION_STRATEGY_H_

#include <memory>

#include "cc/input/scroll_snap_data.h"

namespace cc {

enum class SnapStopAlwaysFilter { … };
enum class SnapTargetsPrioritization { … };

// This class represents an abstract strategy that decide which snap selection
// should be considered valid. There are concrete implementations for three core
// scrolling types: scroll with end position only, scroll with direction only,
// and scroll with end position and direction.
class CC_EXPORT SnapSelectionStrategy { … };

// Examples for intended end position scrolls include
// - a panning gesture, released without momentum
// - manupulating the scrollbar "thumb" explicitly
// - programmatically scrolling via APIs such as scrollTo()
// - tabbing through the document's focusable elements
// - navigating to an anchor within the page
// - homing operations such as the Home/End keys
// For this type of scrolls, we want to
// * Minimize the distance between the snap position and the end position.
// * Return the end position if that makes a snap area covers the snapport.
class EndPositionStrategy : public SnapSelectionStrategy { … };

// Examples for intended direction scrolls include
// - pressing an arrow key on the keyboard
// - a swiping gesture interpreted as a fixed (rather than inertial) scroll
// For this type of scrolls, we want to
// * Minimize the distance between the snap position and the starting position,
//   so that we stop at the first snap position in that direction.
// * Return the default intended position (using the default step) if that makes
//   a snap area covers the snapport.
class DirectionStrategy : public SnapSelectionStrategy { … };

// Examples for intended direction and end position scrolls include
// - a “fling” gesture, interpreted with momentum
// - programmatically scrolling via APIs such as scrollBy()
// - paging operations such as the PgUp/PgDn keys (or equivalent operations on
//   the scrollbar)
// For this type of scrolls, we want to
// * Minimize the distance between the snap position and the end position.
// * Return the end position if that makes a snap area covers the snapport.
class EndAndDirectionStrategy : public SnapSelectionStrategy { … };

}  // namespace cc

#endif  // CC_INPUT_SNAP_SELECTION_STRATEGY_H_