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// Copyright 2017 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 UI_EVENTS_BLINK_FLING_BOOSTER_H_
#define UI_EVENTS_BLINK_FLING_BOOSTER_H_
#include "third_party/blink/public/platform/web_gesture_event.h"
namespace ui {
// This class is used to track fling state and provide "fling boosting".
// Boosting is a feature where successive flings can repeatedly increase the
// fling velocity so that users can scroll through long documents. This
// boosting logic occurs only in certain circumstances so we track the state
// and conditions in this class. The FlingController will request the velocity
// for all flings from this class; if FlingBooster decides the fling should be
// boosted it'll add the new fling's velocity to the previous one's.
class FlingBooster {
public:
FlingBooster() = default;
gfx::Vector2dF GetVelocityForFlingStart(
const blink::WebGestureEvent& gesture_start);
void ObserveGestureEvent(const blink::WebGestureEvent& gesture_event);
void ObserveProgressFling(const gfx::Vector2dF& current_velocity);
private:
bool ShouldBoostFling(const blink::WebGestureEvent& fling_start_event);
void Reset();
// When non-null, the current gesture stream is being considered for
// boosting. If a fling hasn't occurred by this time, we won't cause a boost.
// Note, however, that we'll extend this time as we see scroll updates.
base::TimeTicks cutoff_time_for_boost_;
// Tracks the (possibly boosted) velocity used at the previous FlingStart.
// When a new fling is started and we decide to boost, we'll add this
// velocity to it.
gfx::Vector2dF previous_fling_starting_velocity_;
// Tracks the current fling's velocity as it decays. We'll prevent boosting
// if this crosses the kMinBoostFlingSpeedSquare threshold.
gfx::Vector2dF current_fling_velocity_;
// These store the current active fling source device and modifier keys (e.g.
// Ctrl) since a new fling start event must have the same source device and
// modifiers to be able to boost the active fling.
blink::WebGestureDevice source_device_ =
blink::WebGestureDevice::kUninitialized;
int modifiers_ = 0;
// Track the last timestamp we've seen a scroll update that we're evaluating
// as a boost. This is used to calculate the velocity; if it's too slow we'll
// avoid boosting.
base::TimeTicks previous_boosting_scroll_timestamp_;
DISALLOW_COPY_AND_ASSIGN(FlingBooster);
};
} // namespace ui
#endif // UI_EVENTS_BLINK_FLING_BOOSTER_H_