RmlUi/Source/Core/ScrollController.cpp

#include "ScrollController.h"
#include "../../Include/RmlUi/Core/ComputedValues.h"
#include "../../Include/RmlUi/Core/Core.h"
#include "../../Include/RmlUi/Core/Element.h"
#include "../../Include/RmlUi/Core/SystemInterface.h"

namespace Rml {

static constexpr float AUTOSCROLL_SPEED_FACTOR = 0.09f;
static constexpr float AUTOSCROLL_DEADZONE = 10.0f;            // [dp]

static constexpr float SMOOTHSCROLL_WINDOW_SIZE = 100.f;       // The window where smoothing is applied, as a distance from scroll start and end. [dp]
static constexpr float SMOOTHSCROLL_MAX_VELOCITY = 10'000.f;   // [dp/s]
static constexpr float SMOOTHSCROLL_VELOCITY_CONSTANT = 800.f; // [dp/s]
static constexpr float SMOOTHSCROLL_VELOCITY_SQUARE_FACTOR = 0.05f;
static constexpr float SMOOTHSCROLL_FIRST_FRAME_DELTA_TIME_MIN = 1.f / 100.f; // To make the scroll feel a bit more responsive. [s]

// Factor to multiply friction by before applying to velocity.
static constexpr float INERTIA_FRICTION_FACTOR = 5.0f;
static constexpr float INERTIA_VELOCITY_CUTOFF = 30.0f;

// Saturate the delta time to some reasonable FPS value, to avoid large steps in case of stuttering or freezing.
static constexpr float DELTA_TIME_MAX = 1.f / 15.f; // [s]

// Determines the autoscroll velocity based on the distance from the scroll-start mouse position. [px/s]
static Vector2f CalculateAutoscrollVelocity(Vector2f target_delta, float dp_ratio)
{
	target_delta = target_delta / dp_ratio;
	target_delta = {
		Math::Absolute(target_delta.x) < AUTOSCROLL_DEADZONE ? 0.f : target_delta.x,
		Math::Absolute(target_delta.y) < AUTOSCROLL_DEADZONE ? 0.f : target_delta.y,
	};

	// We use a signed square model for the velocity, which seems to work quite well. This is mostly about feeling and tuning.
	return AUTOSCROLL_SPEED_FACTOR * target_delta * Math::Absolute(target_delta);
}

// Determines the smoothscroll velocity based on the distance to the target, and the distance scrolled so far. [px/s]
static Vector2f CalculateSmoothscrollVelocity(Vector2f target_delta, Vector2f scrolled_distance, float dp_ratio)
{
	scrolled_distance = Math::Absolute(scrolled_distance) / dp_ratio;
	target_delta = target_delta / dp_ratio;

	const Vector2f target_delta_abs = Math::Absolute(target_delta);
	Vector2f target_delta_signum = {
		target_delta.x > 0.f ? 1.f : (target_delta.x < 0.f ? -1.f : 0.f),
		target_delta.y > 0.f ? 1.f : (target_delta.y < 0.f ? -1.f : 0.f),
	};

	// The window provides velocity smoothing near the start and end of the scroll.
	const Tween tween(Tween::Exponential, Tween::Out);
	const Vector2f alpha_in = Math::Min(scrolled_distance / SMOOTHSCROLL_WINDOW_SIZE, Vector2f(1.f));
	const Vector2f alpha_out = Math::Min(target_delta_abs / SMOOTHSCROLL_WINDOW_SIZE, Vector2f(1.f));
	const Vector2f smooth_window = {
		0.35f + 0.65f * tween(alpha_in.x) * tween(alpha_out.x),
		0.35f + 0.65f * tween(alpha_in.y) * tween(alpha_out.y),
	};

	const Vector2f velocity_constant = Vector2f(SMOOTHSCROLL_VELOCITY_CONSTANT);
	const Vector2f velocity_square = SMOOTHSCROLL_VELOCITY_SQUARE_FACTOR * target_delta_abs * target_delta_abs;

	// Short scrolls are dominated by the smoothed constant velocity, while the square term is added for quick longer scrolls.
	return dp_ratio * target_delta_signum * smooth_window * Math::Min(velocity_constant + velocity_square, Vector2f(SMOOTHSCROLL_MAX_VELOCITY));
}

void ScrollController::ActivateAutoscroll(Element* in_target, Vector2i start_position)
{
	Reset();
	if (!in_target)
		return;
	target = in_target;
	mode = Mode::Autoscroll;
	autoscroll_start_position = start_position;
	UpdateTime();
}

void ScrollController::ActivateSmoothscroll(Element* in_target, Vector2f delta_distance, ScrollBehavior scroll_behavior)
{
	Reset();
	if (!in_target)
		return;

	target = in_target;

	// Do instant scroll if preferred.
	if (smoothscroll_prefer_instant && scroll_behavior != ScrollBehavior::Smooth)
	{
		PerformScrollOnTarget(delta_distance);
		target = nullptr;
		return;
	}

	mode = Mode::Smoothscroll;
	UpdateTime();
	IncrementSmoothscrollTarget(delta_distance);

	// If the target is scrolled to its edge already, simply cancel the smoothscroll operation.
	if (HasSmoothscrollReachedTarget())
		Reset();
}

void ScrollController::ActivateInertia(Element* in_target, Vector2f velocity)
{
	Reset();
	if (!in_target || (velocity.x == 0 && velocity.y == 0))
		return;

	target = in_target;
	inertia_scroll_velocity = velocity;
	mode = Mode::Inertia;
	UpdateTime();
}

void ScrollController::InstantScrollOnTarget(Element* in_target, Vector2f delta_distance)
{
	if (!in_target)
		return;

	// instant scroll element without changing the current target

	Element* safe_target = target;

	target = in_target;
	PerformScrollOnTarget(delta_distance);
	target = safe_target;
}

bool ScrollController::Update(Vector2i mouse_position, float dp_ratio)
{
	const float dt = (mode == Mode::None ? 0.f : UpdateTime());

	switch (mode)
	{
	case Mode::Smoothscroll: UpdateSmoothscroll(dt, dp_ratio); break;
	case Mode::Autoscroll: UpdateAutoscroll(dt, mouse_position, dp_ratio); break;
	case Mode::Inertia: UpdateInertia(dt); break;
	case Mode::None: break;
	}

	return mode != Mode::None;
}

void ScrollController::UpdateAutoscroll(float dt, Vector2i mouse_position, float dp_ratio)
{
	RMLUI_ASSERT(mode == Mode::Autoscroll && target);

	const Vector2f scroll_delta = Vector2f(mouse_position - autoscroll_start_position);
	const Vector2f scroll_velocity = CalculateAutoscrollVelocity(scroll_delta, dp_ratio);

	autoscroll_accumulated_length += scroll_velocity * dt;

	// Only submit the integer part of the scroll length, accumulate and store fractional parts to enable sub-pixel-per-frame scrolling speeds.
	Vector2f scroll_length_integral;
	autoscroll_accumulated_length.x = Math::DecomposeFractionalIntegral(autoscroll_accumulated_length.x, &scroll_length_integral.x);
	autoscroll_accumulated_length.y = Math::DecomposeFractionalIntegral(autoscroll_accumulated_length.y, &scroll_length_integral.y);

	if (scroll_velocity != Vector2f(0.f))
		autoscroll_moved = true;

	PerformScrollOnTarget(scroll_length_integral);
}

void ScrollController::UpdateSmoothscroll(float dt, float dp_ratio)
{
	RMLUI_ASSERT(mode == Mode::Smoothscroll && target);

	const Vector2f target_delta = Vector2f(smoothscroll_target_distance - smoothscroll_scrolled_distance);
	const Vector2f velocity = CalculateSmoothscrollVelocity(target_delta, smoothscroll_scrolled_distance, dp_ratio);

	if (smoothscroll_scrolled_distance == Vector2f{0})
		dt = Math::Max(dt, SMOOTHSCROLL_FIRST_FRAME_DELTA_TIME_MIN);

	Vector2f scroll_distance_fractional = smoothscroll_speed_factor * velocity * dt + smoothscroll_accumulated_fractional_distance;

	Vector2f scroll_distance_integral;
	smoothscroll_accumulated_fractional_distance.x = Math::DecomposeFractionalIntegral(scroll_distance_fractional.x, &scroll_distance_integral.x);
	smoothscroll_accumulated_fractional_distance.y = Math::DecomposeFractionalIntegral(scroll_distance_fractional.y, &scroll_distance_integral.y);

	for (int i = 0; i < 2; i++)
	{
		// Clamp the distance to the target in case of overshooting integration.
		if (target_delta[i] > 0.f)
			scroll_distance_integral[i] = Math::Min(scroll_distance_integral[i], target_delta[i]);
		else if (target_delta[i] < 0.f)
			scroll_distance_integral[i] = Math::Max(scroll_distance_integral[i], target_delta[i]);
		else
			scroll_distance_integral[i] = 0.f;
	}

#if 0
	// Useful debugging output for velocity model tuning.
	Log::Message(Log::LT_INFO, "Scroll  y0 %8.2f   y1 %8.2f    dt %1.4f   v %8.2f   dy %8.2f    frac %1.2f", smoothscroll_scrolled_distance.y,
		target_delta.y, dt, velocity.y, scroll_distance_integral.y, smoothscroll_accumulated_fractional_distance.y);
#endif

	smoothscroll_scrolled_distance += scroll_distance_integral;
	PerformScrollOnTarget(scroll_distance_integral);

	if (HasSmoothscrollReachedTarget())
		Reset();
}

void ScrollController::UpdateInertia(float dt)
{
	RMLUI_ASSERT(mode == Mode::Inertia && target);

	if (inertia_scroll_velocity.x == 0.0f && inertia_scroll_velocity.y == 0.0f)
	{
		Reset();
		return;
	}

	// Apply and dampen inertia.
	Vector2f scroll_delta = inertia_scroll_velocity * dt;
	PerformScrollOnTarget(scroll_delta);

	float dampening = 1.0f - INERTIA_FRICTION_FACTOR * dt;
	inertia_scroll_velocity *= dampening;

	if (Math::Absolute(inertia_scroll_velocity.x) < INERTIA_VELOCITY_CUTOFF)
		inertia_scroll_velocity.x = 0.0f;
	if (Math::Absolute(inertia_scroll_velocity.y) < INERTIA_VELOCITY_CUTOFF)
		inertia_scroll_velocity.y = 0.0f;
}

bool ScrollController::HasSmoothscrollReachedTarget() const
{
	constexpr float epsilon = 0.1f;
	return (smoothscroll_target_distance - smoothscroll_scrolled_distance).SquaredMagnitude() < epsilon;
}

void ScrollController::PerformScrollOnTarget(Vector2f delta_distance)
{
	RMLUI_ASSERT(target);
	auto overflow_is_scrollable = [](Style::Overflow overflow) { return overflow == Style::Overflow::Auto || overflow == Style::Overflow::Scroll; };
	auto& computed_values = target->GetComputedValues();

	if (delta_distance.x != 0.f && overflow_is_scrollable(computed_values.overflow_x()))
		target->SetScrollLeft(target->GetScrollLeft() + delta_distance.x);
	if (delta_distance.y != 0.f && overflow_is_scrollable(computed_values.overflow_y()))
		target->SetScrollTop(target->GetScrollTop() + delta_distance.y);
}

void ScrollController::IncrementSmoothscrollTarget(Vector2f delta_distance)
{
	auto OppositeDirection = [](float a, float b) { return (a < 0.f && b > 0.f) || (a > 0.f && b < 0.f); };
	Vector2f delta = smoothscroll_target_distance - smoothscroll_scrolled_distance;

	// Reset movement state if we start scrolling in the opposite direction.
	for (int i = 0; i < 2; i++)
	{
		if (OppositeDirection(delta_distance[i], delta[i]))
		{
			smoothscroll_target_distance[i] = 0.f;
			smoothscroll_scrolled_distance[i] = 0.f;
			smoothscroll_accumulated_fractional_distance[i] = 0.f;
		}
	}

	// Clamp the delta distance to the scrollable area.
	const Vector2f scroll_offset = {target->GetScrollLeft(), target->GetScrollTop()};
	const Vector2f max_offset = {target->GetScrollWidth() - target->GetClientWidth(), target->GetScrollHeight() - target->GetClientHeight()};

	const Vector2f target_offset = scroll_offset + smoothscroll_target_distance - smoothscroll_scrolled_distance;
	const Vector2f clamped_delta = Math::Clamp(delta_distance + target_offset, Vector2f(0.f), max_offset) - target_offset;

	smoothscroll_target_distance += clamped_delta;
}

void ScrollController::Reset()
{
	mode = Mode::None;
	target = nullptr;

	autoscroll_start_position = Vector2i{};
	autoscroll_accumulated_length = Vector2f{};
	autoscroll_moved = false;

	smoothscroll_target_distance = Vector2f{};
	smoothscroll_scrolled_distance = Vector2f{};
	smoothscroll_accumulated_fractional_distance = Vector2f{};
	// Keep smoothscroll configuration parameters.

	inertia_scroll_velocity = Vector2f{};
}

void ScrollController::SetDefaultScrollBehavior(ScrollBehavior scroll_behavior, float speed_factor)
{
	smoothscroll_prefer_instant = (scroll_behavior == ScrollBehavior::Instant);
	smoothscroll_speed_factor = speed_factor;
}

String ScrollController::GetAutoscrollCursor(Vector2i mouse_position, float dp_ratio) const
{
	RMLUI_ASSERT(mode == Mode::Autoscroll);

	const Vector2f scroll_delta = Vector2f(mouse_position - autoscroll_start_position);
	const Vector2f scroll_velocity = CalculateAutoscrollVelocity(scroll_delta, dp_ratio);

	if (scroll_velocity == Vector2f(0.f))
		return "rmlui-scroll-idle";

	String result = "rmlui-scroll";

	if (scroll_velocity.y > 0.f)
		result += "-up";
	else if (scroll_velocity.y < 0.f)
		result += "-down";

	if (scroll_velocity.x > 0.f)
		result += "-right";
	else if (scroll_velocity.x < 0.f)
		result += "-left";

	return result;
}

bool ScrollController::HasAutoscrollMoved() const
{
	return mode == Mode::Autoscroll && autoscroll_moved;
}

float ScrollController::UpdateTime()
{
	const double previous_tick = previous_update_time;
	previous_update_time = GetSystemInterface()->GetElapsedTime();

	const float dt = float(previous_update_time - previous_tick);
	return Math::Min(dt, DELTA_TIME_MAX);
}

} // namespace Rml