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Telegram/lib_ui/ui/dragging_scroll_manager.cpp

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+// This file is part of Desktop App Toolkit,
+// a set of libraries for developing nice desktop applications.
+//
+// For license and copyright information please follow this link:
+// https://github.com/desktop-app/legal/blob/master/LEGAL
+//
+#include "ui/dragging_scroll_manager.h"
+
+#include "base/timer.h"
+
+namespace Ui {
+namespace {
+
+// 37px per 15ms while select-by-drag.
+inline constexpr auto kMaxScrollSpeed = 37;
+
+} // namespace
+
+DraggingScrollManager::DraggingScrollManager() = default;
+
+void DraggingScrollManager::scrollByTimer() {
+	const auto d = (_delta > 0)
+		? std::min(_delta * 3 / 20 + 1, kMaxScrollSpeed)
+		: std::max(_delta * 3 / 20 - 1, -kMaxScrollSpeed);
+	_scrolls.fire_copy(d);
+}
+
+void DraggingScrollManager::checkDeltaScroll(int delta) {
+	_delta = delta;
+	if (_delta) {
+		if (!_timer) {
+			_timer = std::make_unique<base::Timer>([=] { scrollByTimer(); });
+		}
+		_timer->callEach(15);
+	} else {
+		cancel();
+	}
+}
+
+void DraggingScrollManager::checkDeltaScroll(
+		const QPoint &point,
+		int top,
+		int bottom) {
+	const auto diff = point.y() - top;
+	checkDeltaScroll((diff < 0)
+		? diff
+		: (point.y() >= bottom)
+		? (point.y() - bottom + 1)
+		: 0);
+}
+
+void DraggingScrollManager::cancel() {
+	if (_timer) {
+		_timer->cancel();
+		_timer = nullptr;
+	}
+}
+
+rpl::producer<int> DraggingScrollManager::scrolls() const {
+	return _scrolls.events();
+}
+
+} // namespace Ui