Added new Ui::СontinuousScroll.

Telegram/SourceFiles/ui/chat/continuous_scroll.cpp

@@ -0,0 +1,74 @@
+/*
+This file is part of Telegram Desktop,
+the official desktop application for the Telegram messaging service.
+
+For license and copyright information please follow this link:
+https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
+*/
+#include "ui/chat/continuous_scroll.h"
+
+#include <QScrollBar>
+#include <QWheelEvent>
+
+namespace Ui {
+
+void ContinuousScroll::wheelEvent(QWheelEvent *e) {
+	if (_tracking
+		&& !e->angleDelta().isNull()
+		&& (e->angleDelta().y() < 0)
+		&& (scrollTopMax() == scrollTop())) {
+		_addContentRequests.fire({});
+		if (base::take(_contentAdded)) {
+			viewportEvent(e);
+		}
+		return;
+	}
+	ScrollArea::wheelEvent(e);
+}
+
+void ContinuousScroll::setTrackingContent(bool value) {
+	if (_tracking == value) {
+		return;
+	}
+	_tracking = value;
+	reconnect();
+}
+
+void ContinuousScroll::reconnect() {
+	if (!_tracking) {
+		_connection.release();
+		return;
+	}
+	const auto handleAction = [=](int action) {
+		const auto scroll = verticalScrollBar();
+		const auto step = (action == QAbstractSlider::SliderSingleStepAdd)
+			? scroll->singleStep()
+			: (action == QAbstractSlider::SliderPageStepAdd)
+			? scroll->pageStep()
+			: 0;
+		if (!action) {
+			return;
+		}
+		const auto newTop = scrollTop() + step;
+		if (newTop > scrollTopMax()) {
+			_addContentRequests.fire({});
+			if (base::take(_contentAdded)) {
+				scroll->setSliderPosition(newTop);
+			}
+		}
+	};
+	_connection = QObject::connect(
+		verticalScrollBar(),
+		&QAbstractSlider::actionTriggered,
+		handleAction);
+}
+
+void ContinuousScroll::contentAdded() {
+	_contentAdded = true;
+}
+
+rpl::producer<> ContinuousScroll::addContentRequests() const {
+	return _addContentRequests.events();
+}
+
+} // namespace Ui

Telegram/SourceFiles/ui/chat/continuous_scroll.h

@@ -0,0 +1,41 @@
+/*
+This file is part of Telegram Desktop,
+the official desktop application for the Telegram messaging service.
+
+For license and copyright information please follow this link:
+https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
+*/
+#pragma once
+
+#include "ui/widgets/scroll_area.h"
+#include "base/qt_connection.h"
+
+namespace Ui {
+
+// This class is designed for seamless scrolling of
+// on-demand augmented content.
+class ContinuousScroll final : public ScrollArea {
+public:
+	using ScrollArea::ScrollArea;
+
+	[[nodiscard]] rpl::producer<> addContentRequests() const;
+	void contentAdded();
+
+	void setTrackingContent(bool value);
+
+protected:
+	void wheelEvent(QWheelEvent *e) override;
+
+private:
+	void reconnect();
+
+	base::qt_connection _connection;
+
+	bool _contentAdded = false;
+	bool _tracking = false;
+
+	rpl::event_stream<> _addContentRequests;
+
+};
+
+} // namespace Ui

Telegram/cmake/td_ui.cmake

@@ -141,6 +141,8 @@ PRIVATE
     ui/chat/chat_style.h
     ui/chat/chat_theme.cpp
     ui/chat/chat_theme.h
+    ui/chat/continuous_scroll.cpp
+    ui/chat/continuous_scroll.h
     ui/chat/forward_options_box.cpp
     ui/chat/forward_options_box.h
     ui/chat/group_call_bar.cpp