LibGUI: Add `AbstractZoomPanWidget` widget :^)

This is an abstract widget that is meant to handle all the panning /
zooming functionality so that all applications implementing it do
not have to try to do their own coordinate math.

Userland/Libraries/LibGUI/AbstractZoomPanWidget.cpp

@@ -0,0 +1,183 @@
+/*
+ * Copyright (c) 2022, Mustafa Quraish <mustafa@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "AbstractZoomPanWidget.h"
+
+namespace GUI {
+
+constexpr float wheel_zoom_factor = 8.0f;
+
+void AbstractZoomPanWidget::set_scale(float new_scale)
+{
+    if (m_original_rect.is_null())
+        return;
+
+    m_scale = clamp(new_scale, m_min_scale, m_max_scale);
+    Gfx::IntSize new_size;
+    new_size.set_width(m_original_rect.width() * m_scale);
+    new_size.set_height(m_original_rect.height() * m_scale);
+    m_content_rect.set_size(new_size);
+
+    if (on_scale_change)
+        on_scale_change(m_scale);
+
+    relayout();
+}
+
+void AbstractZoomPanWidget::scale_by(float delta)
+{
+    float new_scale = m_scale * AK::exp2(delta);
+    set_scale(new_scale);
+}
+
+void AbstractZoomPanWidget::scale_centered(float new_scale, Gfx::IntPoint const& center)
+{
+    if (m_original_rect.is_null())
+        return;
+
+    new_scale = clamp(new_scale, m_min_scale, m_max_scale);
+    if (new_scale == m_scale)
+        return;
+
+    Gfx::FloatPoint focus_point {
+        center.x() - width() / 2.0f,
+        center.y() - height() / 2.0f
+    };
+    m_origin = (m_origin + focus_point) * (new_scale / m_scale) - focus_point;
+    set_scale(new_scale);
+}
+
+void AbstractZoomPanWidget::start_panning(Gfx::IntPoint const& position)
+{
+    m_saved_cursor = override_cursor();
+    set_override_cursor(Gfx::StandardCursor::Drag);
+    m_pan_start = m_origin;
+    m_pan_mouse_pos = position;
+    m_is_panning = true;
+}
+
+void AbstractZoomPanWidget::stop_panning()
+{
+    m_is_panning = false;
+    set_override_cursor(m_saved_cursor);
+}
+
+void AbstractZoomPanWidget::pan_to(Gfx::IntPoint const& position)
+{
+    // NOTE: `position` here (and `m_pan_mouse_pos`) are both in frame coordinates, not
+    // content coordinates, by design. The derived class should not have to keep track of
+    // the (zoomed) content coordinates itself, but just pass along the mouse position.
+    auto delta = position - m_pan_mouse_pos;
+    m_origin = m_pan_start.translated(-delta.x(), -delta.y());
+    relayout();
+}
+
+Gfx::FloatPoint AbstractZoomPanWidget::frame_to_content_position(Gfx::IntPoint const& frame_position) const
+{
+    Gfx::FloatPoint content_position;
+    content_position.set_x(((float)frame_position.x() - (float)m_content_rect.x()) / m_scale);
+    content_position.set_y(((float)frame_position.y() - (float)m_content_rect.y()) / m_scale);
+    return content_position;
+}
+
+Gfx::FloatRect AbstractZoomPanWidget::frame_to_content_rect(Gfx::IntRect const& frame_rect) const
+{
+    Gfx::FloatRect content_rect;
+    content_rect.set_location(frame_to_content_position(frame_rect.location()));
+    content_rect.set_width((float)frame_rect.width() / m_scale);
+    content_rect.set_height((float)frame_rect.height() / m_scale);
+    return content_rect;
+}
+
+Gfx::FloatPoint AbstractZoomPanWidget::content_to_frame_position(Gfx::IntPoint const& content_position) const
+{
+    Gfx::FloatPoint frame_position;
+    frame_position.set_x(m_content_rect.x() + ((float)content_position.x() * m_scale));
+    frame_position.set_y(m_content_rect.y() + ((float)content_position.y() * m_scale));
+    return frame_position;
+}
+
+Gfx::FloatRect AbstractZoomPanWidget::content_to_frame_rect(Gfx::IntRect const& content_rect) const
+{
+    Gfx::FloatRect frame_rect;
+    frame_rect.set_location(content_to_frame_position(content_rect.location()));
+    frame_rect.set_width((float)content_rect.width() * m_scale);
+    frame_rect.set_height((float)content_rect.height() * m_scale);
+    return frame_rect;
+}
+
+void AbstractZoomPanWidget::mousewheel_event(GUI::MouseEvent& event)
+{
+    float new_scale = scale() / AK::exp2(event.wheel_delta() / wheel_zoom_factor);
+    scale_centered(new_scale, event.position());
+}
+
+void AbstractZoomPanWidget::mousedown_event(GUI::MouseEvent& event)
+{
+    if (!m_is_panning && event.button() == GUI::MouseButton::Middle) {
+        start_panning(event.position());
+        event.accept();
+        return;
+    }
+}
+
+void AbstractZoomPanWidget::resize_event(GUI::ResizeEvent& event)
+{
+    relayout();
+    GUI::Widget::resize_event(event);
+}
+
+void AbstractZoomPanWidget::mousemove_event(GUI::MouseEvent& event)
+{
+    if (!m_is_panning)
+        return;
+    pan_to(event.position());
+    event.accept();
+}
+
+void AbstractZoomPanWidget::mouseup_event(GUI::MouseEvent& event)
+{
+    if (m_is_panning && event.button() == GUI::MouseButton::Middle) {
+        stop_panning();
+        event.accept();
+        return;
+    }
+}
+
+void AbstractZoomPanWidget::relayout()
+{
+    if (m_original_rect.is_null())
+        return;
+
+    Gfx::IntSize new_size = m_content_rect.size();
+
+    Gfx::IntPoint new_location;
+    new_location.set_x((width() / 2) - (new_size.width() / 2) - m_origin.x());
+    new_location.set_y((height() / 2) - (new_size.height() / 2) - m_origin.y());
+    m_content_rect.set_location(new_location);
+
+    handle_relayout(m_content_rect);
+}
+
+void AbstractZoomPanWidget::reset_view()
+{
+    m_origin = { 0, 0 };
+    set_scale(1.0f);
+}
+
+void AbstractZoomPanWidget::set_content_rect(Gfx::IntRect const& content_rect)
+{
+    m_content_rect = enclosing_int_rect(content_to_frame_rect(content_rect));
+    update();
+}
+
+void AbstractZoomPanWidget::set_scale_bounds(float min_scale, float max_scale)
+{
+    m_min_scale = min_scale;
+    m_max_scale = max_scale;
+}
+
+}

Userland/Libraries/LibGUI/AbstractZoomPanWidget.h

@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2022, Mustafa Quraish <mustafa@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGUI/Frame.h>
+#include <LibGfx/Point.h>
+#include <LibGfx/Rect.h>
+
+namespace GUI {
+
+class AbstractZoomPanWidget : public GUI::Frame {
+    C_OBJECT(AbstractZoomPanWidget);
+
+public:
+    void set_scale(float scale);
+    float scale() const { return m_scale; }
+    void set_scale_bounds(float min_scale, float max_scale);
+
+    void scale_by(float amount);
+    void scale_centered(float new_scale, Gfx::IntPoint const& center);
+
+    bool is_panning() const { return m_is_panning; }
+    void start_panning(Gfx::IntPoint const& position);
+    void stop_panning();
+
+    void pan_to(Gfx::IntPoint const& position);
+
+    // Should be overridden by derived classes if they want updates.
+    virtual void handle_relayout(Gfx::IntRect const&) { update(); }
+    void relayout();
+
+    Gfx::FloatPoint frame_to_content_position(Gfx::IntPoint const& frame_position) const;
+    Gfx::FloatRect frame_to_content_rect(Gfx::IntRect const& frame_rect) const;
+    Gfx::FloatPoint content_to_frame_position(Gfx::IntPoint const& content_position) const;
+    Gfx::FloatRect content_to_frame_rect(Gfx::IntRect const& content_rect) const;
+
+    virtual void mousewheel_event(GUI::MouseEvent& event) override;
+    virtual void mousedown_event(GUI::MouseEvent& event) override;
+    virtual void resize_event(GUI::ResizeEvent& event) override;
+    virtual void mousemove_event(GUI::MouseEvent& event) override;
+    virtual void mouseup_event(GUI::MouseEvent& event) override;
+
+    void set_original_rect(Gfx::IntRect const& rect) { m_original_rect = rect; }
+    void set_content_rect(Gfx::IntRect const& content_rect);
+    void set_origin(Gfx::FloatPoint const& origin) { m_origin = origin; }
+
+    void reset_view();
+
+    Gfx::IntRect content_rect() const { return m_content_rect; }
+
+    Function<void(float)> on_scale_change;
+
+private:
+    Gfx::IntRect m_original_rect;
+    Gfx::IntRect m_content_rect;
+
+    Gfx::IntPoint m_pan_mouse_pos;
+    Gfx::FloatPoint m_origin;
+    Gfx::FloatPoint m_pan_start;
+    bool m_is_panning { false };
+
+    float m_min_scale { 0.1f };
+    float m_max_scale { 10.0f };
+    float m_scale { 1.0f };
+
+    AK::Variant<Gfx::StandardCursor, NonnullRefPtr<Gfx::Bitmap>> m_saved_cursor { Gfx::StandardCursor::None };
+};
+
+}

Userland/Libraries/LibGUI/CMakeLists.txt

@@ -9,6 +9,7 @@ set(SOURCES
     AbstractSlider.cpp
     AbstractTableView.cpp
     AbstractView.cpp
+    AbstractZoomPanWidget.cpp
     Action.cpp
     ActionGroup.cpp
     Application.cpp