From 9ea2131adffe83559b160d9a17b1fbe2dffb25fe Mon Sep 17 00:00:00 2001
From: Andreas Kling <awesomekling@gmail.com>
Date: Sun, 10 Feb 2019 06:51:01 +0100
Subject: [PATCH] LibGUI: Support dragging the GScrollBar scrubber.

This was pleasantly easy to implement with the global mouse tracking. :^)
---
 LibGUI/GScrollBar.cpp | 45 +++++++++++++++++++++++++++++++++++++------
 LibGUI/GScrollBar.h   | 12 +++++++++---
 2 files changed, 48 insertions(+), 9 deletions(-)

diff --git a/LibGUI/GScrollBar.cpp b/LibGUI/GScrollBar.cpp
index c2b585c140..bc5f77abde 100644
--- a/LibGUI/GScrollBar.cpp
+++ b/LibGUI/GScrollBar.cpp
@@ -55,23 +55,28 @@ Rect GScrollBar::down_button_rect() const
     return { 0, height() - button_size(), button_size(), button_size() };
 }
 
-Rect GScrollBar::pgup_rect() const
+Rect GScrollBar::upper_gutter_rect() const
 {
     return { 0, button_size(), button_size(), scrubber_rect().top() - button_size() };
 }
 
-Rect GScrollBar::pgdn_rect() const
+Rect GScrollBar::lower_gutter_rect() const
 {
     auto scrubber_rect = this->scrubber_rect();
     return { 0, scrubber_rect.bottom(), button_size(), height() - button_size() - scrubber_rect.bottom() };
 }
 
+int GScrollBar::scrubbable_range_in_pixels() const
+{
+    return height() - button_size() * 3;
+}
+
 Rect GScrollBar::scrubber_rect() const
 {
     int range_size = m_max - m_min;
     if (range_size == 0)
         return { 0, button_size(), button_size(), height() - button_size() * 2 };
-    float available_y = height() - button_size() * 3;
+    float available_y = scrubbable_range_in_pixels();
     float y_step = available_y / range_size;
     float y = button_size() + (y_step * m_value);
     return { 0, (int)y, button_size(), button_size() };
@@ -115,7 +120,7 @@ void GScrollBar::paint_event(GPaintEvent&)
 
     Painter painter(*this);
 
-    painter.fill_rect(rect(), Color::MidGray);
+    painter.fill_rect(rect(), Color(164, 164, 164));
 
     painter.draw_rect(up_button_rect(), Color::DarkGray, true);
     painter.fill_rect_with_gradient(up_button_rect().shrunken(2, 2), Color::LightGray, Color::White);
@@ -141,12 +146,40 @@ void GScrollBar::mousedown_event(GMouseEvent& event)
         set_value(value() + m_step);
         return;
     }
-    if (pgup_rect().contains(event.position())) {
+    if (upper_gutter_rect().contains(event.position())) {
         set_value(value() - m_big_step);
         return;
     }
-    if (pgdn_rect().contains(event.position())) {
+    if (lower_gutter_rect().contains(event.position())) {
         set_value(value() + m_big_step);
         return;
     }
+    if (scrubber_rect().contains(event.position())) {
+        m_scrubbing = true;
+        m_scrub_start_value = value();
+        m_scrub_origin = event.position();
+        set_global_cursor_tracking(true);
+        return;
+    }
+}
+
+void GScrollBar::mouseup_event(GMouseEvent& event)
+{
+    if (event.button() != GMouseButton::Left)
+        return;
+    if (!m_scrubbing)
+        return;
+    m_scrubbing = false;
+    set_global_cursor_tracking(false);
+}
+
+void GScrollBar::mousemove_event(GMouseEvent& event)
+{
+    if (!m_scrubbing)
+        return;
+    float delta = event.y() - m_scrub_origin.y();
+    float scrubbable_range = scrubbable_range_in_pixels();
+    float value_steps_per_scrubbed_pixel = (m_max - m_min) / scrubbable_range;
+    float new_value = m_scrub_start_value + (value_steps_per_scrubbed_pixel * delta);
+    set_value(new_value);
 }
diff --git a/LibGUI/GScrollBar.h b/LibGUI/GScrollBar.h
index 48f1199bb1..4fd458afd3 100644
--- a/LibGUI/GScrollBar.h
+++ b/LibGUI/GScrollBar.h
@@ -24,19 +24,25 @@ public:
 private:
     virtual void paint_event(GPaintEvent&) override;
     virtual void mousedown_event(GMouseEvent&) override;
+    virtual void mouseup_event(GMouseEvent&) override;
+    virtual void mousemove_event(GMouseEvent&) override;
     virtual const char* class_name() const override { return "GScrollBar"; }
 
     int button_size() const { return 16; }
     Rect up_button_rect() const;
     Rect down_button_rect() const;
-    Rect pgup_rect() const;
-    Rect pgdn_rect() const;
+    Rect upper_gutter_rect() const;
+    Rect lower_gutter_rect() const;
     Rect scrubber_rect() const;
+    int scrubbable_range_in_pixels() const;
 
     int m_min { 0 };
     int m_max { 0 };
     int m_value { 0 };
     int m_step { 1 };
     int m_big_step { 5 };
-};
 
+    bool m_scrubbing { false };
+    int m_scrub_start_value { 0 };
+    Point m_scrub_origin;
+};