Scrub numbers by dragging their labels in the tool bars and layer opacity (12.12c1)

src/UI/NumericScrub.cpp

@@ -0,0 +1,76 @@
+#include "UI/NumericScrub.h"
+#include <QApplication>
+#include <QMouseEvent>
+#include <QWidget>
+#include <algorithm>
+#include <cmath>
+
+NumericScrub::NumericScrub(QWidget *label, Options options) : QObject(label), m_label(label), m_options(std::move(options))
+{
+    // A disabled label shows its parent's cursor, as Qt does.
+    label->setCursor(Qt::SizeHorCursor);
+    // Buttonless moves must arrive to end a lost release.
+    label->setMouseTracking(true);
+    label->installEventFilter(this);
+}
+
+bool NumericScrub::eventFilter(QObject *watched, QEvent *event)
+{
+    const QEvent::Type type = event->type();
+    // A double click's second press is a press too.
+    const bool press = type == QEvent::MouseButtonPress || type == QEvent::MouseButtonDblClick;
+    if (!press && type != QEvent::MouseMove && type != QEvent::MouseButtonRelease)
+        return false;
+    const auto *mouse = static_cast<QMouseEvent *>(event);
+    if (watched != m_label) {
+        // A popup opened mid-drag takes the release.
+        if (type == QEvent::MouseButtonRelease && mouse->button() == Qt::LeftButton)
+            end();
+        return false;
+    }
+    if (press) {
+        // Filters see a disabled label's presses before Qt drops them.
+        if (mouse->button() != Qt::LeftButton || !m_label->isEnabled())
+            return false;
+        // The label holds the grab: its own points stay exact.
+        m_press = mouse->position();
+        qApp->installEventFilter(this);
+        return true;
+    }
+    if (!m_press)
+        return false;
+    if (type == QEvent::MouseButtonRelease) {
+        if (mouse->button() == Qt::LeftButton)
+            end();
+        return true;
+    }
+    // Qt can lose a release: a buttonless move ends.
+    if (!mouse->buttons().testFlag(Qt::LeftButton)) {
+        end();
+        return true;
+    }
+    const QPointF translation = mouse->position() - *m_press;
+    if (!m_start) {
+        if (std::hypot(translation.x(), translation.y()) < 1)
+            return true;
+        m_start = m_options.value();
+        m_options.onStart();
+    }
+    double proposed = *m_start + translation.x() * m_options.sensitivity;
+    if (m_options.step && *m_options.step > 0)
+        proposed = std::round(proposed / *m_options.step) * *m_options.step;
+    m_options.set(std::min(m_options.high, std::max(m_options.low, proposed)));
+    return true;
+}
+
+void NumericScrub::end()
+{
+    if (!m_press)
+        return;
+    qApp->removeEventFilter(this);
+    m_press.reset();
+    if (m_start) {
+        m_start.reset();
+        m_options.onEnd();
+    }
+}