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

src/UI/NumericScrub.h

@@ -0,0 +1,39 @@
+#pragma once
+#include <QObject>
+#include <QPointF>
+#include <functional>
+#include <optional>
+
+class QWidget;
+
+// Swift's NumericScrub: a label dragged sideways changes its value.
+class NumericScrub : public QObject {
+public:
+    struct Options {
+        double sensitivity;
+        double low;
+        double high;
+        // Dragged values snap to multiples of this, when set.
+        std::optional<double> step;
+        std::function<double()> value;
+        std::function<void(double)> set;
+        std::function<void()> onStart = [] {};
+        std::function<void()> onEnd = [] {};
+    };
+
+    // Lives as the label's child and filters its events.
+    NumericScrub(QWidget *label, Options options);
+    // Ends a drag as a release would.
+    void end();
+
+protected:
+    bool eventFilter(QObject *watched, QEvent *event) override;
+
+private:
+    QWidget *const m_label;
+    const Options m_options;
+    // The press, until the release; Swift's gesture begins there.
+    std::optional<QPointF> m_press;
+    // Swift's startValue: set once the drag moves a point.
+    std::optional<double> m_start;
+};