core/transformwatcher: add TransformWatcher

src/core/transformwatcher.hpp

@@ -0,0 +1,84 @@
+#pragma once
+
+#include <qlist.h>
+#include <qobject.h>
+#include <qqmlintegration.h>
+#include <qquickitem.h>
+#include <qquickwindow.h>
+#include <qtmetamacros.h>
+
+#ifdef QS_TEST
+class TestTransformWatcher;
+#endif
+
+///! Monitor of all geometry changes between two objects.
+/// The TransformWatcher monitors all properties that affect the geometry
+/// of two `Item`s relative to eachother.
+///
+/// > [!INFO] The algorithm responsible for determining the relationship
+/// > between `a` and `b` is biased towards `a` being a parent of `b`,
+/// > or `a` being closer to the common parent of `a` and `b` than `b`.
+class TransformWatcher: public QObject {
+	Q_OBJECT;
+	// clang-format off
+	Q_PROPERTY(QQuickItem* a READ a WRITE setA NOTIFY aChanged);
+	Q_PROPERTY(QQuickItem* b READ b WRITE setB NOTIFY bChanged);
+	/// Known common parent of both `a` and `b`. Defaults to `null`.
+	///
+	/// This property can be used to optimize the algorithm that figures out
+	/// the relationship between `a` and `b`. Setting it to something that is not
+	/// a common parent of both `a` and `b` will prevent the path from being determined
+	/// correctly, and setting it to `null` will disable the optimization.
+	Q_PROPERTY(QQuickItem* commonParent READ commonParent WRITE setCommonParent NOTIFY commonParentChanged);
+	/// This property is updated whenever the geometry of any item in the path from `a` to `b` changes.
+	///
+	/// Its value is undefined, and is intended to trigger an expression update.
+	Q_PROPERTY(QObject* transform READ transform NOTIFY transformChanged);
+	// clang-format on
+	QML_ELEMENT;
+
+public:
+	explicit TransformWatcher(QObject* parent = nullptr): QObject(parent) {}
+
+	[[nodiscard]] QQuickItem* a() const;
+	void setA(QQuickItem* a);
+
+	[[nodiscard]] QQuickItem* b() const;
+	void setB(QQuickItem* b);
+
+	[[nodiscard]] QQuickItem* commonParent() const;
+	void setCommonParent(QQuickItem* commonParent);
+
+	[[nodiscard]] QObject* transform() const { return nullptr; } // NOLINT
+
+signals:
+	void transformChanged();
+
+	void aChanged();
+	void bChanged();
+	void commonParentChanged();
+
+private slots:
+	void recalcChains();
+
+private:
+	void resolveChains(QQuickItem* a, QQuickItem* b, QQuickItem* commonParent);
+	void resolveChains();
+	void linkItem(QQuickItem* item) const;
+	void linkChains();
+	void unlinkChains();
+
+	QQuickItem* mA = nullptr;
+	QQuickItem* mB = nullptr;
+	QQuickItem* mCommonParent = nullptr;
+
+	// a -> traverse parent chain -> parent window -> global scope -> child window -> traverse child chain -> b
+	QList<QQuickItem*> parentChain;
+	QList<QQuickItem*> childChain;
+	QQuickWindow* parentWindow = nullptr;
+	QQuickWindow* childWindow = nullptr;
+
+#ifdef QS_TEST
+	friend class TestTransformWatcher;
+#endif
+};