Message attached property

Create Message attached property to propagate parameters like room, timeline, index and maxContentWidth down to the message content avoiding lots of boilerplate

src/messageattached.h

@@ -0,0 +1,78 @@
+// SPDX-FileCopyrightText: 2025 James Graham <james.h.graham@protonmail.com>
+// SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+
+#pragma once
+
+#include <QQmlEngine>
+#include <QQuickAttachedPropertyPropagator>
+#include <QQuickItem>
+
+#include "neochatroom.h"
+
+class MessageAttached : public QQuickAttachedPropertyPropagator
+{
+    Q_OBJECT
+    QML_NAMED_ELEMENT(Message)
+    QML_ATTACHED(MessageAttached)
+    QML_UNCREATABLE("")
+
+    /**
+     * @brief The room that the message comes from.
+     */
+    Q_PROPERTY(NeoChatRoom *room READ room WRITE setRoom NOTIFY roomChanged FINAL)
+
+    /**
+     * @brief The timeline for the current message.
+     */
+    Q_PROPERTY(QQuickItem *timeline READ timeline WRITE setTimeline NOTIFY timelineChanged FINAL)
+
+    /**
+     * @brief The index of the message in the timeline
+     */
+    Q_PROPERTY(int index READ index WRITE setIndex NOTIFY indexChanged FINAL)
+
+    /**
+     * @brief The width available to the message content.
+     */
+    Q_PROPERTY(qreal maxContentWidth READ maxContentWidth WRITE setMaxContentWidth NOTIFY maxContentWidthChanged FINAL)
+
+public:
+    explicit MessageAttached(QObject *parent = nullptr);
+
+    static MessageAttached *qmlAttachedProperties(QObject *object);
+
+    NeoChatRoom *room() const;
+    void setRoom(NeoChatRoom *room);
+
+    QQuickItem *timeline() const;
+    void setTimeline(QQuickItem *timeline);
+
+    int index() const;
+    void setIndex(int index);
+
+    qreal maxContentWidth() const;
+    void setMaxContentWidth(qreal maxContentWidth);
+
+Q_SIGNALS:
+    void roomChanged();
+    void timelineChanged();
+    void indexChanged();
+    void maxContentWidthChanged();
+
+protected:
+    void propagateMessage(MessageAttached *message);
+    void attachedParentChange(QQuickAttachedPropertyPropagator *newParent, QQuickAttachedPropertyPropagator *oldParent) override;
+
+private:
+    QPointer<NeoChatRoom> m_room;
+    bool m_explicitRoom = false;
+
+    QPointer<QQuickItem> m_timeline;
+    bool m_explicitTimeline = false;
+
+    int m_index;
+    bool m_explicitIndex = false;
+
+    qreal m_maxContentWidth = -1;
+    bool m_explicitMaxContentWidth = false;
+};