Add write support with automatic type coercion to OpcUaMonitoredNode

writeValue() Q_INVOKABLE coerces QML JS types to the exact C++ type
expected by the OPC UA node (auto-detected from DataType attribute via
opcUaDataTypeToQOpcUaType). Handles all scalar types, booleans, and
comma-separated array input. Adds writable property derived from
AccessLevel bits. Demo shows inline TextField + Write button for
writable nodes, "(READ-ONLY)" for others.

3 files changed, 277 insertions, 9 deletions

diff --git a/demo/NodePage.qml b/demo/NodePage.qml
index 1468817..442a6fd 100644
--- a/demo/NodePage.qml
+++ b/demo/NodePage.qml
@@ -91,21 +91,28 @@ Page {
         // Column headers
         RowLayout {
             Layout.fillWidth: true
-            spacing: 0
+            Layout.leftMargin: 12
+            Layout.rightMargin: 12
+            spacing: 12
             Label {
                 text: "Identifier"
                 font.bold: true
-                Layout.preferredWidth: 200
-                leftPadding: 12
+                Layout.preferredWidth: 160
             }
             Label {
                 text: "Value"
                 font.bold: true
+                Layout.preferredWidth: 160
+            }
+            Label {
+                text: "Write"
+                font.bold: true
                 Layout.fillWidth: true
             }
         }
 
         Rectangle {
+            id: separator
             Layout.fillWidth: true
             height: 1
             color: "#ccc"
@@ -128,6 +135,10 @@ Page {
                     id: node
                     nodeId: delegate.modelData
                     monitored: nodePage.StackView.status === StackView.Active
+                    onWriteCompleted: (success, message) => {
+                        var short_id = node.nodeId.substring(node.nodeId.indexOf(";s=") + 3);
+                        nodePage.logFunction(short_id + ": " + message);
+                    }
                 }
 
                 background: Rectangle {
@@ -137,19 +148,45 @@ Page {
 
                 contentItem: RowLayout {
                     spacing: 12
+
+                    // Column 1: Identifier
                     Label {
                         text: {
                             var idx = node.nodeId.indexOf(";s=");
                             return idx >= 0 ? node.nodeId.substring(idx + 3) : node.nodeId;
                         }
-                        Layout.preferredWidth: 188
+                        Layout.preferredWidth: 160
                         elide: Text.ElideRight
                     }
+
+                    // Column 2: Live value (always visible)
                     Label {
                         text: node.value !== undefined && String(node.value) !== "" ? String(node.value) : "—"
-                        Layout.fillWidth: true
+                        Layout.preferredWidth: 160
                         elide: Text.ElideRight
                     }
+
+                    // Column 3: Edit area (writable) or READ-ONLY label
+                    TextField {
+                        id: editField
+                        visible: node.writable
+                        Layout.fillWidth: true
+                        placeholderText: "Enter value..."
+                        onAccepted: node.writeValue(text)
+                    }
+                    Button {
+                        id: writeButton
+                        visible: node.writable
+                        text: "Write"
+                        onClicked: node.writeValue(editField.text)
+                    }
+                    Label {
+                        visible: !node.writable
+                        text: "(READ-ONLY)"
+                        color: "gray"
+                        font.italic: true
+                        Layout.fillWidth: true
+                    }
                 }
 
                 ToolTip.visible: hovered
diff --git a/src/OpcUaMonitoredNode.cpp b/src/OpcUaMonitoredNode.cpp
index b158881..518ddd5 100644
--- a/src/OpcUaMonitoredNode.cpp
+++ b/src/OpcUaMonitoredNode.cpp
@@ -33,7 +33,8 @@ OpcUaMonitoredNode::setNodeId (const QString &id)
     return;
 
   teardownNode ();
-  if (OpcUaClient::instance () && OpcUaClient::instance ()->connected ())
+  auto *client = OpcUaClient::instance ();
+  if (client && client->connected ())
     setupNode ();
 }
 
@@ -58,6 +59,12 @@ OpcUaMonitoredNode::value () const
   return m_value;
 }
 
+bool
+OpcUaMonitoredNode::writable () const
+{
+  return m_writable;
+}
+
 OpcUaNodeInfo
 OpcUaMonitoredNode::info () const
 {
@@ -113,6 +120,8 @@ OpcUaMonitoredNode::setupNode ()
            &OpcUaMonitoredNode::handleAttributeUpdated);
   connect (m_node, &QOpcUaNode::valueAttributeUpdated, this,
            &OpcUaMonitoredNode::handleValueUpdated);
+  connect (m_node, &QOpcUaNode::attributeWritten, this,
+           &OpcUaMonitoredNode::handleAttributeWritten);
 
   m_node->readAttributes (
       QOpcUa::NodeAttribute::Value | QOpcUa::NodeAttribute::DisplayName
@@ -134,6 +143,14 @@ OpcUaMonitoredNode::teardownNode ()
 
   m_info = OpcUaNodeInfo{};
   emit infoChanged ();
+
+  m_valueType = QOpcUa::Types::Undefined;
+
+  if (m_writable)
+    {
+      m_writable = false;
+      emit writableChanged ();
+    }
 }
 
 /* ======================================
@@ -163,15 +180,24 @@ OpcUaMonitoredNode::handleAttributeUpdated (QOpcUa::NodeAttribute attr,
       break;
     case QOpcUa::NodeAttribute::DataType:
       {
-        auto ns0 = QOpcUa::namespace0IdFromNodeId (value.toString ());
+        const QString rawId = value.toString ();
+        m_valueType = QOpcUa::opcUaDataTypeToQOpcUaType (rawId);
+        auto ns0 = QOpcUa::namespace0IdFromNodeId (rawId);
         m_info.dataType = QOpcUa::namespace0IdName (ns0);
         if (m_info.dataType.isEmpty ())
-          m_info.dataType = value.toString ();
+          m_info.dataType = rawId;
       }
       break;
     case QOpcUa::NodeAttribute::AccessLevel:
       {
         quint32 bits = value.toUInt ();
+        bool newWritable
+            = (bits & quint32 (QOpcUa::AccessLevelBit::CurrentWrite)) != 0;
+        if (m_writable != newWritable)
+          {
+            m_writable = newWritable;
+            emit writableChanged ();
+          }
         QStringList flags;
         if (bits & quint32 (QOpcUa::AccessLevelBit::CurrentRead))
           flags << QStringLiteral ("Read");
@@ -187,7 +213,7 @@ OpcUaMonitoredNode::handleAttributeUpdated (QOpcUa::NodeAttribute attr,
       }
       break;
     default:
-      return;
+      return; // Skip infoChanged for attributes we don't track.
     }
   emit infoChanged ();
 }
@@ -223,3 +249,189 @@ OpcUaMonitoredNode::handleClientConnectedChanged ()
       teardownNode ();
     }
 }
+
+void
+OpcUaMonitoredNode::handleAttributeWritten (QOpcUa::NodeAttribute attr,
+                                            QOpcUa::UaStatusCode statusCode)
+{
+  if (attr != QOpcUa::NodeAttribute::Value)
+    return;
+
+  bool ok = (statusCode == QOpcUa::Good);
+  emit writeCompleted (ok, ok ? QStringLiteral ("Write successful")
+                              : QOpcUa::statusToString (statusCode));
+}
+
+/* ======================================
+ *  Write support
+ * ====================================== */
+
+/**
+ * @brief Convert a QML value to the C++ type matching m_valueType.
+ *
+ * Returns an invalid QVariant if conversion fails (e.g. non-numeric
+ * string for an integer node, or out-of-range for narrow types).
+ * Arrays are handled by recursing over each element.
+ */
+QVariant
+OpcUaMonitoredNode::coerceValue (const QVariant &input) const
+{
+  if (input.metaType ().id () == QMetaType::QVariantList)
+    {
+      QVariantList list = input.toList ();
+      QVariantList result;
+      result.reserve (list.size ());
+      for (const QVariant &item : list)
+        {
+          QVariant coerced = coerceValue (item);
+          if (!coerced.isValid ())
+            return {};
+          result.append (coerced);
+        }
+      return QVariant::fromValue (result);
+    }
+
+  switch (m_valueType)
+    {
+    case QOpcUa::Boolean:
+      {
+        if (input.metaType ().id () == QMetaType::Bool)
+          return QVariant (input.toBool ());
+        QString s = input.toString ().trimmed ().toLower ();
+        if (s == QStringLiteral ("true") || s == QStringLiteral ("1"))
+          return QVariant (true);
+        if (s == QStringLiteral ("false") || s == QStringLiteral ("0"))
+          return QVariant (false);
+        return {};
+      }
+    case QOpcUa::SByte:
+      {
+        bool ok = false;
+        qint32 v = input.toInt (&ok);
+        if (!ok || v < -128 || v > 127)
+          return {};
+        return QVariant::fromValue (static_cast<qint8> (v));
+      }
+    case QOpcUa::Byte:
+      {
+        bool ok = false;
+        quint32 v = input.toUInt (&ok);
+        if (!ok || v > 255)
+          return {};
+        return QVariant::fromValue (static_cast<quint8> (v));
+      }
+    case QOpcUa::Int16:
+      {
+        bool ok = false;
+        qint32 v = input.toInt (&ok);
+        if (!ok || v < -32768 || v > 32767)
+          return {};
+        return QVariant::fromValue (static_cast<qint16> (v));
+      }
+    case QOpcUa::UInt16:
+      {
+        bool ok = false;
+        quint32 v = input.toUInt (&ok);
+        if (!ok || v > 65535)
+          return {};
+        return QVariant::fromValue (static_cast<quint16> (v));
+      }
+    case QOpcUa::Int32:
+      {
+        bool ok = false;
+        qint32 v = input.toInt (&ok);
+        return ok ? QVariant::fromValue (v) : QVariant{};
+      }
+    case QOpcUa::UInt32:
+      {
+        bool ok = false;
+        quint32 v = input.toUInt (&ok);
+        return ok ? QVariant::fromValue (v) : QVariant{};
+      }
+    case QOpcUa::Int64:
+      {
+        bool ok = false;
+        qint64 v = input.toLongLong (&ok);
+        return ok ? QVariant::fromValue (v) : QVariant{};
+      }
+    case QOpcUa::UInt64:
+      {
+        bool ok = false;
+        quint64 v = input.toULongLong (&ok);
+        return ok ? QVariant::fromValue (v) : QVariant{};
+      }
+    case QOpcUa::Float:
+      {
+        bool ok = false;
+        float v = input.toFloat (&ok);
+        return ok ? QVariant::fromValue (v) : QVariant{};
+      }
+    case QOpcUa::Double:
+      {
+        bool ok = false;
+        double v = input.toDouble (&ok);
+        return ok ? QVariant::fromValue (v) : QVariant{};
+      }
+    case QOpcUa::String:
+      return QVariant (input.toString ());
+    case QOpcUa::DateTime:
+      {
+        QDateTime dt = input.toDateTime ();
+        return dt.isValid () ? QVariant::fromValue (dt) : QVariant{};
+      }
+    case QOpcUa::ByteString:
+      return QVariant::fromValue (input.toByteArray ());
+    default:
+      return input;
+    }
+}
+
+void
+OpcUaMonitoredNode::writeValue (const QVariant &value)
+{
+  if (!m_node)
+    {
+      emit writeCompleted (false, QStringLiteral ("Node not connected"));
+      return;
+    }
+  if (!m_writable)
+    {
+      emit writeCompleted (false, QStringLiteral ("Node is read-only"));
+      return;
+    }
+  if (m_valueType == QOpcUa::Types::Undefined)
+    {
+      emit writeCompleted (false,
+                           QStringLiteral ("Data type not yet resolved"));
+      return;
+    }
+
+  QVariant toWrite = value;
+
+  // QML text fields always send QString.  For array nodes (current
+  // value is QVariantList) split "1, 2, 3" into a list first so
+  // coerceValue can handle each element individually.
+  if (m_value.metaType ().id () == QMetaType::QVariantList
+      && value.metaType ().id () == QMetaType::QString)
+    {
+      QStringList parts
+          = value.toString ().split (QLatin1Char (','), Qt::SkipEmptyParts);
+      QVariantList list;
+      list.reserve (parts.size ());
+      for (const QString &part : parts)
+        list.append (QVariant (part.trimmed ()));
+      toWrite = QVariant::fromValue (list);
+    }
+
+  QVariant coerced = coerceValue (toWrite);
+  if (!coerced.isValid ())
+    {
+      emit writeCompleted (
+          false, QStringLiteral ("Cannot convert value to node data type"));
+      return;
+    }
+
+  if (!m_node->writeValueAttribute (coerced, m_valueType))
+    emit writeCompleted (false,
+                         QStringLiteral ("Write request failed to dispatch"));
+}
diff --git a/src/OpcUaMonitoredNode.h b/src/OpcUaMonitoredNode.h
index ac5983a..5895bea 100644
--- a/src/OpcUaMonitoredNode.h
+++ b/src/OpcUaMonitoredNode.h
@@ -57,6 +57,7 @@ class OpcUaMonitoredNode : public QObject, public QQmlParserStatus
   Q_PROPERTY (
       bool monitored READ monitored WRITE setMonitored NOTIFY monitoredChanged)
   Q_PROPERTY (QVariant value READ value NOTIFY valueChanged)
+  Q_PROPERTY (bool writable READ writable NOTIFY writableChanged)
   Q_PROPERTY (OpcUaNodeInfo info READ info NOTIFY infoChanged)
 
 public:
@@ -69,8 +70,19 @@ public:
   void setMonitored (bool monitored);
 
   QVariant value () const;
+  bool writable () const;
   OpcUaNodeInfo info () const;
 
+  /**
+   * @brief Write a value to the OPC UA node.
+   *
+   * Coerces @a value to the node's data type (auto-detected from the
+   * server's DataType attribute).  For array nodes, a comma-separated
+   * string is split and each element coerced individually.
+   * Result is reported asynchronously via writeCompleted().
+   */
+  Q_INVOKABLE void writeValue (const QVariant &value);
+
   void classBegin () override;
   void componentComplete () override;
 
@@ -78,23 +90,30 @@ signals:
   void nodeIdChanged ();
   void monitoredChanged ();
   void valueChanged ();
+  void writableChanged ();
   void infoChanged ();
+  void writeCompleted (bool success, const QString &message);
 
 private slots:
   void handleAttributeUpdated (QOpcUa::NodeAttribute attr,
                                const QVariant &value);
   void handleValueUpdated (const QVariant &value);
   void handleClientConnectedChanged ();
+  void handleAttributeWritten (QOpcUa::NodeAttribute attr,
+                               QOpcUa::UaStatusCode statusCode);
 
 private:
   void setupNode ();
   void teardownNode ();
+  QVariant coerceValue (const QVariant &input) const;
 
   QString m_nodeId;
   bool m_monitored = true;
   bool m_componentComplete = false;
 
   QOpcUaNode *m_node = nullptr;
+  QOpcUa::Types m_valueType = QOpcUa::Types::Undefined;
+  bool m_writable = false;
   QVariant m_value;
   OpcUaNodeInfo m_info;
 };