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Thread: Loading additional QML files

  1. #1
    Join Date
    Feb 2015
    Posts
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    Qt products
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    Platforms
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    Default Loading additional QML files

    Hello,

    I'm struggling with dynamic loading of *.qml files. Although my real application is way bigger I have managed to reduce the problem to following code:

    (1) Single c++ application loading main.qml and then loading WindControlItem.qml

    Qt Code:
    1. /*
    2.  * main.cpp
    3.  */
    4. #include <QGuiApplication>
    5. #include <QQmlApplicationEngine>
    6. #include <QQuickItem>
    7.  
    8. int main(int argc, char *argv[])
    9. {
    10. QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    11. QGuiApplication app(argc, argv);
    12. QQmlApplicationEngine engine;
    13. engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    14. if (engine.rootObjects().isEmpty())
    15. return -1;
    16.  
    17. // Root object for finding QML Items by their objectName
    18. QObject* root = engine.rootObjects()[0];
    19. assert(root != nullptr);
    20.  
    21. // The QML Item to which we want to inject our QML-Item-loaded-from-file
    22. QQuickItem* centralPane = qobject_cast<QQuickItem*>(root->findChild<QObject*>("centralPane"));
    23. assert(centralPane != nullptr);
    24.  
    25. // Load the QML file to a component
    26. QString qml_path = "WeatherCtrlItem.qml";
    27. QQmlComponent comp(&engine, QUrl::fromLocalFile(qml_path));
    28.  
    29. // Create an instance of the component
    30. QObject* obj = comp.create();
    31. assert(obj != nullptr);
    32.  
    33. // Up-cast it to a QQuickItem
    34. QQuickItem *item = qobject_cast<QQuickItem*>(obj);
    35. assert(item != nullptr);
    36.  
    37. // Born C++, die QML
    38. engine.setObjectOwnership(item, QQmlEngine::JavaScriptOwnership);
    39.  
    40. // Set QObject-parent to the central pane
    41. item->setParent(centralPane);
    42.  
    43. // Set Visual-parent to the central pane
    44. item->setParentItem(centralPane);
    45.  
    46. return app.exec();
    47. }
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    (2) Content of main.qml

    Qt Code:
    1. /*
    2.  * main.qml
    3.  */
    4.  
    5. import QtQuick 2.10
    6. import QtQuick.Controls 1.4
    7.  
    8. ApplicationWindow {
    9. id: root
    10. visible: true
    11. width: 800
    12. height: 600
    13.  
    14. menuBar: MenuBar {
    15. Menu {
    16. title: "File"
    17. MenuItem {
    18. text: "Exit"
    19. onTriggered: Qt.exit(0)
    20. }
    21. } // Menu
    22. } // MenuBar
    23.  
    24. Rectangle {
    25. anchors.left: parent.left
    26. width: (parent.width * 0.2)
    27. height: parent.height
    28. }
    29.  
    30. // I want this item to eventually become a parent of another dynamically loaded
    31. // item -- that's why I give it an objectName.
    32. Rectangle {
    33. objectName: "centralPane"
    34. width: parent.width * 0.6
    35. height: parent.height
    36. anchors.horizontalCenter: parent.horizontalCenter
    37. }
    38.  
    39. Rectangle {
    40. width: (parent.width * 0.2)
    41. height: parent.height
    42. anchors.right: parent.right
    43. }
    44.  
    45. } // ApplicationWindow
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    (3) Content of WindControlItem.qml

    Qt Code:
    1. /*
    2.  * WindControlItem.qml
    3.  */
    4. import QtQuick 2.10
    5. import QtQuick.Controls 1.4
    6.  
    7. Rectangle {
    8. id: recRoot
    9. color: "#778899"
    10. anchors.fill: parent
    11.  
    12. GroupBox {
    13. id: gpbLayers
    14. title: "Wind layers"
    15. anchors.fill: parent
    16.  
    17. Grid {
    18. id: grdFields
    19. anchors.fill: parent
    20. columns: 2
    21.  
    22. Text {
    23. text: "speed"
    24. }
    25.  
    26. TextField {
    27. id: fieldSpeed
    28. placeholderText: "meters/sec"
    29. }
    30.  
    31. Text {
    32. text: "direction"
    33. }
    34.  
    35. TextField {
    36. id: fieldDir
    37. placeholderText: "degrees"
    38. }
    39.  
    40. Button{
    41. text: "Add layer"
    42. }
    43.  
    44. } // Grid
    45.  
    46. } // GroupBox
    47.  
    48. } // Item root
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    When I start the application the GUI displays correctly. The problem is that after I close the ApplicationWindow lots of warnings are printed to the output:

    Qt Code:
    1. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/GroupBox.qml:184: ReferenceError: parent is not defined
    2. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/GroupBox.qml:204: ReferenceError: parent is not defined
    3. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/GroupBox.qml:205: ReferenceError: parent is not defined
    4. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/GroupBox.qml:206: ReferenceError: parent is not defined
    5. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/GroupBox.qml:216: ReferenceError: parent is not defined
    6. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/Private/Control.qml:90: ReferenceError: parent is not defined
    7. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/Private/AbstractCheckable.qml:123: ReferenceError: parent is not defined
    8. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/Private/Control.qml:90: ReferenceError: parent is not defined
    9. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/TextField.qml:656: ReferenceError: parent is not defined
    10. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/Private/TextInputWithHandles.qml:104: ReferenceError: parent is not defined
    11. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/Private/TextInputWithHandles.qml:131: ReferenceError: parent is not defined
    12. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/Private/Control.qml:90: ReferenceError: parent is not defined
    13. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/TextField.qml:656: ReferenceError: parent is not defined
    14. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/Private/TextInputWithHandles.qml:104: ReferenceError: parent is not defined
    15. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/Private/TextInputWithHandles.qml:131: ReferenceError: parent is not defined
    16. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/Private/Control.qml:90: ReferenceError: parent is not defined
    17. file:///C:/tools/Qt/5.12.0/msvc2017/qml/QtQuick/Controls/Private/BasicButton.qml:194: ReferenceError: parent is not defined
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    I have tried to debug the code with both Qt 5.11.3 and Qt 5.12.0 but the resulting behavior is identical.


    Finally my questions:

    (1) Am I using correct approach for dynamic loading of Items defined in *.QML file?

    (2) What could be the cause for the reference errors?


    Than you for your answers!

  2. #2
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    Default Re: Loading additional QML files

    Quote Originally Posted by jforejtn View Post
    (1) Am I using correct approach for dynamic loading of Items defined in *.QML file?
    Your case looks much more like the use case of a Loader.

    A general rule of thumb is that you do not attempt to access QML created objects by objectName and findChild/findChildren.
    This kind of dependency inversion (C++ code depending on specific objects in QML) is almost always a bad idea.

    Quote Originally Posted by jforejtn View Post
    (2) What could be the cause for the reference errors?
    Maybe order of deletion gone wrong.
    Try to not set JavaScriptOwnership, the QObject parent should be enough.

    But again, this looks like a classic Loader scenario to me.

    Cheers,
    _

  3. The following user says thank you to anda_skoa for this useful post:

    jforejtn (4th February 2019)

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