Q_PROPERTY

I'm confused about Q_PROPERTY

can any body tell what "Q_PROPERTY (QString text text READ WRITE setText NOTIFY textChanged)" means

Hello

Q_PROPERTY is a macro and a part of the system of Qt properties. Its purpose is to allow us to bind the properties that we use in our QML to C++ data by appropriate methods. Your example combines three things a QML property whose id is "text". Let's review each of them:

Text READ: it means "to get a value for the text property, call a C++ method text()".

WRITE setText: in other words, "if we change the value of the text property to QML by assigning a value, communicate to C++ by calling a method, setText (QString NewText).

TextChanged NOTIFY: This means that "If the value of the data in my C++ object is modified by a non - QML action, notify me that it has changed so that I can update the value of the property in QML by transmitting the signal of Qt, whose name is «textChanged()»»

There is still information on the QML and C++ integration in our documentation here:

http://developer.BlackBerry.com/native/documentation/Cascades/dev/integrating_cpp_qml/

Here is an example of my own as well:

Q_OBJECT

    // a Q_PROPERTY declaration is a binding between a property we can use in QML and underlying C++ methods and signals

Q_PROPERTY(
        // the name of our property as used from QML
        QString my_property
        // a method that will be called to populate our QML property with a value from C++
        READ getMyProperty
        // a method that will be called when we want to change the value of our corresponding C++ variable
        WRITE setMyProperty
        // a signal that will be emitted if the underlying data value changes. This signal will be automatically connected to our QML property
        // so that anywhere our property has been used, it's value will be updated
        NOTIFY signalMyPropertyChanged)

Figure 1 declaring a Q_PROPERTY

And then we declare signals and the associated methods:

public:

    // for my Q_PROPERTY
    QString getMyProperty();
    void setMyProperty(QString text);

    Q_INVOKABLE void resetMyProperty();

signals:
    // signal that is emitted if my property value changes
    void signalMyPropertyChanged();

Figure 2: declare the methods and signals used by the Q_PROPERTY

private:
    QString _some_text;

Figure 3: Our C++ data we bind to our property

Note that I also used Q_INVOKABLE authorizing the call directly from the resetMyProperty() to the specified method of QML.

QString ApplicationUI::getMyProperty() {
    qDebug() << "XXXX getMyProperty called";
    return _some_text;
}

void ApplicationUI::setMyProperty(QString some_text) {
    qDebug() << "XXXX setMyProperty called:" << some_text;
    _some_text = some_text;
    qDebug() << "XXXX setMyProperty emiting signalMyPropertyChanged";
    emit signalMyPropertyChanged();
}

void ApplicationUI::resetMyProperty() {
    qDebug() << "XXXX resetMyProperty called";
    setMyProperty("Hello I am a property");
}

Figure 4: implementation of C++ functions

    QmlDocument *qml = QmlDocument::create("asset:///main.qml").parent(this);

    // expose the property to QML
    qml->setContextProperty("my_prop", this);

Figure 5 expose our C++ object to QML

import bb.cascades 1.2

Page {
    Container {

        TextField {
            id: tf
            objectName: "tf"
            // this causes the READ method bound to our property to be called
            text: my_prop.my_property
        }
        Button {
            text: "Change Property"
            onClicked: {
                // this causes the WRITE method bound to our property to be called
                my_prop.my_property = tf.text
            }
        }
        Button {
            text: "Reset"
            onClicked: {
                // this calls a C++ method that was exposed using Q_INVOKABLE
                my_prop.resetMyProperty();
            }
        }
    }
}

Figure 6 the QML

When I run my application, I see that TextField tf is generated automatically and this is accomplished by the system calling the getMyProperty READING method. So, if I change the text and click the button of change of ownership, I see that the WRITE method is called. If I click on Reset, the Q_INVOKABLE method is called directly from QML and it my signal, so notify the QML bound property value has changed. This causes a call to the READ method and my text field in QML is updated.

XXXX getMyProperty called
XXXX setMyProperty called: "Hello"
XXXX setMyProperty emiting signalMyPropertyChanged
XXXX getMyProperty called
XXXX resetMyProperty called
XXXX setMyProperty called: "Hello I am a property"
XXXX setMyProperty emiting signalMyPropertyChanged
XXXX getMyProperty called

Figure 7: out of the call display console

Hope that helps

Martin

Tags: BlackBerry Developers

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    Hello again, here's the example as promised.

    To use the native filebrowse dialog box, the native code must run in its own thread to prevent the user interface in the Cascades to block. This is achieved by encapsulating all the dialog box code in a class derived from QThread.  The class I wrote is called FileBrowseDialog

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    #ifndef FILEBROWSEDIALOG_HPP_
#define FILEBROWSEDIALOG_HPP_

#include 
#include 
#include 

/*
 * The file browse dialog displays a dialog to browse and select
 * files from shared folders on the system.
 */
class FileBrowseDialog : public QThread
{
    Q_OBJECT

    /*
     * QML property to allow multiple selection
     */
    Q_PROPERTY(bool multiselect READ getMultiSelect WRITE setMultiSelect)

    /*
     * QML property to read the selected filenames
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    Q_PROPERTY(QVariant filepaths READ getFilePaths)

    /*
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     */
    Q_PROPERTY(QVariant filters READ getFilters WRITE setFilters)
public:
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    FileBrowseDialog(QObject* parent = 0);
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    /*
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public:
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    QVariant getFilters() const;
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    /*
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    bool getMultiSelect() const;
    void setMultiSelect(bool value);

signals:
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     * Signal emitted when the OK button has been clicked on the browse dialog
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     */
    void selectionCompleted();

    /*
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     */
    void selectionCancelled();

protected:
    /*
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     */
    virtual void run();

protected:
    dialog_instance_t m_dialog;
    bool m_multiSelect;
    QVariantList m_filePaths;
    QVariantList m_filters;
};

#endif /* FILEBROWSEDIALOG_HPP_ */

    FileBrowseDialog.cpp

    #include "FileBrowseDialog.hpp"
#include 
#include 

FileBrowseDialog::FileBrowseDialog(QObject* parent)
    : QThread(parent)
    , m_multiSelect(false)
{
    m_filters.push_back(QString("*.*"));
}

FileBrowseDialog::~FileBrowseDialog()
{
}

void FileBrowseDialog::show()
{
    if (!isRunning())
    {
        m_filePaths.clear();
        start();
    }
}

QVariant FileBrowseDialog::getFilePaths() const
{
    return m_filePaths;
}

bool FileBrowseDialog::getMultiSelect() const
{
    return m_multiSelect;
}

void FileBrowseDialog::setMultiSelect(bool value)
{
    m_multiSelect = value;
}

QVariant FileBrowseDialog::getFilters() const
{
    return m_filters;
}

void FileBrowseDialog::setFilters(QVariant const& value)
{
    m_filters = value.toList();
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}

void FileBrowseDialog::run()
{
    bps_initialize();

    //request all dialog events
    dialog_request_events(0);
    if (dialog_create_filebrowse(&m_dialog) != BPS_SUCCESS)
    {
        qDebug() << "Failed to create file browse dialog.";
        emit selectionCancelled();
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    //set the selection filters
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        for (QVariantList::iterator it = m_filters.begin(); it != m_filters.end(); ++it, ++i)
        {
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            if (!filter.trimmed().isEmpty())
            {
                int length = (filter.length() + 1) * sizeof(char);
                ext[i] = new char[length];
                strncpy(ext[i], filter.toAscii(), length);
            }
        }
        if (dialog_set_filebrowse_filter(m_dialog, (const char**)ext, m_filters.count()) != BPS_SUCCESS)
        {
            qDebug() << "unable to set file browse dialog extensions";
        }
        for (i = 0; i < m_filters.count(); i++)
        {
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        }
        delete ext;
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    if (dialog_show(m_dialog) != BPS_SUCCESS)
    {
        qDebug() << "Failed to show file browse dialog.";
        dialog_destroy(m_dialog);
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        return;
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    bool shutdown = false;
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        if (event)
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            {
                //0=ok, 1=cancel
                int selectedIndex = dialog_event_get_selected_index(event);

                if (selectedIndex == 1)
                {
                    int count;
                    char** filepaths;
                    if (BPS_SUCCESS == dialog_event_get_filebrowse_filepaths(event, &filepaths, &count))
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                        for (int i = 0; i < count; i++)
                        {
                            qDebug() << "selected file: " << filepaths[i];
                            m_filePaths.push_back(QString(filepaths[i]));
                        }
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                    }
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                }
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                {
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                }

                qDebug() << "Got file browse dialog click";
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        }
    }

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    }
}

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    -Filters: a read/write value is where you can specify one or more filters to file (for example, ".doc", "*.jpg") etc.

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    #include 
#include 
#include 

#include "app.hpp"
#include "FileBrowseDialog.hpp"

using namespace bb::cascades;

App::App()
{
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    qml->setContextProperty("cs", this);

    AbstractPane *root = qml->createRootNode();
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}

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    import bb.cascades 1.0
import Dialog.FileBrowse 1.0

Page {
    content: Container {
        Label { id: filebrowseDialogLabel }
        Button {
            text : "File Browse Dialog"
            onClicked: {
                filebrowseDialog.show();
            }
        }
        attachedObjects: [
            FileBrowseDialog {
                id: filebrowseDialog
                multiselect : true
                filters : ["*.doc","*.jpg","*.txt"]
                onSelectionCompleted: {
                    if(filebrowseDialog.filepaths.length>0)
                        filebrowseDialogLabel.text = filebrowseDialog.filepaths[0];
                    else
                        filebrowseDialogLabel.text = "no file selected";
                }
                onSelectionCancelled: {
                    filebrowseDialogLabel.text = "file browse dialog was cancelled";
                }
            }
        ]
    }
}

    And it's pretty much just invoke the native dialog file navigation in stunts. Please note save the file would follow a similar model, but I found that this dialog box was not particularly useful because it displays only a simple dialogbox with a text file name entry.

    See you soon

    Swann

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