Cascades Development

[Meghasanghvi]

Hi,

I make application to read accelerometer data. I create UI using C++ and when user click on the button application start taking users reading of accelerometer. I tried to update textarea or label text and update the x,y position of the component. How can i do that?

I attached my app.cpp file in which i want to update the button or label text and move image to different x,y coordinate.

[rakesh86shankar]

You can have global variables  for button,absolute layout properties and you can update it when you recieve the signal right.

Regards

Rakesh Shankar.P

[Meghasanghvi]

Ya i want to update its value but inside the button click event UI is not updated.

I also tried with Qthread for my example but in this also i have to check how worker thread and GUI thread is working together,

[vladest]

hi

its absolutely incorrect to try to catch all events in an event handler

you have to redesign your application

you have to give Qt's event dispatcher chance to get a piece of CPU

[Meghasanghvi]

Hi vladest,

I am updating my UI using the signal-slot mechanismof GUI and worker thread. Is it not a correct way to do multitaksing in Casacde. 

Below is my code for signal-slot mechanism.

QObject::connect(myThread, SIGNAL(signalGUI(float,float,float)), this,
SLOT(showUpdatedData(float,float,float)));

Thanks,

Megha.

[Meghasanghvi · Accepted solution]

I am successfully implemented multitasking on Cascade.

Below is my code in which both thread work in parallel.

Step 1: Extend the class with the Thread class

Step 2: Emit the signal whenever you want to update your UI from the Thread class

Step 3: 

QObject::connect(myThread, SIGNAL(signalGUI(float,float,float)), this,
SLOT(showUpdatedData(float,float,float)));

myThread is a object of thread class which we createad by exteding the Qthread.

signalGUI is a signal which we emit from the worker thread.

showUpdatedData is a slot in which we write the code to update the GUI.

This way we can implement multitasking on cascade.

[casantos]

Ressurrecting an old thread: I'm trying to load an image in a background thread and use it to update an ImageView. contained in an item of a ListView with grid layout. My code is like this:

== Worker.hpp ==

class Worker: public QObject {
Q_OBJECT
public:
    Worker(const QString& imagePath);

public slots:
    // Update image and text for the current item
    void doWork();

signals:
    void workDone(const Image& image);

private:
    const QString mImagePath;
};

== Worker.cpp ==

static bb::ImageData fromQImage(const QImage &qImage) {

    [...]
    return imageData;
}

Worker::Worker(const QString& imagePath) :
        mImagePath(imagePath) {
            ;
}

static QSize scaledSize(252, 252);

void Worker::doWork() {
    QImageReader reader(mImagePath);
    if (reader.canRead()) {
        QSize size = reader.size();
        int width = size.width();
        int height = size.height();
        if (width != height) {
            int aleft, atop, awidth, aheight;
            if (width < height) {
                aleft = 0;
                atop = (height - width) / 2, awidth = aheight = width;
            } else {
                aleft = (width - height) / 2;
                atop = 0, awidth = aheight = height;
            }
            QRect clipRect(aleft, atop, awidth, aheight);
            reader.setClipRect(clipRect);
        }
        reader.setScaledSize(scaledSize);
        Image img(fromQImage(reader.read()));
        emit workDone(img);
    }
}

== GalleryItem.hpp ==

class GalleryItem: public Container, public ListItemListener {
Q_OBJECT

public:
    GalleryItem(Container *parent = 0);
    ~GalleryItem();

public slots:
    void updateItem(const QString& text, const QString& imagePath);
    void select(bool select);
    void reset(bool selected, bool activated);
    void activate(bool activate);

private:
    ImageView *mThumbView;
    QThread *mWorkerThread;
    Label *mLabel;
    Container *mHighlight;
};

== GalleryItem.cpp ==

void GalleryItem::updateItem(const QString& text, const QString& imagePath) {
    Worker *worker = new Worker(imagePath);
    mWorkerThread = new QThread(this);
    worker->moveToThread(mWorkerThread);

    connect(mWorkerThread, SIGNAL(started()), worker, SLOT(doWork()));
    connect(mWorkerThread, SIGNAL(finished()), worker, SLOT(deleteLater()));
    connect(worker, SIGNAL(workDone(Image)), mThumbView, SLOT(setImage(Image)), Qt::QueuedConnection);
    connect(worker, SIGNAL(workDone(Image)), mWorkerThread, SLOT(quit()));
    // Starts an event loop, and emits workerThread->started()
    mWorkerThread->start();
    mLabel->setText(text);
}

===

If I leave the code as is I get a bunch of messages like this in the console:

ApplicationPrivate::context: ERROR called from non-UI thread QThread(0x81f30b0)
ApplicationPrivate::context: Method called from non-UI thread

Then the application crashes. If I comment out the call to moveToThread(). Ths looks strange, since it seems to be the right way to use QThread, according to the documentation (http://qt-project.org/doc/qt-4.8/qthread.html).

This is also the apprach taken in the ImageLoader example (https://github.com/blackberry/Cascades-Samples/tree/master/imageloader).