How to Safely Update a Qt Main Window from a Secondary Thread?

Qt - Updating Main Window from a Secondary Thread

Problem:

In a multithreaded Qt application, an attempt to update the main window's UI (mainwindow.ui) from a thread other than the main thread results in an error. Specifically, the following code fails:

mythread::run()
{
    QPixmap i1 (":/notes/pic/4mdodiyez.jpg");
    QLabel *label = new QLabel();
    label->setPixmap(i1);
    ana->ui->horizontalLayout_4->addWidget(label);
}

Solution:

Modifying the UI from a secondary thread is not directly possible in Qt due to thread safety concerns. The recommended approach is to move the UI modifications to a slot in the main window and connect a signal from the secondary thread to that slot.

Implementation:

Implement a worker class to handle the updates:

class GUIUpdater : public QObject
{
    Q_OBJECT

public:
    explicit GUIUpdater(QObject *parent = 0) : QObject(parent) {}
    void newLabel(const QString &image) { emit requestNewLabel(image); }

signals:
    void requestNewLabel(const QString &);
};

Create and move the GUIUpdater object to a secondary thread:

QThread *thread = new QThread(this);
GUIUpdater *updater = new GUIUpdater();
updater->moveToThread(thread);

Connect the updater's requestNewLabel signal to a slot that creates the label in the main window:

connect(updater, SIGNAL(requestNewLabel(QString)), this, SLOT(createLabel(QString)));

Invoke the newLabel method from the secondary thread to trigger the update:

updater->newLabel("h:/test.png");

In the main window's slot:

void createLabel(const QString &imgSource) {
    QPixmap i1(imgSource);
    QLabel *label = new QLabel(this);
    label->setPixmap(i1);
    layout->addWidget(label);
}

This solution allows for safe and efficient UI updates from secondary threads while maintaining Qt's thread safety guarantees.

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