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

Qt - Updating Main Window from a Different Thread

Introduction:

Multithreaded Qt applications can encounter situations where processes running in the main window need to be updated from a secondary thread. However, directly accessing the main window UI from another thread is not advisable. This article explores how to safely update the main window from a different thread by leveraging thread signals and slots.

Problem Statement:

In the provided code, the mythread class attempts to add a QLabel to the main window's horizontalLayout_4 from a separate thread. However, the ana->ui->horizontalLayout_4->addWidget(label) line fails to compile.

Solution:

To address this issue, a more suitable approach is to place UI modifications within a slot in the main window and connect a thread signal to that slot. This ensures that GUI functionality remains in the main thread and can be signaled from other threads.

Here's an example:

Main UI (MainUI.h and MainUI.cpp):

<code class="cpp">class MainUI : public QWidget
{
    Q_OBJECT

public:
    explicit MainUI(QWidget *parent = 0) : QWidget(parent)
    {
        // Initialize layout and thread
        layout = new QHBoxLayout(this);
        setLayout(layout);

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

        // Connect thread signal to main window slot
        connect(updater, SIGNAL(requestNewLabel(QString)), this, SLOT(createLabel(QString)));
        connect(thread, SIGNAL(destroyed()), updater, SLOT(deleteLater()));

        // Initiate label creation from thread
        updater->newLabel("path/to/image.png");
    }

public slots:
    void createLabel(const QString &imgSource)
    {
        // Create and add label to layout in main thread
        QPixmap i1(imgSource);
        QLabel *label = new QLabel(this);
        label->setPixmap(i1);
        layout->addWidget(label);
    }

private:
    QHBoxLayout *layout;
};</code>

Worker Object (GUIUpdater.h and GUIUpdater.cpp):

<code class="cpp">class GUIUpdater : public QObject
{
    Q_OBJECT

public:
    explicit GUIUpdater(QObject *parent = 0) : QObject(parent) {}

    void newLabel(const QString &image)
    {
        // Emit signal to main window
        emit requestNewLabel(image);
    }

signals:
    void requestNewLabel(const QString &);
};</code>

In this example:

• MainUI is the main window that receives UI updates.
• GUIUpdater is a worker object created in a separate thread.
• When GUIUpdater::newLabel is called from the separate thread, it emits the requestNewLabel signal.
• The MainUI.createLabel slot is connected to the requestNewLabel signal, allowing GUI updates to be performed safely within the main thread.

By using this approach, UI modifications are performed in the main thread while initiating the process from a separate thread. This ensures thread safety and prevents potential issues when accessing the main window's UI from different threads.

The above is the detailed content of How to Safely Update a Qt Main Window from a Different Thread?. For more information, please follow other related articles on the PHP Chinese website!