How can I change GUI from std::thread?

In Qt, like in many other GUI frameworks, the GUI may be updated from the main thread only. This means if you want to update the GUI from another thread, you have to communicate that to the main thread, which in turn will update the GUI.

For more details refer to these articles:

• Multithreading in Qt
• Qt: Threading Basics
• Qt: Multithreading Technologies

^{Additional resources in other languages: Правильная работа с потоками в Qt.}

As you asked for the demo with QThread, in the comments, then here it is.

As GUI, I have a mainwindow with two simple buttons and I want to show, hide the big buttons with a QThread (instead of just the slot clicked) I emit an intermediate signal from the clicked to hide/show the button.

The role of the QThreadis only to emit the signal to sigShowHide with argument trueor false.

The main UI thread treats this signal by showing or hiding the button by calling the slot onShowHideButtonThreaded which reacts to the signal sigShowHide

Here are the code files:

mainwindows.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QThread>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

signals:
        
       void sigShowHide(bool);

public slots:
       void onShowHideButtonThreaded(bool);
       void onButton1Click();


private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindows.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QObject::connect(ui->pushButton, &QPushButton::clicked, this, &MainWindow::onButton1Click);
    QObject::connect(this,&MainWindow::sigShowHide, this, &MainWindow::onShowHideButtonThreaded);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::onShowHideButtonThreaded(bool a)
{
    qDebug() << " the main thread id = " << QThread::currentThread() << "set the visibility ";
    ui->pushButton_2->setVisible(a);
}

void MainWindow::onButton1Click()
{
    qDebug()<< "clicked";
    qDebug() << " the main thread id = " << QThread::currentThread();
    QThread* l_thread = QThread::create([&]()
    {
        qDebug() << "Running Thread " << QThread::currentThreadId() << " to emit signal only ";
        emit sigShowHide( !this->ui->pushButton_2->isVisible());
    });
    l_thread->start();
  }

`

The main.cpp

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

An example of execution is :

the main thread id = QThread(0x116ee18)
Running Thread 0x1ed8 to emit signal only 
the main thread id = QThread(0x116ee18) set the visibility 

As stated in @rustyx's answer: In Qt, like in many other GUI frameworks, the GUI may be updated from the main thread only.

I was also stuck on this problem, and here are my two solutions:

1. Use QMetaObject::invokeMethod.

   QMetaObject::invokeMethod(ui->SettingsLabel, "setVisible", Q_ARG(bool, true));
   

   QMetaObject::invokeMethod is a thread-safe API, it has a Qt::ConnectionType type parameter, which has a default value Qt::AutoConnection.

   Descriptions of some Qt::ConnectionType values:

   Qt::AutoConnection:

   (Default) If the receiver lives in the thread that emits the signal, Qt::DirectConnection is used. Otherwise, Qt::QueuedConnection is used. The connection type is determined when the signal is emitted.

   Qt::DirectConnection:

   The slot is invoked immediately when the signal is emitted. The slot is executed in the signalling thread.

   Qt::QueuedConnection:

   The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.

   Qt::BlockingQueuedConnection:

   Same as Qt::QueuedConnection, except that the signalling thread blocks until the slot returns. This connection must not be used if the receiver lives in the signalling thread, or else the application will deadlock.

   It is clear from the description that the API will put your call request into a queue, and then the main thread will take it out from the queue to finally process your call request.

   And if you need to get the return value of the target method, you need to explicitly pass Qt::BlockingQueuedConnection to the Qt::ConnectionType type parameter. Because the indirect default value Qt::QueuedConnection will not wait for the call to complete, you will likely get the wrong return value.

   But this solution has a trouble that is it only supports signal and slot functions, NOT non-signal and non-slot functions.

2. Use connect.

   // In "MainWindow" class declaration
   //
   class MainWindow : public QMainWindow
   {
       // ...
   Q_SIGNALS:
       void setSettingsLabelVisibleSafety(bool value);
       // ...
   };
   
   // In "MainWindow" constructor
   //
   connect(this, &MainWindow::setSettingsLabelVisibleSafety, this,
       [this](bool value) {
           ui->SettingsLabel->setVisible(value);
       }
   );
   
   // In other thread
   //
   setSettingsLabelVisibleSafety(value);
   

   connect also has a Qt::ConnectionType type parameter, which has a default value Qt::AutoConnection if you pass the receiver parameter explicitly, otherwise the default value is Qt::DirectConnection. So you need to either explicitly pass the receiver parameter or explicitly pass Qt::AutoConnection value to the Qt::ConnectionType type parameter.

   If the target method has a return value, Qt::BlockingQueuedConnection needs to be passed explicitly as well.

   This solution supports signal and slot functions, as well as non-signal and non-slot functions.

Both of the above solutions require you to call qRegisterMetaType to register the user-defined types (if any).

qRegisterMetaType<YourType>("YourType");

Obviously, I prefer the second solution.