How to Safely Update a Qt MainWindow from a Separate Thread?

Qt - Updating Main Window with Second Thread

In multithreaded Qt applications, updating the main UI (mainwindow.ui) from a separate thread can pose a challenge. This article addresses a common issue: Inability to access UI elements (such as ana->ui->horizontalLayout_4 in the given code) from within a custom thread.

The solution involves utilizing Qt's signal-slot mechanism, ensuring that UI modifications are performed only from within the main thread. Here's how to implement this approach:

1. Create a Slot in the Main Window:

   • Declare a slot in the main window class, such as createLabel(const QString &imgSource). This slot will handle UI modifications from other threads.

2. Define a Worker Object:

   • Create a class that inherits from QObject (instead of QThread) and define a method, newLabel(const QString &image), to emit a signal with the image source as a parameter. This class will serve as the worker object.

3. Move the Worker Object to the Second Thread:

   • Create an instance of the worker object and move it to the second thread using the moveToThread() method.

4. Connect Signals and Slots:

   • Connect the requestNewLabel signal emitted by the worker object to the createLabel slot in the main window.

5. Invoke the Worker Method:

   • From the second thread, call the newLabel method of the worker object, passing the image source as an argument. This will trigger the signal-slot connection and update the main UI from the main thread.

By implementing this approach, UI modifications from other threads can be safely handled through signals and slots, ensuring synchronization with the main thread. This prevents potential UI inconsistencies or crashes caused by directly accessing UI elements from non-main threads.

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