How to Ensure Real-Time UI Updates with Progress Bar During PyQt Background Operations?

PyQt: Connecting a Signal to a Slot to Initiate Background Operations with Progress Bar Updates

In PyQt, connecting signals to slots is crucial for initiating background operations while updating UI elements. A common scenario involves updating a progress bar during a long-running task.

Consider the following code snippet:

<code class="python">def update_progress_bar(new, old):
    fraction = (new - start) / (stop - start)
    progress.setValue(fraction * 100)

obj.value_changed.connect(update_progress_bar)</code>

In this example, the value_changed signal is emitted each time the value in obj changes, and the update_progress_bar slot is connected to it. This slot updates the progress bar's value based on the new value.

However, if the signal and slot connection is made after moving the worker object (in this case, a Scanner object) to a separate thread, the progress bar is only updated at the end of the operation. This is because both the signal and slot are running on the same thread, causing the slot to be executed after the background operation completes.

To resolve this issue, it is crucial to ensure that the signal-slot connection is made before moving the worker object to a separate thread. When the signal is emitted, Qt automatically determines the appropriate connection type based on where the signal is emitted and where the receiver is located. This ensures that the slot is executed on the correct thread.

Furthermore, it is important to mark the Python method used as the slot using the @pyqtSlot decorator. This decorator ensures that the method is recognized as a Qt slot, allowing it to be properly connected to signals.

<code class="python">from QtCore import pyqtSlot

class Scanner(QObject):

    @pyqtSlot()
    def scan(self):
        # Perform background operation
        progress.setValue(100)</code>

By following these guidelines, you can ensure that signals are emitted and slots are executed in the correct threads, enabling reliable background operations with real-time UI updates.

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