PyQt: Does Signal Connection Order Matter When Moving Objects Between Threads?

PyQt: Connecting a Signal to a Slot to Initiate a Background Operation

In the context of multithreaded PyQt applications, connecting signals to slots is crucial for effective communication between thread-separated components. The issue arises when connecting signals to slots intended to execute background operations while updating UI elements.

In a specific example provided, a Scanner object is created in a separate thread to perform a background operation (scan_value) and update a progress bar in the main thread (progress). When the scan button is clicked, the signal is connected to the Scanner's scan slot.

The question raised is whether the order of these actions matters, particularly in terms of whether the signal connection occurs before or after moving the Scanner object to the other thread. According to the Qt documentation, the connection type is determined when the signal is emitted, and if the signal is emitted from a different thread, it is queued for later processing.

However, the provided code exhibits a problem, where the progress bar is only updated at the very end if the signal connection is made before the object is moved to the other thread. This discrepancy suggests that the issue may lie with the slot function rather than the signal emission.

In PyQt, the slot function must be explicitly decorated with the @pyqtSlot decorator to be recognized as a valid slot for signal connection. If the slot function is not properly annotated, the signal may not be correctly handled.

Edit:

It's important to note that this behavior regarding signal connection type determination only applies to recent versions of Qt (4.4 and later). In earlier versions, the connection type is determined at the time of connection, making the order of actions crucial.

Additionally, it's worth addressing the specific issue encountered in PyQt. PyQt uses proxy objects to wrap non-Qt slot functions when connecting to Qt signals. These proxy objects are automatically moved to the same thread as the receiving object, which can cause problems if the connection is made before the receiving object is moved.

To avoid this issue, it's recommended to use the @pyqtSlot decorator to create a proper Qt slot, eliminating the need for a proxy object and ensuring reliable signal handling.

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