Does the Order of Connecting a Signal to a Slot Matter When Moving the Receiver to a Separate Thread in PyQt?

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

You have a scenario where you need to perform a background operation and update a progress bar in the user interface. The background operation, represented by scan_value, iterates through a range of values and emits a value_changed signal upon each change. While the obj.value_changed signal is connected to an appropriate slot that updates the progress bar, a question arises regarding the order of operations.

Initially, the code follows a conventional approach, connecting the signal to the slot before moving the Scanner object to a separate thread. However, a modification is proposed, where the signal connection occurs after initiating the thread and moving the Scanner object.

The Question:

Does the sequence of connecting the signal before or after moving the receiving object to another thread impact the behavior of the progress bar updates?

The Answer:

According to Qt documentation, the type of connection is determined based on where the signal is actually emitted. By default, QtCore.Qt.AutoConnection is utilized, ensuring that the signal behaves as Qt.QueuedConnection if emitted from a different thread and Qt.DirectConnection if emitted from the same thread.

However, the crucial point to consider is the slot implementation. In PyQt, the slot function requires the @pyqtSlot decorator to explicitly mark it as a Qt slot. This avoids relying on a proxy object, which can lead to problems if the signal connection is established before moving the receiving object to a separate thread.

In essence, the problematic scenario occurs when a proxy object is used to wrap a regular Python function and connect it to a Qt signal. Since PyQt automatically attempts to move the proxy to the same thread as the receiving object, if the connection is made prematurely, the proxy may remain in the main thread, causing unexpected behavior.

To resolve this issue, use the @pyqtSlot decorator to create Qt slots directly, eliminating the need for proxy objects. This method is consistent across both PyQt and PySide, ensuring proper behavior regardless of the order of signal connection and thread operations.

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