Monitoring a variable or an object changed asynchronously by a Callback function

[jefftee]

While I'm sure it's true that a queued connection (Qt::QueuedConnection) is slower, Qt also provides a direct connection (Qt::DirectConnection) which is a function call. The default is Qt::AutoConnection, in which Qt will use the appropriate direct or queued connection.

When the signal is emitted from a different thread than the receiving slot, a queued connection must be used. I believe all other cases, a direct connection will be used for the same thread, unless you override the parameter on the connect statement and specify a Qt::QueuedConnection.