The event is called before the thread starts executing with peInit parameter. It is then called several times while the thread is executing with peCycle parameter. When the thread has finished executing it is called with peCleanUp parameter. 

The event is executed within the main VCL thread and any any visual components can be safelly updated from within this event. If ShowDialog property is True, there is no need to react on peCycle parameter, because a progress indicator will be provided for the user. peCycle parameter should be used only to update the user defined progress indicators. 

The OnProgressUpdate will be called in time intervals defined with UpdateInterval property. The Counter property can be used to check how many loops were executed, to compute the current progress. If InternalLoop is True, then this formula can be used: Counter/(Max-Min)*100, to compute execution progress in percent. The same formula can be used, if InternalLoop is false and Max, Min and Counter properties reflect the actual number of loops or progress.

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