Попробуй сперва вызвать
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Если я правильно понял это:
The minimum and maximum is the number of steps in the operation for which this progress dialog shows progress. For example, if the operation is to examine 50 files, this value minimum value would be 0, and the maximum would be 50.
Before examining the first file, call setValue(0). As each file is processed call setValue(1), setValue(2), etc., finally calling setValue(50) after examining the last file.