This callback type is used by g_file_measure_disk_usage() to make periodic progress reports when measuring the amount of disk spaced used by a directory.
These calls are made on a best-effort basis and not all types of GFile will support them. At the minimum, however, one call will always be made immediately.
In the case that there is no support, reporting will be set to FALSE (and the other values undefined) and no further calls will be made. Otherwise, the reporting will be TRUE and the other values all-zeros during the first (immediate) call. In this way, you can know which type of progress UI to show without a delay.
For g_file_measure_disk_usage() the callback is made directly. For g_file_measure_disk_usage_async() the callback is made via the default main context of the calling thread (ie: the same way that the final async result would be reported).
current_size is in the same units as requested by the operation (see G_FILE_MEASURE_APPARENT_SIZE).
The frequency of the updates is implementation defined, but is ideally about once every 200ms.
The last progress callback may or may not be equal to the final result. Always check the async result to get the final value.