* various file-sharing operations. On each file (or search),
* the callback is guaranteed to be called once with "START"
* and once with STOPPED; calls with PROGRESS, ERROR or COMPLETED
* are optional and depend on the circumstances; parent operations
* will be STARTED before child-operations and STOPPED after
* various file-sharing operations. On each file (or search),
* the callback is guaranteed to be called once with "START"
* and once with STOPPED; calls with PROGRESS, ERROR or COMPLETED
* are optional and depend on the circumstances; parent operations
* will be STARTED before child-operations and STOPPED after