Fired before all the transfers begin.
Object_OnPreAllTransfers()
Multiple transfers may occur when ADF(Auto Document Feeder) is enabled or there are more than one frames in a page. In those cases, more than one OnPreTransfer Event will be fired, and only one OnPreAllTransfers Event will be fired.
This is the place to call CancelAllPendingTransfers().
OnPreTransfer Event, OnPostAllTransfers Event, OnPostTransfer Event, OnTransferCancelled Event, OnTransferError Event.