Key message we could find in the source code:

1. after sap.ca.ui.utils.busydialog.requireBusyDialog() is called in application, we can only see the rotating wheel in UI after 1.5 seconds - defined in BUSYDIALOG_TIMEOUT.

2. the requireBusyDialog and releaseBusyDialog must be called in pair. There is private variable _iRequiredCount maintained inside the implementation. When require function is called, _iRequiredCount++; release for _iRequiredCount--.

3. it is obsolete. New application should use sap.m.BusyDialog instead.

request url: 

And connectionManager is responsible to call requireBusyDialog:

iRequiredCount changed from 0 to 1:

2. OpportunityAttachment data is requested. ConnectionManager calls another requireBusyDialog, iRequiredCount changed from 1 tp 2:

3. in detail view, ETAG refresh operations makes iRequiredCount changed from 2 to 3:

4. At this time, the response of request in step1 has been returned to frontend, ConnectionManager calls releaseBusyDialog: 

iRequiredCount changes from 3 to 2:

Since now iRequiredCount still > 0, the busyDialog will not be closed. 

5. the response for request in step2 is returnd, iRequiredCount changes from 2 to 1:

6. the ETAG request sent in step3 has response now, iRequiredCount changes from 1 to 0:

now busy dialog is closed: