Only when this function returns GST_ITERATOR_OK, elem will contain a valid
value. elem must have been initialized to the type of the iterator or
initialized to zeroes with Value.unset. The caller is responsible for
unsetting or resetting elem with Value.unset or Value.reset
after usage.
When this function returns GST_ITERATOR_DONE, no more elements can be
retrieved from it.
A return value of GST_ITERATOR_RESYNC indicates that the element list was
concurrently updated. The user of it should call Iterator.resync to
get the newly updated list.
A return value of GST_ITERATOR_ERROR indicates an unrecoverable fatal error.
Get the next item from the iterator in elem.
Only when this function returns GST_ITERATOR_OK, elem will contain a valid value. elem must have been initialized to the type of the iterator or initialized to zeroes with Value.unset. The caller is responsible for unsetting or resetting elem with Value.unset or Value.reset after usage.
When this function returns GST_ITERATOR_DONE, no more elements can be retrieved from it.
A return value of GST_ITERATOR_RESYNC indicates that the element list was concurrently updated. The user of it should call Iterator.resync to get the newly updated list.
A return value of GST_ITERATOR_ERROR indicates an unrecoverable fatal error.