start
Called before writing to the file begins so the provider can prepare file output.
Return
true if operation succeeded, false if writing should be aborted.
Parameters
write
Determines how file data should be written to the output. For APPEND_TO_FILE the data should be added to the end of existing file data. For REWRITE_FILE PSPDFKit will write full PDF document and the data should be written to a temporary file. The temporary file should replace then original file in finishWrite callback method. Original file must not be written to when rewriting is in progress - PSPDFKit will issue read calls while writing and attempting to write output to the same input will result in a corrupted file.