Starts the asynchronous execution of an export session.
- iOS 4.0+
- macOS 10.7+
- tvOS 9.0+
A block that is invoked when writing is complete or in the event of writing failure.
This method starts an asynchronous export operation and returns immediately.
status signals the terminal state of the export session, and if a failure occurs,
error describes the problem.
If internal preparation for export fails,
handler is invoked synchronously. The handler may also be called asynchronously, after the method returns, in the following cases:
If a failure occurs during the export, including failures of loading, re-encoding, or writing media data to the output.
cancelExport() is invoked.
After the export session succeeds, having completely written its output to the