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CreateStreamedFileAsync Method creates a
StorageFile to represent the specified stream of data.This method lets the app produce the data on-demand by specifying a function to be invoked when the StorageFile that represents the stream is first accessed.
StreamedFileDataRequestedHandler produce the data stream represented by the returned StorageFile and lets the app produce data on-demand instead of writing the contents of the file at creation time.
For more details about how to create and write a file with StorageFile. I suggest you could refer to the Doc:Create, write, and read a file
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