write(snapshot:to:previous:progress:)

Declaration

@concurrent func write(snapshot: sending Self.Snapshot, to destination: sending Self.Destination, previous: sending Self.Snapshot?, progress: consuming Subprogress) async throws

Parameters

• snapshot:

  The snapshot to write to disk.

• destination:

  The file URL to write to.

• previous:

  The last successfully written snapshot, or nil on the first save. Compare to content to write only what changed in package documents.

• progress:

  A Subprogress value to report writing progress. Consume it once with reporter(totalCount:) and call complete(count:) as units finish.

Discussion

SwiftUI calls this method in the background after obtaining a snapshot via snapshot(contentType:). Perform all serialization and disk access here.

For most documents, use FileWrapperDocumentWriter instead of implementing a custom writer. Only implement write yourself when you need capabilities FileWrapperDocumentWriter doesn’t provide — such as direct URL access for Core Graphics, AVFoundation, or other frameworks that operate on file paths:

@concurrent
func write(content image: sending CGImage, to destination: URL,
    previous: sending CGImage?, progress: consuming Subprogress
) async throws {
    guard let imageDestination =
        CGImageDestinationCreateWithURL(
            destination as CFURL,
            UTType.jpeg.identifier as CFString,
            1, nil
        ) else {
        throw CocoaError(.fileWriteUnknown)
    }
    CGImageDestinationAddImage(
        imageDestination, image, nil
    )
    guard CGImageDestinationFinalize(
        imageDestination
    ) else {
        throw CocoaError(.fileWriteUnknown)
    }
}

See Also

Writing a document

• Snapshot
• Destination