fileWrapper(configuration:)

Serializes a document snapshot to a file wrapper.

Declaration

func fileWrapper(configuration: Self.WriteConfiguration) throws -> FileWrapper

Parameters

configuration:
Information about a file that already exists for the document, if any.

Return Value

The destination to serialize the document contents to. The value can be a newly created FileWrapper or an update of the one provided in the configuration input.

Discussion

To store a document — for example, in response to a Save command — SwiftUI calls this method. Use it to serialize the document’s data and create or modify a file wrapper with the serialized data:

func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
    let data = try JSONEncoder().encode(model)
    return FileWrapper(regularFileWithContents: data)
}

To store a document as a package — a directory containing multiple files — track which parts of the model changed and reuse WriteConfiguration/existingFile as the base directory wrapper to avoid rewriting unchanged files. The example below stores collection metadata in a file and each note in its own <uuid>.json file at the package root and only rewrites the files that changed:

struct NoteCollection: FileDocument {
    var metadata: Metadata
    var notes: [Note] = []

    // ...

    func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
        let documentPackage = configuration.existingFile
            ?? FileWrapper(directoryWithFileWrappers: [:])
        let encoder = JSONEncoder()

        // Update the metadata if it changed.
        if metadata.isChanged {
            if let old = documentPackage.fileWrappers?[Metadata.fileName] {
                documentPackage.removeFileWrapper(old)
            }
            documentPackage.addRegularFile(
                withContents: try encoder.encode(metadata),
                preferredFilename: Metadata.fileName)
        }

        // Sync note wrappers with the notes collection.
        let currentNotes = Set(notes.map(\.filename))
        let notesFiles = documentPackage.fileWrappers ?? [:]

        // Remove notes no longer in the collection.
        for (filename, wrapper) in notesFiles {
            if !currentNotes.contains(filename)
                && filename != Metadata.fileName {
                documentPackage.removeFileWrapper(wrapper)
            }
        }

        // Write notes that are new or changed.
        for note in notes where note.isChanged {
            if let old = documentPackage.fileWrappers?[note.filename] {
                documentPackage.removeFileWrapper(old)
            }
            documentPackage.addRegularFile(
                withContents: try encoder.encode(note),
                preferredFilename: note.filename)
        }
        return documentPackage
    }
}