# fileWrapper(configuration:) Serializes a document snapshot to a file wrapper. ``` 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 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 `.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 } } ``` > Note: SwiftUI calls this method on a background thread. Don’t > make user interface changes from that thread. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)