---
title: "fileWrapper(configuration:)"
framework: swiftui
role: symbol
role_heading: Instance Method
path: "swiftui/filedocument/filewrapper(configuration:)"
---

# fileWrapper(configuration:)

Serializes a document snapshot to a file wrapper.

## Declaration

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

## Parameters

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

## Return Value

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

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     } } note: SwiftUI calls this method on a background thread. Don’t make user interface changes from that thread.

## See Also

### Writing a document

- [writableContentTypes](swiftui/filedocument/writablecontenttypes.md)
- [FileDocument.WriteConfiguration](swiftui/filedocument/writeconfiguration.md)