MultipartWriter

protocol MultipartWriter<OutboundBody> : Sendable

Overview

Writers conforming to this protocol can serialize multipart data by writing boundaries, headers, and body chunks in the proper multipart format. The protocol supports both streaming and buffered writing approaches.

Implementing a Custom Writer

Only write(bytes:) has to be implemented. Boundaries, header fields, and whole parts are all written in terms of it by the default implementations, so a writer that sends a message somewhere is as small as this:

struct StdoutMultipartWriter: MultipartWriter {
    typealias OutboundBody = [UInt8]

    let boundary: String

    func write(bytes: some Collection<UInt8> & Sendable) async throws {
        print(String(decoding: bytes, as: UTF8.self), terminator: "")
    }
}

// Usage example:
var writer = StdoutMultipartWriter(boundary: "boundary123")
try await writer.writeBoundary()
try await writer.writeHeaders([.contentType: "text/plain"])
try await writer.writeBodyChunk(Array("Hello, world!".utf8))
try await writer.finish()