Fix issue with @Multipart & add body encoding tests

Example/main.swift

@@ -34,7 +34,8 @@ let provider = Provider(baseURL: "http://127.0.0.1:3000")
         let start = Date()
         let res = try await next(req)
         let elapsedTime = String(format: "%.2fs", Date().timeIntervalSince(start))
-        print("Got a \(res.statusCode!) for \(req.method) \(req.url!) after \(elapsedTime)")
+        let statusCode = res.statusCode.map { "\($0)" } ?? "N/A"
+        print("Got a \(statusCode) for \(req.method) \(req.url!) after \(elapsedTime)")
         return res
     }
 

PapyrusCore/Sources/Multipart/MultipartEncoder.swift

@@ -16,15 +16,15 @@ public struct MultipartEncoder: RequestEncoder {
 
     public func encode(_ value: some Encodable) throws -> Data {
         guard let parts = value as? [String: Part] else {
-            preconditionFailure("Can only encode `[String: Part]` with `MultipartEncoder`.")
+            preconditionFailure("Can only encode `[String: Part]` with `MultipartEncoder`. Got \(type(of: value)) instead")
         }
 
         let initialBoundary = Data("--\(boundary)\(crlf)".utf8)
         let middleBoundary = Data("\(crlf)--\(boundary)\(crlf)".utf8)
         let finalBoundary = Data("\(crlf)--\(boundary)--\(crlf)".utf8)
 
         var body = Data()
-        for (key, part) in parts {
+        for (key, part) in parts.sorted(by: { $0.key < $1.key }) {
             body += body.isEmpty ? initialBoundary : middleBoundary
             body += partHeaderData(part, key: key)
             body += part.data
@@ -44,7 +44,7 @@ public struct MultipartEncoder: RequestEncoder {
             headers["Content-Type"] = mimeType
         }
 
-        let string = headers.map { "\($0): \($1)\(crlf)" }.joined() + crlf
+        let string = headers.map { "\($0): \($1)\(crlf)" }.sorted().joined() + crlf
         return Data(string.utf8)
     }
 

PapyrusCore/Sources/RequestBuilder.swift

@@ -70,6 +70,7 @@ public struct RequestBuilder {
     public enum Content {
         case value(ContentValue)
         case fields([ContentKey: ContentValue])
+        case multipart([ContentKey: Part])
     }
 
     public static var defaultQueryEncoder: URLEncodedFormEncoder = URLEncodedFormEncoder()
@@ -153,11 +154,26 @@ public struct RequestBuilder {
         body = .value(ContentValue(value))
     }
 
+    public mutating func addField(_ key: String, value: Part, mapKey: Bool = true) {
+        var parts: [ContentKey: Part] = [:]
+        if let body = body {
+            guard case .multipart(let existingParts) = body else {
+                preconditionFailure("Tried to add a multipart Part, \(key), to a request but it already had non multipart fields added to it. If you use @Multipart, all fields on the request must be of type Part.")
+            }
+
+            parts = existingParts
+        }
+
+        let key: ContentKey = mapKey ? .implicit(key) : .explicit(key)
+        parts[key] = value
+        body = .multipart(parts)
+    }
+
     public mutating func addField<E: Encodable>(_ key: String, value: E, mapKey: Bool = true) {
         var fields: [ContentKey: ContentValue] = [:]
         if let body = body {
             guard case .fields(let existingFields) = body else {
-                preconditionFailure("Tried to add a @Field, \(key): \(E.self), to a request, but it already had a @Body, \(body). @Body and @Field are mutually exclusive.")
+                preconditionFailure("Tried to add a Field, \(key): \(E.self), to a request, but it already had Body or Multipart parameters, \(body). Body, Field, and Multipart are mutually exclusive.")
             }
 
             fields = existingFields
@@ -204,6 +220,10 @@ public struct RequestBuilder {
             return nil
         case .value(let value):
             return try requestEncoder.encode(value)
+        case .multipart(let fields):
+            let pairs = fields.map { ($0.mapped(keyMapping), $1) }
+            let dict = Dictionary(uniqueKeysWithValues: pairs)
+            return try requestEncoder.encode(dict)
         case .fields(let fields):
             let pairs = fields.map { ($0.mapped(keyMapping), $1) }
             let dict = Dictionary(uniqueKeysWithValues: pairs)

PapyrusCore/Sources/ResponseDecoder.swift

@@ -7,8 +7,8 @@ public protocol ResponseDecoder: KeyMappable {
 // MARK: application/json
 
 extension ResponseDecoder where Self == JSONDecoder {
-    public static func json(_ decoder: JSONDecoder) -> Self {
-        decoder
+    public static var json: Self {
+        JSONDecoder()
     }
 }
 

PapyrusCore/Tests/RequestBuilderTests.swift

@@ -16,4 +16,100 @@ final class RequestBuilderTests: XCTestCase {
         let req = RequestBuilder(baseURL: "foo/", method: "bar", path: "/baz")
         XCTAssertEqual(try req.fullURL().absoluteString, "foo/baz")
     }
+
+    func testMultipart() throws {
+        var req = RequestBuilder(baseURL: "foo/", method: "bar", path: "/baz")
+        let encoder = MultipartEncoder(boundary: UUID().uuidString)
+        req.requestEncoder = encoder
+        req.addField("a", value: Part(data: Data("one".utf8), fileName: "one.txt", mimeType: "text/plain"))
+        req.addField("b", value: Part(data: Data("two".utf8)))
+        let (body, headers) = try req.bodyAndHeaders()
+        guard let body else {
+            XCTFail()
+            return
+        }
+
+        // 0. Assert Headers
+
+        XCTAssertEqual(headers, [
+            "Content-Type": "multipart/form-data; boundary=\(encoder.boundary)",
+            "Content-Length": "266"
+        ])
+
+        // 1. Assert Body
+
+        XCTAssertEqual(body.string, """
+            --\(encoder.boundary)\r
+            Content-Disposition: form-data; name="a"; filename="one.txt"\r
+            Content-Type: text/plain\r
+            \r
+            one\r
+            --\(encoder.boundary)\r
+            Content-Disposition: form-data; name="b"\r
+            \r
+            two\r
+            --\(encoder.boundary)--\r
+
+            """
+        )
+    }
+
+    func testJSON() async throws {
+        var req = RequestBuilder(baseURL: "foo/", method: "bar", path: "/baz")
+        let encoder = JSONEncoder()
+        encoder.outputFormatting = [.sortedKeys, .prettyPrinted]
+        req.requestEncoder = encoder
+        req.addField("a", value: "one")
+        req.addField("b", value: "two")
+        let (body, headers) = try req.bodyAndHeaders()
+        guard let body else {
+            XCTFail()
+            return
+        }
+
+        // 0. Assert Headers
+
+        XCTAssertEqual(headers, [
+            "Content-Type": "application/json",
+            "Content-Length": "32"
+        ])
+
+        // 1. Assert Body
+
+        XCTAssertEqual(body.string, """
+            {
+              "a" : "one",
+              "b" : "two"
+            }
+            """
+        )
+    }
+
+    func testURLForm() async throws {
+        var req = RequestBuilder(baseURL: "foo/", method: "bar", path: "/baz")
+        req.requestEncoder = URLEncodedFormEncoder()
+        req.addField("a", value: "one")
+        req.addField("b", value: "two")
+        let (body, headers) = try req.bodyAndHeaders()
+        guard let body else {
+            XCTFail()
+            return
+        }
+
+        // 0. Assert Headers
+
+        XCTAssertEqual(headers, [
+            "Content-Type": "application/x-www-form-urlencoded",
+            "Content-Length": "11"
+        ])
+
+        // 1. Assert Body
+        XCTAssertTrue(["a=one&b=two", "b=two&a=one"].contains(body.string))
+    }
+}
+
+extension Data {
+    fileprivate var string: String {
+        String(decoding: self, as: UTF8.self)
+    }
 }