Get rid of some pointless mutable state

Readme.md

@@ -12,17 +12,19 @@ Copy the files you want to use into your project, and then customize them to sui
 Create an encoder and then add parts to it as needed:
 
 ```Swift
-let encoder = MultipartFormEncoder()
-encoder.addPart(.text(name: "email", text: "somebody@example.com"))
-encoder.addPart(.text(name: "password", text: "secret"))
 let avatarData = UIImage(from: somewhere).jpegData(compressionQuality: 1)
-encoder.addPart(.binary(name: "avatar", type: "image/jpeg", data: avatarData, filename: "avatar.jpg"))
+let encoder = MultipartFormEncoder()
+let body = encoder.encode(parts: [
+    .text(name: "email", text: "somebody@example.com"),
+    .text(name: "password", text: "secret"),
+    .binary(name: "avatar", type: "image/jpeg", data: avatarData, filename: "avatar.jpg"),
+])
 ```
 
 The entire form is encoded as `Data` in memory so you may not want to use this for more than a few megabytes at a time:
 
 ```Swift
-let body = encoder.encode()
+let body = encoder.encode(parts: [/* ... */])
 var request = URLRequest(url: URL(string: "https://example.com/accounts")!)
 request.httpMethod = "POST"
 request.httpBody = body.data

Sources/Osiris/MultipartFormEncoder.swift

@@ -55,17 +55,11 @@ extension MultipartFormEncoder {
 final class MultipartFormEncoder {
     let boundary: String
 
-    private var parts: [Part] = []
-
     init(boundary: String? = nil) {
         self.boundary = boundary ?? "LifeIsMadeOfSeconds-\(UUID().uuidString)"
     }
 
-    func addPart(_ part: Part) {
+    func encode(parts: [Part]) -> Body {
-        parts.append(part)
-    }
-
-    func encode() -> Body {
         var bodyData = Data()
         for part in parts {
             // Header

Sources/Osiris/RequestBuilder.swift

@@ -37,10 +37,7 @@ final class RequestBuilder {
 
             case .multipart:
                 let encoder = MultipartFormEncoder()
-                for part in request.parts {
+                let body = encoder.encode(parts: request.parts)
-                    encoder.addPart(part)
-                }
-                let body = encoder.encode()
                 result.addValue(body.contentType, forHTTPHeaderField: "Content-Type")
                 result.addValue("\(body.contentLength)", forHTTPHeaderField: "Content-Length")
                 result.httpBody = body.data

Tests/OsirisTests/MultipartFormEncoderTests.swift

@@ -30,15 +30,14 @@ class MultipartFormEncoderTests: XCTestCase {
     }
 
     func testEncodeNothing() throws {
-        let body = subject.encode()
+        let body = subject.encode(parts: [])
         XCTAssertEqual(body.contentType, "multipart/form-data; boundary=\"SuperAwesomeBoundary\"")
         AssertBodyEqual(body.data, "--SuperAwesomeBoundary--")
     }
 
     func testEncodeText() throws {
-        subject.addPart(.text(name: "name", value: "Tina"))
         AssertBodyEqual(
-            subject.encode().data,
+            subject.encode(parts: [.text(name: "name", value: "Tina")]).data,
             [
                 "--SuperAwesomeBoundary",
                 "Content-Disposition: form-data; name=\"name\"",
@@ -50,9 +49,11 @@ class MultipartFormEncoderTests: XCTestCase {
     }
 
     func testEncodeData() throws {
-        subject.addPart(.binary(name: "video", data: Data("phony video data".utf8), type: "video/mp4", filename: "LiesSex&VideoTape.mp4"))
+        let data = Data("phony video data".utf8)
         AssertBodyEqual(
-            subject.encode().data,
+            subject.encode(parts: [
+                .binary(name: "video", data: data, type: "video/mp4", filename: "LiesSex&VideoTape.mp4"),
+            ]).data,
             [
                 "--SuperAwesomeBoundary",
                 "Content-Disposition: form-data; name=\"video\"; filename=\"LiesSex&VideoTape.mp4\"",
@@ -66,12 +67,15 @@ class MultipartFormEncoderTests: XCTestCase {
     }
 
     func testEncodeEverything() throws {
-        subject.addPart(.text(name: "name", value: "Queso"))
+        let imageData = Data("phony image data".utf8)
-        subject.addPart(.binary(name: "image", data: Data("phony image data".utf8), type: "image/jpeg", filename: "feltcute.jpg"))
+        let videoData = Data("phony video data".utf8)
-        subject.addPart(.text(name: "spot", value: "top of the bbq"))
-        subject.addPart(.binary(name: "video", data: Data("phony video data".utf8), type: "video/mp4", filename: "LiesSex&VideoTape.mp4"))
         AssertBodyEqual(
-            subject.encode().data,
+            subject.encode(parts: [
+                .text(name: "name", value: "Queso"),
+                .binary(name: "image", data: imageData, type: "image/jpeg", filename: "feltcute.jpg"),
+                .text(name: "spot", value: "top of the bbq"),
+                .binary(name: "video", data: videoData, type: "video/mp4", filename: "LiesSex&VideoTape.mp4"),
+            ]).data,
             [
                 "--SuperAwesomeBoundary",
                 "Content-Disposition: form-data; name=\"name\"",