Share a body across all versions of a message.

lib/src/body.dart

Index: lib/src/body.dart
diff --git a/lib/src/body.dart b/lib/src/body.dart
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+++ b/lib/src/body.dart
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+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library shelf.body;
+
+import 'dart:async';
+import 'dart:convert';
+
+/// The body of a request or response.
+///
+/// This tracks whether the body has been read. It's separate from [Message]
+/// because the message may be changed with [Message.change], but each instance
+/// should share a notion of whether the body was read.
+class Body {
+  /// The contents of the message body.
+  ///
+  /// This will be `null` after [read] is called.
+  Stream<List<int>> _stream;
+
+  Body._(this._stream);
+
+  /// Converts [body] to a byte stream and wraps it in a [Body].
+  ///
+  /// [body] may be either a [Body], a [String], a [Stream<List<int>>], or
+  /// `null`. If it's a [String], [encoding] will be used to convert it to a
+  /// [Stream<List<int>>].
+  factory Body(body, [Encoding encoding]) {
+    if (encoding == null) encoding = UTF8;
+
+    if (body is Body) return body;
+
+    var stream;
+    if (body == null) {
+      stream = new Stream.fromIterable([]);
+    } else if (body is String) {
+      stream = new Stream.fromIterable([encoding.encode(body)]);
+    } else if (body is Stream) {
+      stream = body;
+    } else {
+      throw new ArgumentError('Response body "$body" must be a String or a '
+          'Stream.');
+    }
+
+    return new Body._(stream);
+  }
+
+  /// Returns a [Stream] representing the body.
+  ///
+  /// Can only be called once.
+  Stream<List<int>> read() {
+    if (_stream == null) {
+      throw new StateError("The 'read' method can only be called once on a "
+          "shelf.Request/shelf.Response object.");
+    }
+    var stream = _stream;
+    _stream = null;
+    return stream;
+  }
+}