Add a multipart HTTP request class.

pkg/http/lib/src/multipart_file.dart

+// Copyright (c) 2012, 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 multipart_file;
+
+import 'dart:io';
+
+import 'utils.dart';
+
+/// A file to be uploaded as part of a [MultipartRequest]. This doesn't need to
+/// correspond to a physical file.
+class MultipartFile {
+  /// The name of the form field for the file.
+  final String field;
+
+  /// The size of the file in bytes. This must be known in advance, even if this
+  /// file is created from an [InputStream].
+  final int length;
+
+  /// The basename of the file. May be null.
+  final String filename;
+
+  /// The content-type of the file. Defaults to `application/octet-stream`.
+  final ContentType contentType;
+
+  /// The stream that will emit the file's contents.
+  final InputStream _stream;
+
+  /// Whether [finalize] has been called.
+  bool get isFinalized => _isFinalized;
+  bool _isFinalized = false;
+
+  /// Creates a new [MultipartFile] from an [InputStream]. The length of the
+  /// file in bytes must be known in advance. If it's not, read the data from
+  /// the stream and use [MultipartFile.fromBytes] instead.
+  ///
+  /// [contentType] currently defaults to `application/octet-stream`, but in the
+  /// future may be inferred from [filename].
+  MultipartFile(this.field, this._stream, this.length,
+      {this.filename, ContentType contentType})
+    : this.contentType = contentType != null ? contentType :
+          new ContentType("application", "octet-stream");
+
+  /// Creates a new [MultipartFile] from a byte array.
+  ///
+  /// [contentType] currently defaults to `application/octet-stream`, but in the
+  /// future may be inferred from [filename].
+  factory MultipartFile.fromBytes(String field, List<int> value,
+      {String filename, ContentType contentType}) {
+    var stream = new ListInputStream();
+    stream.write(value);
+    stream.markEndOfStream();
+    return new MultipartFile(field, stream, value.length,
+        filename: filename,
+        contentType: contentType);
+  }
+
+  /// Creates a new [MultipartFile] from a string.
+  ///
+  /// The encoding to use when translating [value] into bytes is taken from
+  /// [contentType] if it has a charset set. Otherwise, it defaults to UTF-8.
+  /// [contentType] currently defaults to `text/plain; charset=utf-8`, but in
+  /// the future may be inferred from [filename].
+  factory MultipartFile.fromString(String field, String value,
+      {String filename, ContentType contentType}) {
+    contentType = contentType == null ? new ContentType("text", "plain") :
+        // Make a copy of the original contentType so we can modify charset.

[Bob Nystrom, 2012/11/07 00:50:26]

If this is the motivation, maybe add a clone method to ContentType?

[nweiz, 2012/11/07 20:53:29]

I think there's a broader language-level (or at least all-libraries-level) issue
of "how do I clone an object?". I don't think the solution is to add one-off
clone methods to every class that we happen to want to clone somewhere. I
suspect something better will come out of the discussions on serialization,
since the problems are related.

+        new ContentType.fromString(contentType.toString());
+    var charset = contentType.charset;
+    var encoding = encodingForCharset(contentType.charset, Encoding.UTF_8);
+    contentType.charset = encoding.name;
+
+    return new MultipartFile.fromBytes(field, encodeString(value, encoding),
+        filename: filename,
+        contentType: contentType);
+  }
+
+  // TODO(nweiz): Infer the content-type from the filename.
+  /// Creates a new [MultipartFile] from a [File].
+  ///
+  /// [filename] defaults to the name of the file on disk. [contentType]
+  /// currently defaults to `application/octet-stream`, but in the future may be
+  /// inferred from [filename].
+  static Future<MultipartFile> fromFile(String field, File file,
+      {String filename, ContentType contentType}) {
+    if (filename == null) filename = new Path(file.name).filename;
+    return file.length().transform((length) {
+      return new MultipartFile(field, file.openInputStream(), length,
+          filename: filename,
+          contentType: contentType);
+    });
+  }
+
+  // Finalizes the file in preparation for it being sent as part of a
+  // [MultipartRequest]. This returns an [InputStream] that should emit the body
+  // of the file. The stream may be closed to indicate an empty file.
+  InputStream finalize() {
+    if (finalized) {
+      throw new StateError("Can't finalize a finalized MultipartFile.");
+    }
+    _isFinalized = true;
+    return _stream;
+  }
+}