如何使用 Retrofit 发送 multipart/form-data？

Question

我想从 Android 客户端向 REST 服务器发送 Article。这是来自服务器的 Python 模型：

class Article(models.Model):
    author = models.CharField(max_length=256, blank=False)
    photo = models.ImageField()

以下接口描述了前一个实现：

@POST("/api/v1/articles/")
public Observable<CreateArticleResponse> createArticle(
        @Body Article article
);

现在我想发送包含文章数据的图像。 photo 不是 Android 客户端上 Article 模型的一部分。

@Multipart
@POST("/api/v1/articles/")
public Observable<CreateArticleResponse> createArticle(
        @Part("article") Article article,
        @Part("photo") TypedFile photo
);

API 已准备好并使用 cURL 成功测试。

$ curl -vX POST http://localhost:8000/api/v1/articles/ \
    -H "Content-Type: multipart/form-data" \
    -H "Accept:application/json" \
    -F "author=cURL" \
    -F "photo=@/home/user/Desktop/article-photo.png"

当我从 Android 客户端通过 createArticle() 发送数据时，我收到一个 HTTP 400 状态，表明 字段是 required/missing.

D  <--- HTTP 400 http://192.168.1.1/articles/ (2670ms)
D  Date: Mon, 20 Apr 2015 12:00:00 GMT
D  Server: WSGIServer/0.1 Python/2.7.8
D  Vary: Accept, Cookie
D  X-Frame-Options: SAMEORIGIN
D  Content-Type: application/json
D  Allow: GET, POST, HEAD, OPTIONS
D  OkHttp-Selected-Protocol: http/1.0
D  OkHttp-Sent-Millis: 1429545450469
D  OkHttp-Received-Millis: 1429545453120
D  {"author":["This field is required."],"photo":["No file was submitted."]}
D  <--- END HTTP (166-byte body)
E  400 BAD REQUEST

这是在服务器端收到的 request.data：

ipdb> print request.data  
  <QueryDict: {u'article': [u'{"author":"me"}'], \
  u'photo': [<TemporaryUploadedFile: IMG_1759215522.jpg \
  (multipart/form-data)>]}>

如何将 Article 对象转换为符合多部分的数据类型？我读到 Retrofit might allow to use Converters for this. It should be something that implements a retrofit.mime.TypedOutput as far as I understood for the documentation。

Multipart parts use the RestAdapter's converter or they can implement TypedOutput to handle their own serialization.

相关

HTML 4.01 Specification - Form submission - multipart/form-data
Retrofit Annotation Type Part documentation
Upload multipart image data in JSON with Retrofit?
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Retrofit issue #178: Create manual for sending files with retrofit
Retrofit issue #531: Problem uploading file via POST/Multipart
Retrofit issue #658: Not able to send string parameters with image when using Multipart
Retrofit issue #662: Retrofit Form Encoded and Multipart in single request

Answer 1

根据您的 curl 请求，您正在尝试创建如下内容：

POST http://localhost:8000/api/v1/articles/ HTTP/1.1
User-Agent: curl/7.30.0
Host: localhost
Connection: Keep-Alive
Accept: application/json
Content-Length: 183431
Expect: 100-continue
Content-Type: multipart/form-data; boundary=----------------------------23473c7acabb

------------------------------23473c7acabb
Content-Disposition: form-data; name="author"

cURL
------------------------------23473c7acabb
Content-Disposition: form-data; name="photo"; filename="article-photo.png"
Content-Type: application/octet-stream

‰PNG

<!RAW BYTES HERE!>

M\UUÕ+4qUUU¯°WUUU¿×ß¿þ Naa…k¿    IEND®B`‚
------------------------------23473c7acabb--

使用改装适配器可以通过以下方式创建此请求：

@Multipart
@POST("/api/v1/articles/")
Observable<Response> uploadFile(@Part("author") TypedString authorString,
                                @Part("photo") TypedFile photoFile);

用法：

TypedString author = new TypedString("cURL");
File photoFile = new File("/home/user/Desktop/article-photo.png");
TypedFile photoTypedFile = new TypedFile("image/*", photoFile);
retrofitAdapter.uploadFile(author, photoTypedFile)
               .subscribe(<...>);

创建类似的输出：

POST http://localhost:8000/api/v1/articles/ HTTP/1.1
Content-Type: multipart/form-data; boundary=32230279-83af-4480-abfc-88a880b21b19
Content-Length: 709
Host: localhost
Connection: Keep-Alive
Accept-Encoding: gzip
User-Agent: okhttp/2.3.0

--32230279-83af-4480-abfc-88a880b21b19
Content-Disposition: form-data; name="author"
Content-Type: text/plain; charset=UTF-8
Content-Length: 4
Content-Transfer-Encoding: binary

cUrl
--32230279-83af-4480-abfc-88a880b21b19
Content-Disposition: form-data; name="photo"; filename="article-photo.png"
Content-Type: image/*
Content-Length: 254
Content-Transfer-Encoding: binary

<!RAW BYTES HERE!>

--32230279-83af-4480-abfc-88a880b21b19--

这里的主要区别在于您使用 POJO Article article 作为多部分参数，默认情况下由 Converter 转换为 json。而您的服务器需要纯字符串。使用 curl，您发送的是 cURL，而不是 {"author":"cURL"}。

Answer 2

服务器需要一个 "author" 字符串，但您试图向它传递一个 "article" 对象。传递 "String author" 而不是 "Article article."

此外，我认为 "no file submitted" 错误是一个转移注意力的错误，因为该文件显然存在于您的 "request.data."

如何使用 Retrofit 发送 multipart/form-data？

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