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HttpContentEncoding.md

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HTTP: Encoding, Content-Type, and Content-Length

Encoding

Every request instance has an Encoding encoding property that defaults to UTF8. The encoding can be set to something else (e.g. ASCII or LATIN1), but for most cases the default UTF8 should be sufficient.

The selected encoding is significant for three reasons:

  • The HTTP logic in this library may use the selected encoding to encode or decode the request body prior to sending. For example, if you set the request body as a String, it may be encoded to a Uint8List byte list or a ByteBuffer and then sent in that form.

  • The content-length will be calculated from the encoded request body. This is necessary because the number of bytes once encoded can vary based on the encoding used. (See the "Content-Length" section below.)

  • The content-type request header will include the charset parameter and it will be set to the string representation of the encoding. This tells the server how the request body will be encoded so that it can successfully decode the body.

    import 'dart:convert';

...

var request = new transport.Request();

// For a plain-text request, the default encoding is "UTF8" and the default
// mime-type is "text/plain".
print(request.contentType); // plain/text; charset=utf-8

// If we update the encoding, the charset parameter will reflect this change.
request.encoding = LATIN1;
print(request.contentType); // plain/text; charset=iso-8859-1

    Exception: if the contentType property is ever set manually, the charset parameter will no longer be updated when the encoding changes. This is explained below in the "Content-Type" section.

Content-Type

All of the request classes have a default value for the content-type based on the type of data being sent in the request body:

  • transport.Request: text/plain
  • transport.FormRequest: application/x-www-form-urlencoded
  • transport.JsonRequest: application/json
  • transport.MultipartRequest: multipart/form-data
  • transport.StreamedRequest: text/plain

The default content-type for transport.StreamedRequest is text/plain because it is unknown at the time of dispatch what data will be sent in the request body.

The content-type of a request is available via a MediaType contentType property (MediaType is from the http_parser package).

As mentioned above in the "Encoding" section, the charset parameter in the content-type is updated automatically whenever the encoding property changes. However, once you manually set contentType, this behavior will stop. In other words, we are assuming that if you set contentType manually, you are intentionally overriding the defaults and are taking responsibility for setting the charset parameter appropriately.

Content-Length

The content-length is set automatically for all Request classes since the length of the body in bytes is known before sending. The content-length of a request is available as a read-only int contentLength property and is the number of bytes of the request body when encoded.

The exception to this is transport.StreamedRequest since the body is sent asynchronously. When using transport.StreamedRequest, you must set the contentLength value manually.

Consider the following plain-text request:

var request = new transport.Request()
  ..body = 'Hello World ®';

print(request.contentType.toString());
// content-type: text/plain; charset=utf-8
print(request.contentLength);
// 14

As you can see, the content-type is text/plain because the request body is plain-text and the charset is utf-8 because UTF8 encoding is used by default.

If we change the encoding, the content-type and content-length will be updated accordingly:

var request = new transport.Request()
  ..body = 'Hello World ®'
  ..encoding = LATIN1;

print(request.contentType.toString());
// content-type: text/plain; charset=iso-8859-1
print(request.contentLength);
// 13