Dart

MIME Content-Transfer-Encoding Header Field

See more MIME Examples

Explains the Content-Transfer-Encoding header field and how it affects how data is stored in the MIME.

Dart

import 'package:chilkat/chilkat.dart';

void main() {
  // The Content-Transfer-Encoding header field is typically set to one of these values:
  // 
  //   base64
  //   quoted-printable
  //   binary
  //   8bit
  //   7bit

  // If the encoding is base64 or quoted-printable, then the bytes of the content are encoded as such.
  // The values "8bit", "7bit", and "binary" all imply that NO encoding has been performed.

  // Here are some guidelines and hints:
  // 
  // 1) "8bit", "7bit", and "binary" encodings (i.e. NO encoding) results in the smallest MIME.

  // 2) A "binary" encoding implies non-text data.  Binary MIME CANNOT be retrieved as a string.  It can only be retrieved as a byte array.

  // 3) "8bit" and "7bit" encodings imply text.  
  //    7bit implies that the text is us-ascii (all byte values less than or equal to 0x7F).
  //    8bit implies that the text contains non-us-ascii chars.

  // 4) Base64 is the best choice for encoding non-text data, such as PDF's, images, etc.

  // 5) quoted-printable is the best choice for encoding text data where most chars are going to be us-ascii. This would typically 
  //    include all Western European languages.  For Asian, Arabic, Hebrew, etc. where most chars in the text are non-us-ascii,
  //    the most efficient encoding would be base64.  

  // Let's demonstrate with this small JPG image.

  final sbJpgBase64 = CkStringBuilder();
  sbJpgBase64.append('/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g\r\n');
  sbJpgBase64.append('NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf\r\n');
  sbJpgBase64.append('YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj\r\n');
  sbJpgBase64.append('Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA\r\n');
  sbJpgBase64.append('AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY\r\n');
  sbJpgBase64.append('NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+\r\n');
  sbJpgBase64.append('pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq\r\n');
  sbJpgBase64.append('Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc\r\n');
  sbJpgBase64.append('enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg\r\n');
  sbJpgBase64.append('xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9\r\n');
  sbJpgBase64.append('DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA\r\n');
  sbJpgBase64.append('AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA\r\n');
  sbJpgBase64.append('QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx\r\n');
  sbJpgBase64.append('EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==\r\n');

  final mime = CkMime();
  mime.contentType = 'image/jpeg';
  mime.setBodyFromEncoded('base64', sbJpgBase64.getAsString());

  print(mime.getMime());
  print('-');

  // The result:
  // 
  // 	Content-Type: image/jpeg
  // 	Content-Transfer-Encoding: base64
  // 
  // 	/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g
  // 	NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf
  // 	YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj
  // 	Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA
  // 	AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY
  // 	NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+
  // 	pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq
  // 	Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc
  // 	enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg
  // 	xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9
  // 	DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA
  // 	AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA
  // 	QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx
  // 	EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==
  // 

  // Set the Content-Transfer-Encoding to "binary" by setting the Encoding property.
  mime.encoding = 'binary';

  // Try to get the MIME string.  This CANNOT be done.  The bytes of the JPG image do not
  // represent chars, and to return a string means that bytes must be interpreted according
  // to some character encoding (such as utf-8).  Non-text binary bytes can only be 
  // contained in a string IF encoded in some way. Encodings such as Base64, quoted-printable,
  // URL, etc. exist to make it possible to represent binary data in string format.   
  print(mime.getMime());
  print('-');

  // We CAN get the binary MIME as bytes..
  final mimeBytes = mime.getMimeBytes();

  // Regardless of the Content-Transfer-Encoding, the 
  // body content can always be retrieved and the body bytes
  // decoded from whatever encoding is used..
  final jpgBytes = mime.getBodyBinary();

  // To get the body in base64 format, first make sure
  // the Content-Transfer-Encoding is base64, then call GetBodyEncoded.
  mime.encoding = 'base64';
  final jpgBase64 = mime.getBodyEncoded();
  print(jpgBase64);
  print('-');

  // Let's go back to "binary" MIME..
  mime.encoding = 'binary';

  // Let's say we have MIME, and it was loaded directly from a file, or from
  // a byte array.  (It was not loaded from the contents of a string variable.)
  // We don't know whether the MIME contains binary or 8bit encodings, and thus
  // we dont' know if the MIME is safe to get as a string.
  // The Convert8Bit method can be called to recursively traverse the MIME and set
  // all 8bit or binary encodings to "base64".  This makes the MIME safe for storing in
  // a string.
  mime.convert8Bit();
  final mimeStr = mime.getMime();
  print(mimeStr);
  print('-');
}