React Native

Add a Custom Header to a Related Item

Demonstrates the Chilkat Email.AddRelatedHeader method, which adds a custom MIME header field to an existing related item identified by its zero-based index. This example first adds a related style sheet (which becomes index 0) and captures its generated Content-ID, builds an HTML body that references the style sheet by that cid:, then attaches an extra header field to the related item and prints the resulting MIME.

Background: Like attachments, each related item (an inline image, style sheet, etc.) is a MIME part with its own small header block describing that part — Content-Type, Content-ID, Content-Location, and so on. AddRelatedHeader lets you insert additional fields into that per-part block, which is occasionally required for interoperability with clients that look for specific custom headers on embedded resources.

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React Native

import { Email, StringBuilder } from '@chilkat/react-native'

function chilkatExample() {
  // Demonstrates the AddRelatedHeader method, which adds a custom MIME header field to an
  // existing related item, identified by its zero-based index.

  const email = new Email();
  email.subject = 'Related item with a custom header';

  // Add a related item first (a style sheet); capture its generated Content-ID.  It becomes
  // related-item index 0.
  let cid: string;
  try {
    cid = email.addRelatedString('styles.css', 'body { color: black; }', 'utf-8');
  } catch {
    console.log(email.lastErrorText);
    return;
  }

  // Build the HTML body, referencing the related style sheet by its Content-ID.  A
  // placeholder is used and then replaced with the actual Content-ID.
  const sbHtml = new StringBuilder();
  sbHtml.append('<html><head><link rel="stylesheet" href="cid:PLACEHOLDER_CID"/></head><body>Styled content.</body></html>');
  const numReplaced = sbHtml.replace('PLACEHOLDER_CID', cid);
  email.setHtmlBody(sbHtml.getAsString());

  // Add a custom header field to the first related item (index 0).
  email.addRelatedHeader(0, 'X-Custom-Related-Header', 'some value');

  // The custom header now appears in the related item's MIME part.
  console.log(email.getMime());
}