Function streamDocument

examples/pwa/pwa.js:411–473

* @param {string} url * @param {!WritableStreamDefaultWriter} writer * @return {!Promise}

(url, writer)

 * @return {!Promise}
 */
function streamDocument(url, writer) {
  // Try native first.
  if (window.fetch && window.TextDecoder && window.ReadableStream) {
    return fetch(url).then((response) => {
      // This should be a lot simpler with transforming streams and pipes,
      // but, TMK, these are not supported anywhere yet.
      const /** !ReadableStreamDefaultReader */ reader =
          response.body.getReader();
      const decoder = new TextDecoder();
      function readChunk(chunk) {
        const text = decoder.decode(chunk.value || new Uint8Array(), {
          stream: !chunk.done,
        });
        if (text) {
          writer.write(text);
        }
        if (chunk.done) {
          writer.close();
        } else {
          return reader.read().then(readChunk);
        }
      }
      return reader.read().then(readChunk);
    });
  }

  // Polyfill via XHR.
  return new Promise((resolve, reject) => {
    const xhr = new XMLHttpRequest();
    xhr.open('GET', url, true);
    xhr.setRequestHeader('Accept', 'text/html');
    let pos = 0;
    xhr.onreadystatechange = () => {
      if (xhr.readyState < /* STATUS_RECEIVED */ 2) {
        return;
      }
      if (xhr.status < 100 || xhr.status > 599) {
        xhr.onreadystatechange = null;
        reject(new Error(`Unknown HTTP status ${xhr.status}`));
        return;
      }
      if (
        xhr.readyState == /* LOADING */ 3 ||
        xhr.readyState == /* COMPLETE */ 4
      ) {
        const s = xhr.responseText;
        const chunk = s.substring(pos);
        pos = s.length;
        writer.write(chunk);
        if (xhr.readyState == /* COMPLETE */ 4) {
          writer.close().then(resolve);
        }
      }
    };
    xhr.onerror = () => {
      reject(new Error('Network failure'));
    };
    xhr.onabort = () => {