Presentation API

<!-- controller.html -->
<button id="presentBtn" style="display: none;">Present</button>
<script>
  // The Present button is visible if at least one presentation display is available
  var presentBtn = document.getElementById("presentBtn");
  // It is also possible to use relative presentation URL e.g. "presentation.html"
  var presUrls = ["http://example.com/presentation.html",
                  "http://example.net/alternate.html"];
  // show or hide present button depending on display availability
  var handleAvailabilityChange = function(available) {
    presentBtn.style.display = available ? "inline" : "none";
  };
  // Promise is resolved as soon as the presentation display availability is
  // known.
  var request = new PresentationRequest(presUrls);
  request.getAvailability().then(function(availability) {
    // availability.value may be kept up-to-date by the controlling UA as long
    // as the availability object is alive. It is advised for the web developers
    // to discard the object as soon as it's not needed.
    handleAvailabilityChange(availability.value);
    availability.onchange = function() { handleAvailabilityChange(this.value); };
  }).catch(function() {
    // Availability monitoring is not supported by the platform, so discovery of
    // presentation displays will happen only after request.start() is called.
    // Pretend the devices are available for simplicity; or, one could implement
    // a third state for the button.
    handleAvailabilityChange(true);
  });
</script>

<!-- controller.html -->
<script>
  presentBtn.onclick = function () {
    // Start new presentation.
    request.start()
      // The connection to the presentation will be passed to setConnection on
      // success.
      .then(setConnection);
      // Otherwise, the user canceled the selection dialog or no screens were
      // found.
  };
</script>

<!-- controller.html -->
<button id="reconnectBtn" style="display: none;">Reconnect</button>
<script>
  var reconnect = function () {
    // read presId from localStorage if exists
    var presId = localStorage["presId"];
    // presId is mandatory when reconnecting to a presentation.
    if (!!presId) {
      request.reconnect(presId)
        // The new connection to the presentation will be passed to
        // setConnection on success.
        .then(setConnection);
        // No connection found for presUrl and presId, or an error occurred.
    }
  };
  // On navigation of the controller, reconnect automatically.
  document.addEventListener("DOMContentLoaded", reconnect);
  // Or allow manual reconnection.
  reconnectBtn.onclick = reconnect;
</script>

<!-- controller.html -->
<!-- Setting presentation.defaultRequest allows the page to specify the
     PresentationRequest to use when the controlling UA initiates a
     presentation. -->
<script>
  navigator.presentation.defaultRequest = new PresentationRequest(presUrls);
  navigator.presentation.defaultRequest.onconnectionavailable = function(evt) {
    setConnection(evt.connection);
  };
</script>

<!-- controller.html -->
<button id="disconnectBtn" style="display: none;">Disconnect</button>
<button id="stopBtn" style="display: none;">Stop</button>
<button id="reconnectBtn" style="display: none;">Reconnect</button>
<script>
  let connection;
  // The Disconnect and Stop buttons are visible if there is a connected presentation
  const stopBtn = document.querySelector("#stopBtn");
  const reconnectBtn = document.querySelector("#reconnectBtn");
  const disconnectBtn = document.querySelector("#disconnectBtn");
  stopBtn.onclick = _ => {
    connection && connection.terminate();
  };
  disconnectBtn.onclick = _ => {
    connection && connection.close();
  };
  function setConnection(newConnection) {
    // Disconnect from existing presentation, if not attempting to reconnect
    if (connection && connection != newConnection && connection.state != 'closed') {
      connection.onclosed = undefined;
      connection.close();
    }
    // Set the new connection and save the presentation ID
    connection = newConnection;
    localStorage["presId"] = connection.id;
    function showConnectedUI() {
      // Allow the user to disconnect from or terminate the presentation
      stopBtn.style.display = "inline";
      disconnectBtn.style.display = "inline";
      reconnectBtn.style.display = "none";
    }
    function showDisconnectedUI() {
      disconnectBtn.style.display = "none";
      stopBtn.style.display = "none";
      reconnectBtn.style.display = localStorage["presId"] ? "inline" : "none";
    }
    // Monitor the connection state
    connection.onconnect = _ => {
      showConnectedUI();
      // Register message handler
      connection.onmessage = message => {
        console.log(`Received message: ${message.data}`);
      };
      // Send initial message to presentation page
      connection.send("Say hello");
    };
    connection.onclose = _ => {
      connection = null;
      showDisconnectedUI();
    };
    connection.onterminate = _ => {
      // Remove presId from localStorage if exists
      delete localStorage["presId"];
      connection = null;
      showDisconnectedUI();
    };
  };
</script>

<!-- presentation.html -->
<script>
  var addConnection = function(connection) {
    this.onmessage = function (message) {
      if (message.data == "say hello")
        this.send("hello");
    };
  };
  navigator.presentation.receiver.connectionList.then(function (list) {
    list.connections.map(function (connection) {
      addConnection(connection);
    });
    list.onconnectionavailable = function (evt) {
      addConnection(evt.connection);
    };
  });
</script>

<!-- controller.html -->
<script>
  connection.send("{string: '你好，世界!', lang: 'zh-CN'}");
  connection.send("{string: 'こんにちは、世界!', lang: 'ja'}");
  connection.send("{string: '안녕하세요, 세계!', lang: 'ko'}");
  connection.send("{string: 'Hello, world!', lang: 'en-US'}");
</script>
<!-- presentation.html -->
<script>
  connection.onmessage = function (message) {
    var messageObj = JSON.parse(message.data);
    var spanElt = document.createElement("SPAN");
    spanElt.lang = messageObj.lang;
    spanElt.textContent = messageObj.string;
    document.appendChild(spanElt);
  };
</script>