RTCPeerConnection : EventTarget

Spec:
https://w3c.github.io/webrtc-pc/#interface-definition

----
Details:

<p>
Although WebRTC is a peer to peer protocol, the peers need some information about
each other before they can communicate. Typically this is done with a Signaling
Server, but any mechanism to transfer data can be used. The following
demonstrates setting up the peer connection using the clipboard (via copy/paste)
to transfer the connection data.
</p>
<h3>Offerer</h3>
This example demonstrates the peer that creates the offer.
<htmlexample>
<button id="copy" disabled>Copy Offer/Candidates to Clipboard</button><br>
<input id="paste" placeholder="Paste Answer/Candidates here" disabled>
<table>
<tr><td>local</td><td>remote</td></tr>
<tr>
  <td><video id="local" autoplay></video></td>
  <td><video id="remote" autoplay></video></td>
</tr>
</table>

<script>
  // Use an offscreen canvas as the video source.
  const canvas = document.createElement('canvas');
  const context = canvas.getContext('2d');
  let color = 0;
  const draw = () => {
    color = (color + 2) % 255;
    context.fillStyle = `rgb(${color}, 0, 0)`;
    context.fillRect(0, 0, 300, 150);
    requestAnimationFrame(draw);
  }
  draw();

  const localVideo = document.getElementById('local');
  const stream = canvas.captureStream();
  localVideo.srcObject = stream;

  const peerConnection = new RTCPeerConnection({ iceServers: [ { urls: 'stun:stun.l.google.com:19302' } ] });

  stream.getTracks().forEach(t => peerConnection.addTrack(t, stream));

  // Save a list of ice candidates to send to the peer
  const iceCandidates = [];
  peerConnection.onicecandidate = e => {
    if (e.candidate) {
      iceCandidates.push(e.candidate);
    }
  };

  peerConnection.ontrack = e => { 
    document.getElementById('remote').srcObject = e.streams[0];
  };

  (async () => {
    const copyButton = document.getElementById('copy');

    peerConnection.onicegatheringstatechange = () => {
      if (peerConnection.iceGatheringState === 'complete') {
        copyButton.disabled = false;
        console.log('Offer ready, click copy button.');
      }
    };

    const offer = await peerConnection.createOffer();
    await peerConnection.setLocalDescription(offer);

    const pasteInput = document.getElementById('paste');

    copyButton.onclick = () => {
      navigator.clipboard.writeText(JSON.stringify({ offer, iceCandidates }));
      console.log('Offer copied/candidates. Paste in Answerer example.');

      pasteInput.disabled = false;
    };

    pasteInput.onpaste = async (e) => {
      const { answer, iceCandidates } = JSON.parse(e.clipboardData.getData('text/plain'));
      await peerConnection.setRemoteDescription(answer);
      iceCandidates.forEach(c => peerConnection.addIceCandidate(c));
    };
  })();
</script>

<style>
  video {
    width: 150px;
    height: 100px;
  }
  input {
    width: 250px;
  }
</style>
</htmlexample>

<h3>Answerer</h3>
This example demonstrates the peer that receives the offer and creates the answer.
<htmlexample>
<input id="paste" placeholder="Paste Offer/Candidates here"><br>
<button id="copy" disabled>Copy Answer/Candidates to Clipboard</button><br>
<table>
<tr><td>local</td><td>remote</td></tr>
<tr>
  <td><video id="local" autoplay></video></td>
  <td><video id="remote" autoplay></video></td>
</tr>
</table>

<script>
  // Use an offscreen canvas as the video source.
  const canvas = document.createElement('canvas');
  const context = canvas.getContext('2d');
  let color = 0;
  const draw = () => {
    color = (color - 2 + 255) % 255;
    context.fillStyle = `rgb(0, 0, ${color})`;
    context.fillRect(0, 0, 300, 150);
    requestAnimationFrame(draw);
  }
  draw();

  const localVideo = document.getElementById('local');
  const stream = canvas.captureStream();
  localVideo.srcObject = stream;

  const peerConnection = new RTCPeerConnection({ iceServers: [ { urls: 'stun:stun.l.google.com:19302' } ] });

  stream.getTracks().forEach(t => peerConnection.addTrack(t, stream));

  // Save a list of ice candidates to send to the peer
  const iceCandidates = [];
  peerConnection.onicecandidate = e => {
    if (e.candidate) {
      iceCandidates.push(e.candidate);
    }
  };

  peerConnection.ontrack = e => { 
    document.getElementById('remote').srcObject = e.streams[0];
  };

  const copyButton = document.getElementById('copy');
   
  document.getElementById('paste').onpaste = async (e) => {
    const { offer, iceCandidates } = JSON.parse(e.clipboardData.getData('text/plain'));

    peerConnection.onicegatheringstatechange = () => {
      if (peerConnection.iceGatheringState === 'complete') {
        copyButton.disabled = false;
        console.log('Answer ready, click copy button.');
      }
    };

    await peerConnection.setRemoteDescription(offer);
    const answer = await peerConnection.createAnswer();
    await peerConnection.setLocalDescription(answer);

    copyButton.onclick = () => {
      navigator.clipboard.writeText(JSON.stringify({ answer, iceCandidates }));
      console.log('Copied answer/candidates. Paste in Offerer example.');
    };

    iceCandidates.forEach(c => peerConnection.addIceCandidate(c));
  };
</script>

<style>
  video {
    width: 150px;
    height: 100px;
  }
  input {
    width: 250px;
  }
</style>
</htmlexample>