问题
I am trying to learn WebRTC, I had achieved connecting two RTCPeerConnection in same page and I am now attempting to separate them into two separate pages and connects them. However, after code are written and exchanged offer and answer, I noticed addIceCandidate() on initiator.html will always throw this with null argument
Error at addIceCandidate from queue: TypeError: Failed to execute 'addIceCandidate' on 'RTCPeerConnection': Candidate missing values for both sdpMid and sdpMLineIndex
at processCandidateQueue (initiator.html:69)
After some reading, I learnt null is used to indicate ICE Candidate gathering finishes and example here: https://webrtc.github.io/samples/src/content/peerconnection/pc1/ Also executes "addIceCandidate" with argument null when gathering finishes. But I do not understand why I am seeing the error I see at this moment.
What I had tried:
- I had tried to write a check such that if candidate is null, skip addIceCandidate.
- Place all connection logic in less buttons to reduce delay between function calls
- Add adapter-latest.js to each page
Result:
- Initiator connection state is "fail", receiver connection state is "new". Failed to stream to receiver page.
- Same error was thrown
- Error is gone, but connection still fails
initiator.html
<!doctype html>
<html lang="en">
<head>
<title>First WebRTC Project</title>
<link href="common.css" rel="stylesheet" />
</head>
<body>
<div class="log-display"></div>
<div class="func-list">
Initiating host
<div class="func">
<button onclick="onPrepareMedia(this)">Prepare media</button>
<video class="dump"></video>
</div>
<div class="func">
<button onclick="onCreatePeerConnection(this)">onCreatePeerConnection()</button>
<textarea class="dump"></textarea>
</div>
<div class="func">
<button onclick="onCreateOffer(this)">onCreateOffer()</button>
<textarea class="dump"></textarea>
</div>
<div class="func">
<button onclick="onSetLocalDescription(this)">onSetLocalDescription()</button>
<textarea class="dump"></textarea>
</div>
<div class="func">
<button onclick="onSetRemoteDescription(this, answerReceived)">onSetRemoteDescription() // set answerReceived variable manually</button>
<textarea class="dump"></textarea>
</div>
</div>
<script src="common.js"></script>
<script>
localStorage.removeItem("FirstWebRTC_offer");
localStorage.removeItem("FirstWebRTC_answer");
var constraints = { video: true, audio: true };
var stream = null;
var peerConn = null;
var offer = null, offerReceived = null;
var answer = null, answerReceived = null;
const offerOptions = {
offerToReceiveAudio: 1,
offerToReceiveVideo: 1
};
candidateQueue = [];
var onIceCandidate = async function(e) {
window.log("onIceCandidate", e);
if(peerConn.remoteDescription) {
var rslt = e.candidate && await peerConn.addIceCandidate(e.candidate).catch(e => onError("addIceCandidate", e));
} else {
candidateQueue.push(e.candidate);
}
window.log(JSON.stringify(rslt));
};
var onIceConnectionStateChange = function(e) {
window.log("onIceConnectionStateChange", e);
};
var onNegotiationNeeded = function(e) {
console.log("-----", e);
}
var processCandidateQueue = async function() {
for(var i in candidateQueue) {
var candidate = candidateQueue[i];
await peerConn.addIceCandidate(candidate).catch(e => onError("addIceCandidate from queue", e));
}
}
async function onPrepareMedia(e) {
stream = await navigator.mediaDevices.getUserMedia(constraints);
e.parentElement.children[1].value = dumpProperty(stream)
video = e.parentElement.children[1];
video.srcObject = stream;
video.play();
}
function onCreatePeerConnection(e) {
peerConn = new RTCPeerConnection({});
// Setup ICE event handlers
peerConn.onicecandidate = onIceCandidate;
peerConn.oniceconnectionstatechange = onIceConnectionStateChange;
peerConn.onnegotiationneeded = onNegotiationNeeded
// Add tracks to be transmitted
stream.getTracks().forEach(track => peerConn.addTrack(track, stream));
e.parentElement.children[1].value = dumpProperty(peerConn)
}
async function onCreateOffer(e) {
offer = await peerConn.createOffer(offerOptions)
localStorage.setItem("FirstWebRTC_offer", JSON.stringify(offer))
e.parentElement.children[1].value = dumpProperty(offer)
}
async function onSetLocalDescription(e) {
var rslt = await peerConn.setLocalDescription(offer)
e.parentElement.children[1].value = dumpProperty(rslt)
}
async function onSetRemoteDescription(e) {
answerReceived = JSON.parse(localStorage.getItem("FirstWebRTC_answer"));
rslt = await peerConn.setRemoteDescription(answerReceived)
e.parentElement.children[1].value = dumpProperty(rslt)
processCandidateQueue();
}
</script>
</body>
</html>
receiver.html
<!doctype html>
<html lang="en">
<head>
<title>First WebRTC Project</title>
<link href="common.css" rel="stylesheet" />
</head>
<body>
<div class="log-display"></div>
<div class="func-list">
Receiving host
<div class="func">
<button >Received video</button>
<video class="dump"></video>
</div>
<div class="func">
<button onclick="onCreatePeerConnection(this)">onCreatePeerConnection()</button>
<textarea class="dump"></textarea>
</div>
<div class="func">
<button onclick="onSetRemoteDescription(this)">onSetRemoteDescription()</button>
<textarea class="dump"></textarea>
</div>
<div class="func">
<button onclick="onCreateAnswer(this)">onCreateAnswer()</button>
<textarea class="dump"></textarea>
</div>
<div class="func">
<button onclick="onSetLocalDescription(this)">onSetLocalDescription()</button>
<textarea class="dump"></textarea>
</div>
</div>
<script src="common.js"></script>
<script>
localStorage.removeItem("FirstWebRTC_offer");
localStorage.removeItem("FirstWebRTC_answer");
var constraints = { video: true, audio: true };
var stream = null;
var peerConn = null;
var offer = null, offerReceived = null;
var answer = null, answerReceived = null;
const offerOptions = {
offerToReceiveAudio: 1,
offerToReceiveVideo: 1
};
var onTrack = function(e) {
console.log(e);
video = document.querySelector("video")
if (video.srcObject !== e.streams[0]) {
video.srcObject = e.streams[0];
video.play();
console.log('received and playing remote stream');
}
}
var onIceCandidate = async function(e) {
window.log("onIceCandidate", e);
var rslt = e.candidate && await peerConn.addIceCandidate(e.candidate).catch(e => onError("addIceCandidate", e));
window.log(JSON.stringify(rslt));
};
var onIceConnectionStateChange = function(e) {
window.log("onIceConnectionStateChange", e);
};
function onCreatePeerConnection(e) {
peerConn = new RTCPeerConnection({});
// Setup ICE event handlers
peerConn.onicecandidate = onIceCandidate;
peerConn.oniceconnectionstatechange = onIceConnectionStateChange;
peerConn.ontrack = onTrack;
e.parentElement.children[1].value = dumpProperty(peerConn);
}
async function onSetRemoteDescription(e) {
offerReceived = JSON.parse(localStorage.getItem("FirstWebRTC_offer"));
rslt = await peerConn.setRemoteDescription(offerReceived);
e.parentElement.children[1].value = dumpProperty(rslt);
}
async function onCreateAnswer(e) {
answer = await peerConn.createAnswer(offerReceived);
localStorage.setItem("FirstWebRTC_answer", JSON.stringify(answer));
e.parentElement.children[1].value = dumpProperty(answer);
}
async function onSetLocalDescription(e) {
var rslt = await peerConn.setLocalDescription(answer);
e.parentElement.children[1].value = dumpProperty(rslt);
}
</script>
</body>
</html>
common.js
function dumpProperty(obj, noJSON) {
var output = JSON.stringify(obj);
if(output == "{}" || noJSON) {
output = ""
for (var property in obj) {
output += property + ': ' + obj[property]+';\n';
}
}
return output;
}
function onError(name, e) {
console.warn("Error at " + name + ": ", e);
}
window.log = function(str, obj) {
var logDisplay = document.getElementsByClassName('log-display')[0];
if(logDisplay) {
var newLog = document.createElement("div");
newLog.innerText = str + " : " + dumpProperty(obj);
logDisplay.appendChild(newLog);
}
console.log(str, obj);
}
common.css
.connection-flow-diagram {
display: flex;
text-align: center;
}
.func-list {
display: flex;
flex-direction: column;
flex-wrap: wrap;
justify-content: space-around;
width: 50%;
margin-left: auto;
margin-right: auto;
text-align: center;
}
.func {
padding: 1rem;
display: flex;
flex-direction: column;
border: 1px dashed black;
}
.func button {
}
.func .dump {
height: 180px;
}
.log-display {
position: fixed;
left: 0;
top: 0;
width: 100vw;
height: 100vh;
pointer-events: none;
color: rgba(0,0,0,0.4);
}
回答1:
Why does supplying addIceCandidate with null result in error while the example code works fine?
It's because your browser is not up to spec.
addIceCandidate(null) is valid in the latest spec, and indistinguishable from addIceCandidate() and addIceCandidate({}). They all signal end-of-candidates from the remote end.
The WebRTC samples work because they use adapter.js, which polyfills the correct spec behavior on older browsers.
回答2:
After some reading, I had figured out why my code does not work. It contains a fatal flaw that is unrelated to the title of this question.
First, answer the question of title. Q"Why does supplying addIceCandidate() with null will result in error?" A: This is because I had read an out-dated article on WebRTC, where some time in the past addIceCandidate() was able to take a null value and it will be happy. However, as of 2019 April 25th, this is no longer the case. Instead, with current implemetation:
If the event's candidate property is null, ICE gathering has finished.
MDN - Event: RTCPeerConnection.onicecandidate
Therefore, to properly handle this case, I need to test for null candidate
onIceCandidateHandler(e)
if e.candidate is not null
signalingMedium.sendToRemote(e.candidate)
else
do nothing
Which is why when I added adapter-latest.js, the error goes away; it replaces addIceCandidate() to guard against null candidate
Second, I mentioned although the error goes away when adapter-latest.js was added. This is because I was doing signaling the wrong way.
Here is the description of icecandidate event from MDN
This happens whenever the local ICE agent needs to deliver a message to the other peer through the signaling server.
...
simply implement this method to use whatever messaging technology you choose to send the ICE candidate to the remote peer.
Where in my own code, I was adding candidate to local peer connection (which is wrong).
var onIceCandidate = async function(e) {
window.log("onIceCandidate", e);
if(peerConn.remoteDescription) {
var rslt = e.candidate && await peerConn.addIceCandidate(e.candidate).catch(e => onError("addIceCandidate", e));
} else {
candidateQueue.push(e.candidate);
}
window.log(JSON.stringify(rslt));
};
Thus connection always fail because I was actually connecting to myself.
I will provide a jsFiddle with corrected code once I fixed the issue.
来源：https://stackoverflow.com/questions/55840870/addicecandidate-with-argument-null-result-in-error