title: How to use RTCMediaConstraints on iOS
body: <p>To create a new offer:</p>
  
  <pre><code>[_peerConnection createOfferWithDelegate:self
                                 constraints:[self defaultOfferConstraints]];
  
  - (RTCMediaConstraints *)defaultOfferConstraints {
    NSArray *mandatoryConstraints = @[
        [[RTCPair alloc] initWithKey:@"OfferToReceiveAudio" value:@"true"],
        [[RTCPair alloc] initWithKey:@"OfferToReceiveVideo" value:@"true"]
    ];
    RTCMediaConstraints* constraints =
        [[RTCMediaConstraints alloc]
            initWithMandatoryConstraints:mandatoryConstraints
                     optionalConstraints:nil];
    return constraints;
  }
  </code></pre>
  
  <p>When i changed true to false,it seems it has nothing to do with <code>RTCSessionDescription</code>.  i printed it in </p>
  
  <pre><code>- (void)peerConnection:(RTCPeerConnection *)peerConnection
      didCreateSessionDescription:(RTCSessionDescription *)sdp
                            error:(NSError *)error
  </code></pre>
  
  <p>Questions:  </p>
  
  <ul>
  <li>Does constraints has something to do with RTCSessionDescription?</li>
  <li>Which keys are RTCPair supported?</li>
  </ul>