The Twitch iOS viewing embed allows you to obtain authentication tokens in your 3rd-party app: https://github.com/twitchtv/twitch-ios-plugin-bin
You’ll need to register your app under your Twitch account on http://www.twitch.tv then pass the Client ID and Redirect URI you create there to the library:
// Log the user in to obtain an auth token for broadcasting
- (void)presentLoginViewForClientID:(NSString *)clientID redirectURI:(NSString *)redirectURI;