title: Generator function not called when using Firebase listner
body: <p>I am using Firebase and Redux saga, and want to attache generator on firebase's listener as below.</p>
  
  <pre><code>var ref = firebase.database().ref(`path`)
  ref.on('value',function* (snapshot){
  
      yield put({ type: 'SOME_ACTION' })
  </code></pre>
  
  <p>When I run this program, callback function is not called. If I remove the *, it will be called.
  Is there any workaround for this?</p>