Can I dispatch an action in reducer?

is it possible to dispatch an action in a reducer itself? I have a progressbar and an audio element. The goal is to update the progressbar when the time gets updated in the audio element. But I don't know where to place the ontimeupdate eventhandler, or how to dispatch an action in the callback of ontimeupdate, to update the progressbar. Here is my code:

//reducer

const initialState = {
    audioElement: new AudioElement('test.mp3'),
    progress: 0.0
}

initialState.audioElement.audio.ontimeupdate = () => {
    console.log('progress', initialState.audioElement.currentTime/initialState.audioElement.duration);
    //how to dispatch 'SET_PROGRESS_VALUE' now?
};


const audio = (state=initialState, action) => {
    switch(action.type){
        case 'SET_PROGRESS_VALUE':
            return Object.assign({}, state, {progress: action.progress});
        default: return state;
    }

}

export default audio;

Dispatching an action within a reducer is an anti-pattern. Your reducer should be without side effects, simply digesting the action payload and returning a new state object. Adding listeners and dispatching actions within the reducer can lead to chained actions and other side effects.

Sounds like your initialized AudioElement class and the event listener belong within a component rather than in state. Within the event listener you can dispatch an action, which will update progress in state.

You can either initialize the AudioElement class object in a new React component or just convert that class to a React component.

class MyAudioPlayer extends React.Component {
  constructor(props) {
    super(props);

    this.player = new AudioElement('test.mp3');

    this.player.audio.ontimeupdate = this.updateProgress;
  }

  updateProgress () {
    // Dispatch action to reducer with updated progress.
    // You might want to actually send the current time and do the
    // calculation from within the reducer.
    this.props.updateProgressAction();
  }

  render () {
    // Render the audio player controls, progress bar, whatever else
    return <p>Progress: {this.props.progress}</p>;
  }
}

class MyContainer extends React.Component {
   render() {
     return <MyAudioPlayer updateProgress={this.props.updateProgress} />
   }
}

function mapStateToProps (state) { return {}; }

return connect(mapStateToProps, {
  updateProgressAction
})(MyContainer);

Note that the updateProgressAction is automatically wrapped with dispatch so you don't need to call dispatch directly.