Redux saga, axios and progress event

Question

Is there clean/short/right way to using together axios promise and uploading progress event?

Suppose I have next upload function:

function upload(payload, onProgress) {
  const url = '/sources/upload';

  const data = new FormData();

  data.append('source', payload.file, payload.file.name);

  const config = {
    onUploadProgress: onProgress,
    withCredentials: true
  };

  return axios.post(url, data, config);
}

This function returned the promise.

Also I have a saga:

function* uploadSaga(action) {
  try {
    const response = yield call(upload, payload, [?? anyProgressFunction ??]);
    yield put({ type: UPLOADING_SUCCESS, payload: response });
  } catch (err) {
    yield put({ type: UPLOADING_FAIL, payload: err });
  }
}

I want to receive progress events and put it by saga. Also I want to catch success (or failed) result of the axios request. Is it possible?

Thanks.

Answer 1

I personally found the accepted answer to be very convoluted, and I was having a hard time implementing it. Other google / SO searches all led to similar type answers. If it worked for you, great, but I found another way using an EventEmitter that I personally find much simpler.

Create an event emitter somewhere in your code:

// emitter.js

import { EventEmitter } from "eventemitter3";

export default new EventEmitter();

In your saga to make the api call, use this emitter to emit an event within the onUploadProgress callback:

// mysagas.js
import eventEmitter from '../wherever/emitter';

function upload(payload) {
  // ...

  const config = {
    onUploadProgress: (progressEvent) = {
      eventEmitter.emit(
        "UPLOAD_PROGRESS", 
        Math.floor(100 * (progressEvent.loaded / progressEvent.total))
      );
    }
  };

  return axios.post(url, data, config);
}

Then in your component that needs this upload progress number, you can listen for this event on mount:

// ProgressComponent.jsx
import eventEmitter from '../wherever/emitter';

const ProgressComponent = () => {

  const. [uploadProgress, setUploadProgress] = useState(0);

  useEffect(() => {
    eventEmitter.on(
      "UPLOAD_PROGRESS",
      percent => {
        // latest percent available here, and will fire every time its updated
        // do with it what you need, i.e. update local state, store state, etc
        setUploadProgress(percent)
      }
    );
  
    // stop listening on unmount
    return function cleanup() {
      eventEmitter.off("UPLOAD_PROGRESS")
    }
  }, [])

  return <SomeLoadingBar value={percent} />

}

This worked for me as my application was already making use of a global eventEmitter for other reasons. I found this easier to implement, maybe someone else will too.

Answer 2

So I found the answer, thanks Mateusz Burzyński for the clarification.

We need use eventChannel, but a bit canningly.

Suppose we have api function for uploading file:

function upload(payload, onProgress) {
  const url = '/sources/upload';

  const data = new FormData();

  data.append('source', payload.file, payload.file.name);

  const config = {
    onUploadProgress: onProgress,
    withCredentials: true
  };

  return axios.post(url, data, config);
}

In saga we need to create eventChannel but put emit outside.

function createUploader(payload) {

  let emit;
  const chan = eventEmitter(emitter => {

    emit = emitter;
    return () => {}; // it's necessarily. event channel should 
                     // return unsubscribe function. In our case 
                     // it's empty function
  });

  const uploadPromise = upload(payload, (event) => {
    if (event.loaded.total === 1) {
      emit(END);
    }

    emit(event.loaded.total);
  });

  return [ uploadPromise, chan ];
}

function* watchOnProgress(chan) {
  while (true) {
    const data = yield take(chan);
    yield put({ type: 'PROGRESS', payload: data });
  }
}

function* uploadSource(action) {
  const [ uploadPromise, chan ] = createUploader(action.payload);
  yield fork(watchOnProgress, chan);

  try {
    const result = yield call(() => uploadPromise);
    put({ type: 'SUCCESS', payload: result });
  } catch (err) {
    put({ type: 'ERROR', payload: err });
  }
}