slopeofhope slopeofhope - 3 months ago 30
Javascript Question

When I try to use the result to update the state using yield put API in redux saga gives me an exception why?

so I commented out where it gives me a traceback.

export function* watchFileReader(){
const action = yield take("DROP_FILE")
console.log('action', action)
let file = action.file[0];
readFile(file, function(e){
sessionStorage.removeItem('img')
console.log('alskdjfalsdjkf', e.target.result)
sessionStorage.setItem('img', e.target.result)
// yield put({type: "UDATE", {img: e.target.result})
})
}


update:
this is my promisified function to get the code to work.

function readFileWithPromise(file){
return new Promise((resolve, reject) => {
readFile(file, function(e){
if (e){
resolve(e)
}else{
reject(e)
}
})
})
}

Answer

you can't use yield in callback , there are two ways to avoid this:

  1. cps effect . DOCS LINK

    import { cps , ...  } from 'redux-saga/effects';
    
    export function* watchFileReader(){
      const action = yield take("DROP_FILE")
      let file = action.file[0];
    
      let e = yield cps(readFile,file); <----------------
    
      sessionStorage.removeItem('img')
      sessionStorage.setItem('img', e.target.result)
      yield put({type: "UPDATE", img: e.target.result})
    }
    

    note : function must call cb(null, result) to notify the middleware of a successful result. If fn encounters some error, then it must call cb(error) in order to notify the middleware that an error has occurred.


  2. promisify your function

    function readFileWithPromise(file){
         return new Promise((resolve,reject)=>{
             readFile(file,(err, res) => err ? reject(err) : resolve(res));
        });
    }
    

    and then call it

    import { call, ...  } from 'redux-saga/effects';
    
    export function* watchFileReader(){
      const action = yield take("DROP_FILE")
      let file = action.file[0];
    
      let e = yield call(readFileWithPromise,file); <----------------
    
      sessionStorage.removeItem('img')
      sessionStorage.setItem('img', e.target.result)
      yield put({type: "UPDATE", img: e.target.result})
    }
    

LIVE DEMO for two examples

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