Angular 5 RxJs concatMap,switchMap,mergeMap which?

问题

I have this method to get token via localstorage, if token is not exist or is expired, I will call API to get another token and store to localstorage.

In this case, which map should I use, currently using mergeMap, or other way to do this?

public doGetToken():Observable<Token> {
    return this.loadToken().pipe( //get via localstorage
      map(token=>{
        let valid = this.validateTokenIsValid(token);
        let data = {
          token: token,
          valid: valid
        };
        return data;
      }),
      mergeMap(data=>{
        if (!data.valid) {
          return this.doApiGetToken(data.token).pipe(
            map(
              token=>{
                this.saveToken(token); //save to localstorage
                return token;
              }
            )
          );
        } else {
          return of(data.token);
        }
      })
    );

version: Angular 5, rxjs5

Thank you in advance.

回答1:

If you only make one request, then it doesn't matter which map you use.

mergeMap (also called flatMap), concatMap , exhaustMap or switchMap will behave the same.

These operators behave differently when you emit more than 1 value:

switchMap

will apply the mapping to the latest input received:

Src : -----A----B----C--D-E-------

switchMap (x => x--x) // emit x twice when received

Out:  ------A--A-B--B-C-D-E--E----

concatMap

will finish the mapping before taking another input:

Src : -----A----B----C--D-E-----------

concatMap (x => x--x) // emit x twice when received

Out:  ------A--A-B--B-C--C--D--D-E--E

mergeMap

is like concatMap, but it doesn't wait for mapping to complete. The results can overlap though:

Src : -----A----B----C-D---E-----------

mergeMap (x => x--x) // emit x twice when received

Out:  ------A--A-B--B-C-D-C-D-E--E-----

exhaustMap

is like a reversed switchMap, it gives priority to the output:

Src : -----A--------B----C-D---E-----------

exhaustMap (x => x--x--x) // emit x twice when received

Out:  ------A--A--A--B--B--B-D--D--D-------

For more information :

https://medium.com/@vdsabev/the-simple-difference-between-rxjs-switchmap-and-mergemap-397c311552a5

Marble diagrams :

http://rxmarbles.com/#mergeMap

Edit : I moved the simplification of your code to the bottom to make the general information visible at first sight.

public doGetToken(): Observable<Token> {
  return this.loadToken()
    .pipe( //get via localstorage
      mergeMap(token => {
        if(!this.validateTokenIsValid(token))
          return of(token)
        return this.doApiGetToken(token)
          .pipe(
            tap( token => this.saveToken(token)) //save to localstorage
          );
      })
    )
};

来源：https://stackoverflow.com/questions/51171194/angular-5-rxjs-concatmap-switchmap-mergemap-which