RxJS, Observable, how to preserve value and switch map to another one

Question

// ticker$ will update every 3s
// showHand$ will only triger after user click button
// I would like to take last ticker price as user order price when user click button

let lastPrice: number;

this.ticker$
  // What I am doing now is preserve value to vairable here.
  .do(ticker => lastPrice = ticker.closePrice)
  .switchMap(() => this.showHand$)
  .subscribe(showHand => {
     // use value here
     this.order.price = lastPrice;
     this.order.amount = showHand.amount;
     this.order.type = showHand.type;

     this.submit();
  });

Any segestion about how to prevser value and switch map together, without one line variable like above?

Answer 1

Results selector function is deprecated in version 6 will be removed in version 7.

From docs:

https://github.com/ReactiveX/rxjs/blob/master/docs_app/content/guide/v6/migration.md#result-selectors

with resultSelector (v5.x)

source.pipe(
 switchMap(fn1, fn2)
)

the same functionality without resultSelector, achieved with inner map

source.pipe(
 switchMap((a, i) => fn1(a, i).pipe(
   map((b, ii) => fn2(a, b, i, ii))
 )
)

Answer 2

The behaviour you require is already possible with an overload of SwitchMap with a selectorFunc for the combination of every (outerValue,innerValue) :

this.ticker$
  .switchMap(
    () => this.showHand$,
    (tickerValue, switchMap) => tickerValue
  )
  .subscribe(showHand => { });

Answer 3

I think this is the operator

this.showHand$.take(1)
  .withLatestFrom(this.ticker$)
  .subscribe(([showHand, ticker]) => {
    this.order.price = ticker.closePrice;
    this.order.amount = showHand.amount;
    this.order.type = showHand.type;
    this.submit();      
  });

Note, take(1) will close subscription, but if you want the user to be able to press the button many times, save the subscription to a const and unsubscribe when finished.

Answer 4

There is a little hack to achieve this - basically you have a whole new observable inside the switchmap, and this observable has access to the value passed into the switchmap function. You can use this value in an inner map to preserve the value.

this.ticker$
  .switchMap(ticker => this.showHand$.pipe(map( (hand) => { ticker,hand } )))
  .subscribe( obj => {
     // use value here
     this.order.price = obj.ticker;
     this.order.amount = obj.hand.amount;
     this.order.type = obj.hand.type;

     this.submit();
  });