import { ObservableInput, ObservedValueOf, OperatorFunction } from '../types';
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import { switchMap } from './switchMap';
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import { operate } from '../util/lift';
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// TODO: Generate a marble diagram for these docs.
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/**
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* Applies an accumulator function over the source Observable where the
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* accumulator function itself returns an Observable, emitting values
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* only from the most recently returned Observable.
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*
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* <span class="informal">It's like {@link mergeScan}, but only the most recent
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* Observable returned by the accumulator is merged into the outer Observable.</span>
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*
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* @see {@link scan}
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* @see {@link mergeScan}
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* @see {@link switchMap}
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*
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* @param accumulator
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* The accumulator function called on each source value.
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* @param seed The initial accumulation value.
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* @return A function that returns an observable of the accumulated values.
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*/
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export function switchScan<T, R, O extends ObservableInput<any>>(
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accumulator: (acc: R, value: T, index: number) => O,
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seed: R
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): OperatorFunction<T, ObservedValueOf<O>> {
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return operate((source, subscriber) => {
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// The state we will keep up to date to pass into our
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// accumulator function at each new value from the source.
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let state = seed;
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// Use `switchMap` on our `source` to do the work of creating
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// this operator. Note the backwards order here of `switchMap()(source)`
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// to avoid needing to use `pipe` unnecessarily
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switchMap(
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// On each value from the source, call the accumulator with
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// our previous state, the value and the index.
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(value: T, index) => accumulator(state, value, index),
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// Using the deprecated result selector here as a dirty trick
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// to update our state with the flattened value.
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(_, innerValue) => ((state = innerValue), innerValue)
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)(source).subscribe(subscriber);
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return () => {
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// Release state on finalization
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state = null!;
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};
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});
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}
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