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import { MonoTypeOperatorFunction } from '../types';
import { identity } from '../util/identity';
import { operate } from '../util/lift';
import { OperatorSubscriber } from './OperatorSubscriber';
/**
* Returns a result {@link Observable} that emits all values pushed by the source observable if they
* are distinct in comparison to the last value the result observable emitted.
*
* 1. It will always emit the first value from the source.
* 2. For all subsequent values pushed by the source, they will be compared to the previously emitted values
* using the provided `comparator` or an `===` equality check.
* 3. If the value pushed by the source is determined to be unequal by this check, that value is emitted and
* becomes the new "previously emitted value" internally.
*
* ## Example
*
* A very basic example with no `comparator`. Note that `1` is emitted more than once,
* because it's distinct in comparison to the _previously emitted_ value,
* not in comparison to _all other emitted values_.
*
* ```ts
* import { of } from 'rxjs';
* import { distinctUntilChanged } from 'rxjs/operators';
*
* of(1, 1, 1, 2, 2, 2, 1, 1, 3, 3).pipe(
* distinctUntilChanged()
* )
* .subscribe(console.log);
* // Logs: 1, 2, 1, 3
* ```
*
* ## Example
*
* With a `comparator`, you can do custom comparisons. Let's say
* you only want to emit a value when all of its components have
* changed:
*
* ```ts
* import { of } from 'rxjs';
* import { distinctUntilChanged } from 'rxjs/operators';
*
* const totallyDifferentBuilds$ = of(
* { engineVersion: '1.1.0', transmissionVersion: '1.2.0' },
* { engineVersion: '1.1.0', transmissionVersion: '1.4.0' },
* { engineVersion: '1.3.0', transmissionVersion: '1.4.0' },
* { engineVersion: '1.3.0', transmissionVersion: '1.5.0' },
* { engineVersion: '2.0.0', transmissionVersion: '1.5.0' }
* ).pipe(
* distinctUntilChanged((prev, curr) => {
* return (
* prev.engineVersion === curr.engineVersion ||
* prev.transmissionVersion === curr.transmissionVersion
* );
* })
* );
*
* totallyDifferentBuilds$.subscribe(console.log);
*
* // Logs:
* // {engineVersion: "1.1.0", transmissionVersion: "1.2.0"}
* // {engineVersion: "1.3.0", transmissionVersion: "1.4.0"}
* // {engineVersion: "2.0.0", transmissionVersion: "1.5.0"}
* ```
*
* ## Example
*
* You can also provide a custom `comparator` to check that emitted
* changes are only in one direction. Let's say you only want to get
* the next record temperature:
*
* ```ts
* import { of } from "rxjs";
* import { distinctUntilChanged } from "rxjs/operators";
*
* const temps$ = of(30, 31, 20, 34, 33, 29, 35, 20);
*
* const recordHighs$ = temps$.pipe(
* distinctUntilChanged((prevHigh, temp) => {
* // If the current temp is less than
* // or the same as the previous record,
* // the record hasn't changed.
* return temp <= prevHigh;
* })
* );
*
* recordHighs$.subscribe(console.log);
* // Logs: 30, 31, 34, 35
* ```
*
* @param comparator A function used to compare the previous and current values for
* equality. Defaults to a `===` check.
* @return A function that returns an Observable that emits items from the
* source Observable with distinct values.
*/
export function distinctUntilChanged<T>(comparator?: (previous: T, current: T) => boolean): MonoTypeOperatorFunction<T>;
/**
* Returns a result {@link Observable} that emits all values pushed by the source observable if they
* are distinct in comparison to the last value the result observable emitted.
*
* 1. It will always emit the first value from the source.
* 2. The `keySelector` will be run against all values, including the first value.
* 3. For all values after the first, the selected key will be compared against the key selected from
* the previously emitted value using the `comparator`.
* 4. If the keys are determined to be unequal by this check, the value (not the key), is emitted
* and the selected key from that value is saved for future comparisons against other keys.
*
* ## Example
*
* Selecting update events only when the `updatedBy` field shows
* the account changed hands...
*
* ```ts
* // A stream of updates to a given account
* const accountUpdates$ = of(
* { updatedBy: "blesh", data: [] },
* { updatedBy: "blesh", data: [] },
* { updatedBy: "ncjamieson", data: [] },
* { updatedBy: "ncjamieson", data: [] },
* { updatedBy: "blesh", data: [] }
* );
*
* // We only want the events where it changed hands
* const changedHands$ = accountUpdates$.pipe(
* distinctUntilChanged(undefined, update => update.updatedBy)
* );
*
* changedHands$.subscribe(console.log);
* // Logs:
* // {updatedBy: "blesh", data: Array[0]}
* // {updatedBy: "ncjamieson", data: Array[0]}
* // {updatedBy: "blesh", data: Array[0]}
* ```
*
* @param comparator A function used to compare the previous and current keys for
* equality. Defaults to a `===` check.
* @param keySelector Used to select a key value to be passed to the `comparator`.
* @return A function that returns an Observable that emits items from the
* source Observable with distinct values.
*/
export function distinctUntilChanged<T, K>(
comparator: (previous: K, current: K) => boolean,
keySelector: (value: T) => K
): MonoTypeOperatorFunction<T>;
export function distinctUntilChanged<T, K>(
comparator?: (previous: K, current: K) => boolean,
keySelector: (value: T) => K = identity as (value: T) => K
): MonoTypeOperatorFunction<T> {
// We've been allowing `null` do be passed as the `compare`, so we can't do
// a default value for the parameter, because that will only work
// for `undefined`.
comparator = comparator ?? defaultCompare;
return operate((source, subscriber) => {
// The previous key, used to compare against keys selected
// from new arrivals to determine "distinctiveness".
let previousKey: K;
// Whether or not this is the first value we've gotten.
let first = true;
source.subscribe(
new OperatorSubscriber(subscriber, (value) => {
// We always call the key selector.
const currentKey = keySelector(value);
// If it's the first value, we always emit it.
// Otherwise, we compare this key to the previous key, and
// if the comparer returns false, we emit.
if (first || !comparator!(previousKey, currentKey)) {
// Update our state *before* we emit the value
// as emission can be the source of re-entrant code
// in functional libraries like this. We only really
// need to do this if it's the first value, or if the
// key we're tracking in previous needs to change.
first = false;
previousKey = currentKey;
// Emit the value!
subscriber.next(value);
}
})
);
});
}
function defaultCompare(a: any, b: any) {
return a === b;
}