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import { Observable } from '../Observable';
import { pairwise as higherOrder } from '../operators/pairwise';
/**
* Groups pairs of consecutive emissions together and emits them as an array of
* two values.
*
* <span class="informal">Puts the current value and previous value together as
* an array, and emits that.</span>
*
* <img src="./img/pairwise.png" width="100%">
*
* The Nth emission from the source Observable will cause the output Observable
* to emit an array [(N-1)th, Nth] of the previous and the current value, as a
* pair. For this reason, `pairwise` emits on the second and subsequent
* emissions from the source Observable, but not on the first emission, because
* there is no previous value in that case.
*
* @example <caption>On every click (starting from the second), emit the relative distance to the previous click</caption>
* var clicks = Rx.Observable.fromEvent(document, 'click');
* var pairs = clicks.pairwise();
* var distance = pairs.map(pair => {
* var x0 = pair[0].clientX;
* var y0 = pair[0].clientY;
* var x1 = pair[1].clientX;
* var y1 = pair[1].clientY;
* return Math.sqrt(Math.pow(x0 - x1, 2) + Math.pow(y0 - y1, 2));
* });
* distance.subscribe(x => console.log(x));
*
* @see {@link buffer}
* @see {@link bufferCount}
*
* @return {Observable<Array<T>>} An Observable of pairs (as arrays) of
* consecutive values from the source Observable.
* @method pairwise
* @owner Observable
*/
export function pairwise<T>(this: Observable<T>): Observable<[T, T]> {
return higherOrder()(this) as Observable<[T, T]>;
}
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