es6/operator/scan.js

import { Subscriber } from '../Subscriber';
/* tslint:enable:max-line-length */
/**
 * 对源 Observable 使用累加器函数， 返回生成的中间值， 可选的初始值。
 *
 * <span class="informal">就想是 {@link reduce}, 但是发出目前的累计数当源发出数据的时候。</span>
 *
 * <img src="./img/scan.png" width="100%">
 *
 * 将所有源发出的数据结合起来， 使用一个累加器函数，该函数知道如何将新的值加入到累加器中。
 * 这就像是{@link reduce}， 但是会发出中间的累加值。
 *
 * 返回一个 Observable， 该 Observable 对每个源 Observable 发出的值使用特定的累加器。
 * 如果`seed`值提供了， 这个值会被累加器用作初始值。 如果`seed`值没有被提供， 源数据的第一项会被当做初始值。
 *
 * @example <caption>计数点击次数</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var ones = clicks.mapTo(1);
 * var seed = 0;
 * var count = ones.scan((acc, one) => acc + one, seed);
 * count.subscribe(x => console.log(x));
 *
 * @see {@link expand}
 * @see {@link mergeScan}
 * @see {@link reduce}
 *
 * @param {function(acc: R, value: T, index: number): R} accumulator
 * 对每个源数据调用的累加器函数。
 * @param {T|R} [seed] 初始值。
 * @return {Observable<R>} 带有累加功能的observable。
 * @method scan
 * @owner Observable
 */
export function scan(accumulator, seed) {
    let hasSeed = false;
    // providing a seed of `undefined` *should* be valid and trigger
    // hasSeed! so don't use `seed !== undefined` checks!
    // For this reason, we have to check it here at the original call site
    // otherwise inside Operator/Subscriber we won't know if `undefined`
    // means they didn't provide anything or if they literally provided `undefined`
    if (arguments.length >= 2) {
        hasSeed = true;
    }
    return this.lift(new ScanOperator(accumulator, seed, hasSeed));
}
class ScanOperator {
    constructor(accumulator, seed, hasSeed = false) {
        this.accumulator = accumulator;
        this.seed = seed;
        this.hasSeed = hasSeed;
    }
    call(subscriber, source) {
        return source.subscribe(new ScanSubscriber(subscriber, this.accumulator, this.seed, this.hasSeed));
    }
}
/**
 * We need this JSDoc comment for affecting ESDoc.
 * @ignore
 * @extends {Ignored}
 */
class ScanSubscriber extends Subscriber {
    constructor(destination, accumulator, _seed, hasSeed) {
        super(destination);
        this.accumulator = accumulator;
        this._seed = _seed;
        this.hasSeed = hasSeed;
        this.index = 0;
    }
    get seed() {
        return this._seed;
    }
    set seed(value) {
        this.hasSeed = true;
        this._seed = value;
    }
    _next(value) {
        if (!this.hasSeed) {
            this.seed = value;
            this.destination.next(value);
        }
        else {
            return this._tryNext(value);
        }
    }
    _tryNext(value) {
        const index = this.index++;
        let result;
        try {
            result = this.accumulator(this.seed, value, index);
        }
        catch (err) {
            this.destination.error(err);
        }
        this.seed = result;
        this.destination.next(result);
    }
}
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