es6/operator/skipLast.js

import { Subscriber } from '../Subscriber';
import { ArgumentOutOfRangeError } from '../util/ArgumentOutOfRangeError';
/**
 * 跳过源 Observable 最后发出的的N个值 (N = count)。
 *
 * <img src="./img/skipLast.png" width="100%">
 *
 * `skipLast` 返回一个 Observable，该 Observable 累积足够长的队列以存储最初的N个值 (N = count)。
 * 当接收到更多值时，将从队列的前面取值并在结果序列上产生。 这种情况下值会被延时。
 *
 * @example <caption>跳过有多个值的 Observable 的最后2个值</caption>
 * var many = Rx.Observable.range(1, 5);
 * var skipLastTwo = many.skipLast(2);
 * skipLastTwo.subscribe(x => console.log(x));
 *
 * // Results in:
 * // 1 2 3
 *
 * @see {@link skip}
 * @see {@link skipUntil}
 * @see {@link skipWhile}
 * @see {@link take}
 *
 * @throws {ArgumentOutOfRangeError} 当使用 `skipLast(i)` 时, 如果`i < 0`，则
 * 抛出 ArgumentOutOrRangeError。
 *
 * @param {number} count 源 Observable 中从后往前要跳过的值的数量。
 * @returns {Observable<T>} Observable 跳过源 Observable 发出
 * 的最后几个值。
 * @method skipLast
 * @owner Observable
 */
export function skipLast(count) {
    return this.lift(new SkipLastOperator(count));
}
class SkipLastOperator {
    constructor(_skipCount) {
        this._skipCount = _skipCount;
        if (this._skipCount < 0) {
            throw new ArgumentOutOfRangeError;
        }
    }
    call(subscriber, source) {
        if (this._skipCount === 0) {
            // If we don't want to skip any values then just subscribe
            // to Subscriber without any further logic.
            return source.subscribe(new Subscriber(subscriber));
        }
        else {
            return source.subscribe(new SkipLastSubscriber(subscriber, this._skipCount));
        }
    }
}
/**
 * We need this JSDoc comment for affecting ESDoc.
 * @ignore
 * @extends {Ignored}
 */
class SkipLastSubscriber extends Subscriber {
    constructor(destination, _skipCount) {
        super(destination);
        this._skipCount = _skipCount;
        this._count = 0;
        this._ring = new Array(_skipCount);
    }
    _next(value) {
        const skipCount = this._skipCount;
        const count = this._count++;
        if (count < skipCount) {
            this._ring[count] = value;
        }
        else {
            const currentIndex = count % skipCount;
            const ring = this._ring;
            const oldValue = ring[currentIndex];
            ring[currentIndex] = value;
            this.destination.next(oldValue);
        }
    }
}
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