智能计数器

一个在页面上带有动态更新数字效果的有趣元素就是智能计数器，也可以称之为里程表效果。不采用上下跳数的方式，而是快速地清点到期望的数字，这能达到一种很酷的效果。能做到这点的流行库的其中一个就是由 Hubspot 所写的 odometer 。让我们来看看如何使用短短几行 RxJS 代码来实现类似的效果。

原生 JS

( JSBin | JSFiddle )

// 工具函数
const takeUntilFunc = (endRange, currentNumber) => {
  return endRange > currentNumber
    ? val => val <= endRange
    : val => val >= endRange;
};

const positiveOrNegative = (endRange, currentNumber) => {
  return endRange > currentNumber ? 1 : -1;
};

const updateHTML = id => val => (document.getElementById(id).innerHTML = val);
// 显示
const input = document.getElementById('range');
const updateButton = document.getElementById('update');

const subscription = (function(currentNumber) {
  return fromEvent(updateButton, 'click').pipe(
    map(_ => parseInt(input.value)),
    switchMap(endRange => {
      return timer(0, 20).pipe(
        mapTo(positiveOrNegative(endRange, currentNumber)),
        startWith(currentNumber),
        scan((acc, curr) => acc + curr),
        takeWhile(takeUntilFunc(endRange, currentNumber));
      )
    }),
    tap(v => (currentNumber = v)),
    startWith(currentNumber)
  )
  .subscribe(updateHTML('display'));
})(0);

HTML

<input id="range" type="number">
<button id="update">Update</button>
<h3 id="display">0</h3>

我们可以轻易地获取我们的原生 JS 所写的智能计数器并将其包装在任何流行的基于 UI 库中。下面是 Angular 版本的智能计数器，它接收一个更新结束范围的 Input 输入属性并执行适当的转换。

Angular 版本

( StackBlitz )

import { Component, Input, OnDestroy } from '@angular/core';
import { Subject } from 'rxjs/Subject';
import { timer } from 'rxjs/observable/timer';
import { switchMap, startWith, scan, takeWhile, takeUntil, mapTo } from 'rxjs/operators';

@Component({
  selector: 'number-tracker',
  template: `
    <h3> {{ currentNumber }}</h3>
  `
})
export class NumberTrackerComponent implements OnDestroy {
  @Input()
  set end(endRange: number) {
    this._counterSub$.next(endRange);
  }
  @Input() countInterval = 20;
  public currentNumber = 0;
  private _counterSub$ = new Subject();
  private _onDestroy$ = new Subject();

  constructor() {
    this._counterSub$
      .pipe(
        switchMap(endRange => {
          return timer(0, this.countInterval).pipe(
            mapTo(this.positiveOrNegative(endRange, this.currentNumber)),
            startWith(this.currentNumber),
            scan((acc: number, curr: number) => acc + curr),
            takeWhile(this.isApproachingRange(endRange, this.currentNumber))
          )
        }),
        takeUntil(this._onDestroy$)
      )
      .subscribe((val: number) => this.currentNumber = val);
  }

  private positiveOrNegative(endRange, currentNumber) {
    return endRange > currentNumber ? 1 : -1;
  }

  private isApproachingRange(endRange, currentNumber) {
    return endRange > currentNumber
      ? val => val <= endRange
      : val => val >= endRange;
  }

  ngOnDestroy() {
    this._onDestroy$.next();
    this._onDestroy$.complete();
  }
}

HTML

<p>
  <input type="number"
    (keyup.enter)="counterNumber = vanillaInput.value"
    #vanillaInput>
  <button
    (click)="counterNumber = vanillaInput.value">
    Update number
  </button>
</p>
<number-tracker [end]="counterNumber"></number-tracker>

使用到的操作符

• fromEvent
• map
• mapTo
• scan
• startWith
• switchMap
• takeWhile