如何在角度5的纯管道中使用HTTP调用 [英] How to use HTTP calls in pure pipe in angular 5

问题描述

我正在创建将一种货币值转换为另一种货币值的管道.我正在通过HTTP调用来转换值.

I am creating a pipe to convert one currency value to another. I am making HTTP call to convert value.

@Pipe({
    name: "currencyConverter"
})
export class CurrencyConverterPipe implements PipeTransform {

    transform(value: number, currencyPair: string): number {
        let sourceCurrency = currencyPair.split(":")[0];
        let destinationCurrency = currencyPair.split(":")[1];
        this.currencyConverterService.convert(sourceCurrency, destinationCurrency).subscribe(successData => {
            return successData['fiatValue'] * value;
        }, errorData => {
            console.error(errorData);
        })
    }
}

在HTML

<p> {{ value | currencyConverter:'BTC:USD'}}</p>

我在UI中看不到任何值.当我使管道不纯时，它可以工作，但是它对服务器进行了许多HTTP调用.有什么方法可以在不使用 pure:false 在管道中的情况下进行HTTP调用

I am unable to see any value in UI. When i make my pipe impure, Then it works but it is making many HTTP calls to server. Is there any way to make HTTP call without using pure: false in pipe

推荐答案

该管道不是纯设计的.

为了不引起多个请求，它应该返回相同的observable，除非更改输入，否则不会再执行其他请求，例如:

In order to not cause multiple requests, it should return same observable that won't do additional requests until input is changed, something like:

@Pipe({
  name: "currencyConverter",
  pure: false
})
export class CurrencyConverterPipe  {
  private sourceCurrency;
  private destinationCurrency;
  private valueSubject = new Subject();
  private value$ = this.valueSubject.asObservable().distinctUntilChanged()
  .switchMap(value => {
     return this.currencyConverter
     .convert(sourceCurrency, destinationCurrency))
     .map((data) => data * value);
  });

  constructor(private currencyConverter: ...) {}

  transform(value: number, currencyPair: string): number {
    this.sourceCurrency = currencyPair.split(":")[0];
    this.destinationCurrency = currencyPair.split(":")[1];
    this.valueSubject.next(value);
    return this.value$;
  }
}

由于应该仅将其与 async 管道结合使用，因此将它们连接在一起并扩展 AsyncPipe :

Since it is supposed to be used only in conjunction with async pipe, it makes sense to join them together and extend AsyncPipe:

@Pipe({
  name: "currencyConverterAsync",
  pure: false
})
export class CurrencyConverterPipe extends AsyncPipe {
  private sourceCurrency;
  private destinationCurrency;
  private valueSubject = new Subject();
  private value$ = this.valueSubject.asObservable().distinctUntilChanged()
  .switchMap(value => {
     return this.currencyConverter
     .convert(sourceCurrency, destinationCurrency))
     .map((data) => data * value);
  });

  constructor(cdRef: ChangeDetectorRef, private currencyConverter: ...) {
    super(cdRef);
  }

  transform(value: number, currencyPair: string): number {
    this.sourceCurrency = currencyPair.split(":")[0];
    this.destinationCurrency = currencyPair.split(":")[1];
    this.valueSubject.next(value);

    return super.transform(this.value$);
  }
}

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