使用 Observable 或 Promise 在 Angular 6 中转换管道 [英] Transform pipe in Angular 6 with Observable or Promise
问题描述
我尝试在 angular 6 中设置 Pipe,使用服务(返回 observable 的方法)将文本转换为其他文本
I trying to set Pipe in angular 6 that convert text to other with using service (the method that returns observable)
我尝试了以下代码,但我需要返回一个字符串而不是 Promise
I tried the following code, but I need to return a string instead of Promise
管道:
import { Pipe, PipeTransform } from '@angular/core';
import { TimeZoneService, TimeZone } from '../services/Global/timezone.service';
//import { resolve } from 'dns';
import { reject } from 'q';
import { Observable } from 'rxjs';
@Pipe({
name: 'utcToText'
})
export class UtcToTextPipe implements PipeTransform {
private timezoneLst: TimeZone[] = [];
constructor(private _timezoneSvc : TimeZoneService) {}
async transform(timezone: any, args?: any){
this.timezoneLst = await this._timezoneSvc.getTimeZonesLst().toPromise();
return this.timezoneLst.find(x => x.utc.indexOf(timezone) > -1).text;
}
}
html:
<span>{{subscription.time_zone | utcToText}</span>
有什么办法可以让Promise/Ovservabe的异步方法变成String等同步返回的同步函数?
There is any way to make the asynchronous method of Promise / Ovservabe to a synchronous function that returns synchronous such as String?
非常感谢帮助.
推荐答案
尝试改为返回 Observable
并将 async
链接到现有管道上.此外,您将无法以 API 调用的异步性质返回 string
.
Try instead returning an Observable<string>
and chaining the async
onto your existing pipe. Also you simply will not be able to return string
with the async nature of your API calls.
管道:
import { Pipe, PipeTransform } from '@angular/core';
import { TimeZoneService, TimeZone } from '../services/Global/timezone.service';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';
@Pipe({ name: 'utcToText'})
export class UtcToTextPipe implements PipeTransform {
constructor(private _timezoneSvc : TimeZoneService) {}
transform(timezone: string, args?: any): Observable<string> {
// this assumes getTimeZonesLst() returns an Observable<TimeZone[]>
return this._timezoneSvc.getTimeZonesLst().pipe(
map((timeZones: TimeZone[]) => {
return timeZones.find(x => x.utc.indexOf(timezone) > -1).text;
})
);
}
}
模板:
<span>{{subscription.time_zone | utcToText | async}</span>
这里是一个从 示例 正在运行的异步管道://angular.io/guide/pipes#custom-pipes" rel="noreferrer">指数管道 Angular 文档中的示例.
Here is a example async pipe in action derived from the exponential pipe example in the Angular documentation.
如果您真的由于某种原因需要使用 promises 而不是 observables:
If you really need to use promises instead of observables for some reason:
import { Pipe, PipeTransform } from '@angular/core';
import { TimeZoneService, TimeZone } from '../services/Global/timezone.service';
import { Observable } from 'rxjs';
@Pipe({ name: 'utcToText'})
export class UtcToTextPipe implements PipeTransform {
constructor(private _timezoneSvc : TimeZoneService) {}
transform(timezone: string, args?: any): Promise<string> {
// this assumes getTimeZonesLst() returns an Observable<TimeZone[]>
return this._timezoneSvc.getTimeZonesLst()
.toPromise()
.then((timeZones: TimeZone[]) => {
return timeZones.find(x => x.utc.indexOf(timezone) > -1).text;
})
}
}
希望有帮助!
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