如何一个接一个地调用一个异步函数？

Question

这就是我所拥有的：

  ngOnInit() {
    this._addServ.getDefaultAddress().subscribe((res)=>{
      if(res.status){
        //I need these next two functions to finish their async API requests 
        //before selectNormalDelivery is called
        this.selectShippingAddress();
        this.selectBillingAddress();
        this.selectNormalDelivery();
      }
    })
  }

我需要selectShippingAddress 和selectBillingAddress 来完成他们的通话，然后才能调用selectNormalDelivery。我怎样才能做到这一点？我不想将它们转换为承诺，因为它们在其他时候被调用而无需被链接。这是否涉及.then？

这是函数的代码：

  selectShippingAddress(){
    this.addressService.setShippingAddresses(this.cartService.shippingAddress.address_id).subscribe((res) => {
      console.log('Set shipping address!')
      console.log(res);
    }, err => {
      console.log('Failed to set shipping address')
      console.log(err);
    })
  }

  selectBillingAddress(){
    this.addressService.setPaymentAddresses(this.cartService.billingAddress.address_id).subscribe((res) => {
      console.log('Set billing address!')
      console.log(res);
    }, err => {
      console.log('Failed to set billing address')
      console.log(err);
    })
  }

如果我尝试.then，我会收到错误Property 'then' does not exist on type 'void'.，这是有道理的，因为我没有尝试返回任何内容，我只需要他们为了 API 的缘故完成他们的调用。

建议？

Answer 1

ngOnInit() {
    this._addServ.getDefaultAddress().subscribe((res)=>{
      if(res.status){
        //I need these next two functions to finish their async API requests 
        //before selectNormalDelivery is called
        forkJoin([this.selectShippingAddress(),this.selectBillingAddress()].subscribe(() => {

            this.selectNormalDelivery();
        });
      }
    })
  }

 selectShippingAddress(){
    return this.addressService.setShippingAddresses(this.cartService.shippingAddress.address_id).pipe(map((res) => {
      console.log('Set shipping address!')
      console.log(res);
    }, err => {
      console.log('Failed to set shipping address')
      console.log(err);
    }));
  }

  selectBillingAddress(){
    return this.addressService.setPaymentAddresses(this.cartService.billingAddress.address_id).pipe(map((res) => {
      console.log('Set billing address!')
      console.log(res);
    }, err => {
      console.log('Failed to set billing address')
      console.log(err);
    }));
  }

Answer 2

如果我理解正确，您需要监听一个 observable（来自 getDefaultAddress()），然后处理另外两个 observable（this.addressService.setShippingAddresses() 和 this.addressService.setPaymentAddresses()），然后调用函数 selectNormalDelivery()。

您可以通过使用 RxJS switchMap、catchError、tap 运算符和 forkJoin() 来实现。试试下面的

import { forkJoin, EMPTY } from 'rxjs';
import { tap, catchError, switchMap } from 'rxjs/operators';

ngOnInit() {
  this._addServ.getDefaultAddress().pipe(
    switchMap((res) => {        // <-- map the observable from `getDefaultAddress()` to another observable
      if(res.status) {
        return forkJoin([       // <-- `forkJoin` emits only after the observables complete
          this.selectShippingAddress(), this.selectBillingAddress()
        ])
      }
    })
  ).subscribe(response => {
    this.selectNormalDelivery()
  })
}

selectShippingAddress() {
  return this.addressService.setShippingAddresses(this.cartService.shippingAddress.address_id).pipe(
    tap((res) => {            // <-- `tap` doesn't alter the response
      console.log('Set shipping address!')
      console.log(res);
    }),
    catchError((err) => {     // <-- `catchError` must return an observable
      console.log('Failed to set shipping address')
      console.log(err);
      return EMPTY;
    }))
}


selectShippingAddress() {
  return this.addressService.setPaymentAddresses(this.cartService.shippingAddress.address_id).pipe(
    tap((res) => {
      console.log('Set billing address!')
      console.log(res);
    }),
    catchError((err) => {
      console.log('Failed to set billing address')
      console.log(err);
      return EMPTY;
    }))
}