Angular ngIf：未定义标识符“长度”

Question

我的组件具有属性 transfer$ ，它是可观察到的，可以带有包含错误消息的数组或 object 。 当transfers$为Transfer[]时，我只能输入[items]。 但即使使用条件

<app-checklist *ngIf="transfers?.length > 0" [items]="transfers"></app-checklist>

angular 继续警告：

未定义标识符“长度”。 '转移[] | AppResponseError' 不包含这样的成员

这是为什么？

谢谢

组件.html

<ng-template #loadingTpl>loading</ng-template>

<ng-template #emptyList let-length="length">
    <p *ngIf="length === 0">Empty List</p>
</ng-template>

<ng-template #error let-title="title" let-message="message" let-code="code">
    <div *ngIf="code">
        Error: {{code}} , {{title}} , {{message}}
    </div>
</ng-template>

<div *ngIf="transfers$ | async; let transfers; else loadingTpl">
    <app-checklist *ngIf="transfers?.length > 0" [items]="transfers">
        <div *ngFor="let transfer of transfers">
            ..something here
        </div>
    </app-checklist>
    <ng-container *ngTemplateOutlet="emptyList; context: {length: transfers.length}"></ng-container>
    <ng-container *ngTemplateOutlet="error; context:transfers"></ng-container>
</div>

TransferService.ts：

import { Injectable } from '@angular/core';
import { of } from 'rxjs';

@Injectable()
export class TransfersService {
  transfers$ = of('real observable');
}

组件.ts

import { Component, OnInit } from '@angular/core';
import { Observable } from 'rxjs';
import { TransfersService } from './TransfersService';

export interface AppResponseError = {code:number,title:string,message:string};
export interface Transfer = {id:number,title:string};

@Component({
  selector: 'app-transfer',
})
export class TransferComponent implements OnInit {
  transfers$: Observable<Transfer[] | AppResponseError>;
  constructor(private transfersService: TransfersService) {
    this.transfers$ = this.transfersService.transfers$;
  }

  ngOnInit() {
    this.transfers$.subscribe((data) => {
      console.log(data);
    });
  }
}

Answer 1

您不需要额外的 *ngIf。

<div *ngIf="transfers$ | async; let transfers; else loadingTpl">

这将做的是等到转帐$ | async 已评估，并将结果绑定到 transfer 的值（非美元后缀）。

在 ts 文件中创建下面的 function。

isDataAvailable(arr){
 return Array.isArray(arr) && arr.length >0
}

并像这样在模板中使用它

<app-checklist *ngIf="isDataAvailable(transfers)" [items]="transfers"></app-checklist>

<div *ngIf="transfers$ | async; let transfers; else loadingTpl">
        <app-checklist *ngIf="isDataAvailable(transfers)" [items]="transfers"></app-checklist>
        <div *ngFor="let transfer of transfers">
            ..something here
        </div>
    <ng-container *ngTemplateOutlet="emptyList; context: {length: transfers.length}"></ng-container>
    <ng-container *ngTemplateOutlet="error; context:transfers"></ng-container>
</div>