如何修复“无法读取未定义的属性‘标题’”

Question

我正在关注 Mosh Hamedani 教程“Angular 4：初学者到专业人士”。

我在尝试编辑产品时尝试在表单中显示产品的标题。 该产品存储在 Firebase 数据库中。 我是 Angular 的新手。

但是，当我转到编辑表单时，控制台中出现此错误

ERROR TypeError: Cannot read property 'title' of undefined
    at Object.eval [as updateDirectives] (ProductFormComponent.html:7)
    at Object.debugUpdateDirectives [as updateDirectives] (core.js:23911)
    at checkAndUpdateView (core.js:23307)
    at callViewAction (core.js:23548)
    at execComponentViewsAction (core.js:23490)
    at checkAndUpdateView (core.js:23313)
    at callViewAction (core.js:23548)
    at execEmbeddedViewsAction (core.js:23511)
    at checkAndUpdateView (core.js:23308)
    at callViewAction (core.js:23548)

这是我的表格的一部分：

<div class="form-group">
            <label for="title">Title</label>
            <input #title="ngModel" [(ngModel)]="product.title" <== error here
name="title" id="title" type="text" class="form-control" required>
            <div class="alert alert-danger" *ngIf="title.touched && title.invalid">
              Title is required
            </div>
          </div>

这里是 product-form.component.ts

import { Component, OnInit, OnDestroy } from '@angular/core';
import { CategoryService } from 'src/app/category.service';
import { ProductService } from 'src/app/product.service';
import { Router, ActivatedRoute } from '@angular/router';
import { take } from 'rxjs/operators';

@Component({
  selector: 'app-product-form',
  templateUrl: './product-form.component.html',
  styleUrls: ['./product-form.component.css']
})
export class ProductFormComponent implements OnInit {

  categories$;
  product;

  constructor(
    categoryService: CategoryService,
    private route: ActivatedRoute,
    private productService: ProductService,
    private router: Router) {
    this.categories$ = categoryService.getCategories();

    let id = this.route.snapshot.paramMap.get('id');
     if (id) this.productService.get(id).snapshotChanges().pipe(take(1))
.subscribe(p => this.product = p); <== this does not seem to be working
  }

  save(product) {
    this.productService.create(product);
    this.router.navigate(["/admin/products"]);
  }

  ngOnInit() {
  }

}

如何显示产品的标题？

Answer 1

它有一个简单的解决方案。显示错误是因为在初始化 html 组件时，产品还没有准备好（因为 api 调用需要一些时间来返回数据），所以它是未定义的。因此在html组件中使用?.如下所示。

   <div class="form-group">
            <label for="title">Title</label>
            <input #title="ngModel" [(ngModel)]="product?.title" <== error here
name="title" id="title" type="text" class="form-control" required>
            <div class="alert alert-danger" *ngIf="title.touched && title.invalid">
              Title is required
            </div>
          </div>

    ````

Answer 2

您可以简单地在外部容器上添加 *ngIf="product"。只有在 prodcut 对象初始化时才会显示值。

Answer 3

该错误是因为该值在启动时未初始化。 当变量从订阅中获取其第一个值时会发生这种情况，这需要一些时间才能获取值，因此视图呈现并尝试在该值存在之前获取该值。 我建议首先为您的数据创建一个模型，然后初始化您的模型 查看Creating model classes in typescript 了解如何创建模型。 您可以在启动时对其进行初始化以避免错误，或者创建另一个变量来强制视图等待该变量。

import { Component, OnInit, OnDestroy } from '@angular/core';
import { CategoryService } from 'src/app/category.service';
import { ProductService } from 'src/app/product.service';
import { Router, ActivatedRoute } from '@angular/router';
import { take } from 'rxjs/operators';

@Component({
  selector: 'app-product-form',
  templateUrl: './product-form.component.html',
  styleUrls: ['./product-form.component.css']
})
export class ProductFormComponent implements OnInit {

  categories$;
  product;
initialized : boolean = false

  constructor(
    categoryService: CategoryService,
    private route: ActivatedRoute,
    private productService: ProductService,
    private router: Router) {
    this.categories$ = categoryService.getCategories();

    let id = this.route.snapshot.paramMap.get('id');
     if (id) this.productService.get(id).snapshotChanges().pipe(take(1))
.subscribe(p => {
this.initialized = true
this.product = p

}); }

  save(product) {
    this.productService.create(product);
    this.router.navigate(["/admin/products"]);
  }

  ngOnInit() {
  }

}

那么 html 将如下所示：

<div class="form-group">
            <label for="title">Title</label>
            <input #title="ngModel" *ngIf="initialized" [(ngModel)]="product.title"
name="title" id="title" type="text" class="form-control" required>
            <div class="alert alert-danger" *ngIf="title.touched && title.invalid">
              Title is required
            </div>
          </div>

您可以使用该变量来创建您喜欢的加载指示器

Answer 4

变化

product;

到

product:any = {};

and (valueChanges() 而不是 snapshotChnages())

if (id) this.productService.get(id).snapshotChanges().pipe(take(1))
.subscribe(p => this.product = p); 
  }

到

if (id) this.productService.get(id).valueChanges().pipe(take(1))
.subscribe(p => this.product = p); <== this does not seem to be working
  }

已解决问题

Answer 5

你也可以使用

product:any = {};