如何修复“无法读取未定义的属性'title'”

Question

我正在学习Mosh Hamedani的教程“角度4：初学者到专业”。

当我试图编辑一个产品时，我试图在一个表单中显示它的标题。该产品存储在firebase数据库中。我是新来的Angular。

但是，当我转到编辑表单时，控制台中会出现此错误

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>

这是产品表单.组件.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() {
  }

}

怎么做才能显示产品的标题？

Answer 1

错误是因为该值在启动时没有初始化。当变量从订阅中获取其第一个值时，就会发生这种情况，这需要一些时间来获取值，所以视图呈现并尝试在它存在之前获取值。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。看看如何创建模型的打字稿中的模型类。您可以在启动时初始化它以避免错误，或者创建另一个变量，强制视图等待变量。

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>

您可以使用变量来创建您喜欢的加载指示器

Answer 2

改变

product;

自

product:any = {};

和(valueChanges()而不是snapshotChnages())

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

自

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

已修复问题

Answer 3

您也可以使用

product:any = {};