Angular 7 反应形式无法读取未定义的属性 'controls'

Question

我在 Angular v7 中使用 ReactiveForms。

模板

<div class="login__container">
    <div class="login__form">
        <form novalidate [formGroup]="loginForm" (ngSubmit)="onSubmit()">
            <div class="field">
                <p class="control">
                    <input class="input" type="text" placeholder="Username" formControlName="username" [ngClass]="{ 'is-invalid': submitted && f.username.errors }">
                </p>
            </div>
            <div class="field">
                <p class="control">
                    <input class="input" type="password" placeholder="Password" formControlName="password" [ngClass]="{ 'is-invalid': submitted && f.password.errors }">
                </p>
            </div>
            <div class="field">
                <p class="control">
                    <button class="button is-success" [disabled]="!loginForm.valid">
                        Login
                    </button>
                </p>
            </div>
        </form>
    </div>
</div>

组件

import { Component, OnInit } from '@angular/core'
import { FormBuilder, FormGroup, Validators } from '@angular/forms'

@Component({
    selector: 'app-login-component',
    templateUrl: './login.component.html',
    styleUrls: ['./login.component.scss']
})
export class LoginComponent implements OnInit {
    loginForm: FormGroup
    submitted = false

    constructor(private fB: FormBuilder) {}

    ngOnInit() {
        this.loginForm = this.fB.group({
            username: ['', Validators.required],
            password: ['', Validators.required]
        })
    }

    // convenience getter for easy access to form fields
    get f() {
        return this.loginForm.controls
    }

    onSubmit() {
        this.submitted = true

        // stop here if form is invalid
        if (this.loginForm.invalid) {
            return
        }

        console.log(this.loginForm.value)
    }
}

当我走这条路线时，出现控制台错误

ERROR TypeError: Cannot read property 'controls' of undefined
    at Object.get [as f] (http://localhost:4200/main.js:257:35)
    at http://localhost:4200/vendor.js:92667:9
    at Array.forEach (<anonymous>)
    at deepFreeze (http://localhost:4200/vendor.js:92664:33)
    at http://localhost:4200/vendor.js:92670:7
    at Array.forEach (<anonymous>)
    at deepFreeze (http://localhost:4200/vendor.js:92664:33)
    at http://localhost:4200/vendor.js:92670:7
    at Array.forEach (<anonymous>)
    at deepFreeze (http://localhost:4200/vendor.js:92664:33)

我无法弄清楚为什么会发生这种情况，提交按钮周围的验证似乎有效，所以我只能假设正确引用了这些字段。我怀疑问题出在组件内设置的便利性 getter，但是我看不出这有什么问题，因为自动完成功能为我提供了我期望的值。

Answer 1

检查 html 中的 loginForm 应该很容易修复。

<div class="login__container" *ngIf="loginForm"> <!-- Check here -->
    <div class="login__form">
        <form novalidate [formGroup]="loginForm" (ngSubmit)="onSubmit()">
            <div class="field">
                <p class="control">
                    <input class="input" type="text" placeholder="Username" formControlName="username" [ngClass]="{ 'is-invalid': submitted && f.username.errors }">
                </p>
            </div>
            <div class="field">
                <p class="control">
                    <input class="input" type="password" placeholder="Password" formControlName="password" [ngClass]="{ 'is-invalid': submitted && f.password.errors }">
                </p>
            </div>
            <div class="field">
                <p class="control">
                    <button class="button is-success" [disabled]="!loginForm.valid">
                        Login
                    </button>
                </p>
            </div>
        </form>
    </div>
</div>

Answer 2

您可以简单地引用

之类的字段，而不是您使用的便捷方法

[ngClass]="{ 'is-invalid': submitted && loginForm.controls['username'].errors }"

如果您希望使用

，则会出现显示错误消息的情况

<div *ngIf="submitted && loginForm.controls['username'].hasError('required')" class="error-message">Username is required</div>

我不清楚你目前的方法比像这样直截了当的事情能给你带来什么好处。

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