Angular 12 - TypeError：无法读取 null 的属性（读取“writeValue”）答案

【问题标题】：Angular 12 - TypeError: Cannot read properties of null (reading 'writeValue')Angular 12 - TypeError：无法读取 null 的属性（读取“writeValue”）
【发布时间】：2021-11-24 04:57:02
【问题描述】：

Angular 12 - TypeError：无法读取 null 的属性（读取“writeValue”）

我正在创建一个通用文本输入组件，在为项目提供服务时一切正常，但在构建的项目中出现此错误：

TypeError: Cannot read properties of null (reading 'writeValue')

HTML：

<div class="p-field">
    <label>{{label}} <span class="p-error">{{checkRequired(ngControl.control) ? '*' : ''}}</span></label>
    <input class="full-width" [type]="type" pInputText [formControl]="ngControl.control">

    <small *ngIf="ngControl.control.touched && ngControl.control.errors?.required" class="p-error">{{label}} is required</small>


    <small *ngIf="ngControl.control.errors?.minlength" class="p-error">
        {{label}} must be at least {{ngControl.control.errors.minlength['requiredLength']}}
    </small>

    <small *ngIf="ngControl.control.errors?.maxlength" class="p-error">
        {{label}} must be at most {{ngControl.control.errors.maxlength['requiredLength']}}
    </small>
    <small *ngIf="ngControl.control.errors?.email" class="p-error">
        This email is not valid
    </small>

    <small *ngIf="ngControl.control.errors?.isMatching" class="p-error">Passwords do not match</small>
</div>

组件.ts

import { Component, Input, OnInit, Self } from '@angular/core';
import {AbstractControl, ControlValueAccessor, NgControl} from '@angular/forms';

@Component({
  selector: 'app-text-input',
  templateUrl: './text-input.component.html',
  styleUrls: ['./text-input.component.css']
})
export class TextInputComponent implements ControlValueAccessor {
  @Input() label: string;
  @Input() type = 'text';

  constructor(@Self() public ngControl: NgControl) {
    this.ngControl.valueAccessor = this;
  }

  checkRequired(control) {
    if (control.validator) {
      const validator = control.validator({} as AbstractControl);
      if (validator && validator.required) {
        return true;
      }
    }
  }

  writeValue(obj: any): void {
  }

  registerOnChange(fn: any): void {
  }

  registerOnTouched(fn: any): void {
  }

}

角度信息：

Angular CLI：12.2.6 节点：14.17.3 包管理器：npm 6.14.13 操作系统：win32 x64 角度：12.2.6

【问题讨论】：

标签： angular angular-reactive-forms angular-controlvalueaccessor

【解决方案1】：

在构造函数中使用@Optional() 并进行空值检查：

constructor(@Optional() @Self() public ngControl: NgControl) {
   if (this.ngControl != null) this.ngControl.valueAccessor = this;
}

另外，将正文添加到方法中

writeValue(value: number): void {
    this.value = value;
    this.onChange(this.value);
}

onChange: (_: any) => void = (_: any) => {};

onTouched: () => void = () => {};

registerOnChange(fn: any): void {
    this.onChange = fn;
}

registerOnTouched(fn: any): void {
    this.onTouched = fn;
}

【讨论】：