<!-- novalidate 清除浏览器默认的校验行为 -->
<form [formGroup]="formModel" (ngSubmit)="onSearch()" novalidate><div class="form-group" [class.has-error]="formModel.hasError('minlength','title')"><label for="productTitle">商品名称:</label><input formControlName="title" type="text" id="productTitle" placeholder="商品名称" class="form-control"><span class="help-block" [class.hidden]="!formModel.hasError('minlength','title')">请至少输入三个字</span></div><div class="form-group" [class.has-error]="formModel.hasError('positiveNumber','price')"><label for="productPrice">商品价格:</label><input formControlName="price" type="number" id="productPrice" placeholder="商品价格" class="form-control"><span class="help-block" [class.hidden]="!formModel.hasError('positiveNumber','price')">请输入正数</span></div><div class="form-group"><label for="productCategory">商品类别:</label><select formControlName="category" id="productCategory" class="form-control"><option value="-1">全部分类</option><option *ngFor="let category of categories" [value]="category">{{category}}</option></select></div><div class="form-group"><button type="submit" class="btn btn-primary btn-block">搜索</button></div>
</form>

import { Component, OnInit } from '@angular/core';
import { FormGroup, FormBuilder, Validators,FormControl } from '@angular/forms';
import { ProductService } from '../share/product.service';@Component({selector: 'app-search',templateUrl: './search.component.html',styleUrls: ['./search.component.scss']
})
export class SearchComponent implements OnInit {formModel:FormGroup;categories: string[];constructor(private productService:ProductService) {let fb = new FormBuilder();this.formModel = fb.group({title:['',Validators.minLength(3)], //最少不少于3个字符price:[null,this.positiveNumberValidator], //不能为负数category:['-1'] //默认值为-1 --全部分类
    })}ngOnInit() {this.categories = this.productService.getAllCategories();}positiveNumberValidator(control: FormControl):any{if(!control.value){//如果输入为空则返回空，相当于去空格return null;}let price = parseInt(control.value);if(price>0){ // 如果大于0不显示错误信息return null;}else{  // 不大于0则显示错误信息return {positiveNumber:true};}}
//如果验证通过的话就把表单对应的值打印到控制台
  onSearch(){if(this.formModel.valid){console.log(this.formModel.value);}}}

productService.ts 在服务里声明返回类别方法

getAllCategories():string[]{return ["电子产品","硬件设备","图书"];}