Angular 4 使用< ng-template> 作为标签,而不是Angular2中使用的< template> . Angular 4将< template> 更改为< ng-template> 的原因是因为< template> 之间存在名称冲突标签和html < template> 标准标签.它将完全弃用.这是Angular 4中的一个主要变化.
现在让我们使用模板和 if else 条件并查看输出.
<!--The content below is only a placeholder and can be replaced.--> <div style = "text-align:center"> <h1> Welcome to {{title}}. </h1> </div> <div> Months : <select (change) = "changemonths($event)" name = "month"> <option *ngFor = "let i of months">{{i}}</option> </select> </div> <br/> <div> <span *ngIf = "isavailable;then condition1 else condition2">Condition is valid.</span> <ng-template #condition1>Condition is valid from template</ng-template> <ng-template #condition2>Condition is invalid from template</ng-template> </div> <button (click) = "myClickFunction($event)">Click Me</button>
对于Span标记,我们添加了带 else 条件的 if 语句并将调用模板condition1,否则condition2.
模板的调用如下 :
<ng-template #condition1>Condition is valid from template</ng-template> <ng-template #condition2>Condition is invalid from template</ng-template>
如果条件为真,则调用condition1模板,否则调用condition2.
import { Component } from '@angular/core'; @Component({ selector: 'app-root', templateUrl: './app.component.html', styleUrls: ['./app.component.css'] }) export class AppComponent { title = 'Angular 4 Project!'; //array of months. months = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]; isavailable = false; myClickFunction(event) { this.isavailable = false; } changemonths(event) { alert("Changed month from the Dropdown"); console.log(event); } }
浏览器中的输出如下 :
变量 isavailable 为false,因此打印出condition2模板.如果单击该按钮,将调用相应的模板.如果您检查浏览器,您将看到永远不会在dom中获得span标记.以下示例将帮助您理解相同的内容.
如果您检查浏览器,您将看到dom没有span标记.它具有dom中条件无效的模板.
html中的以下代码行将帮助我们在dom中获取span标记.
<!--The content below is only a placeholder and can be replaced.--> <div style = "text-align:center"> <h1> Welcome to {{title}}. </h1> </div> <div> Months : <select (change) = "changemonths($event)" name = "month"> <option *ngFor = "let i of months">{{i}}</option> </select> </div> <br/> <div> <span *ngIf = "isavailable; else condition2">Condition is valid.</span> <ng-template #condition1>Condition is valid from template</ng-template> <ng-template #condition2>Condition is invalid from template</ng-template> </div> <button (click)="myClickFunction($event)">Click Me</button>
如果我们删除then条件,我们在浏览器中得到"条件有效"消息,并且span标记也是在dom中可用.例如,在 app.component.ts 中,我们将 isavailable 变量设为true.