Angular7 - 模板

Angular 7 使用 <ng-template> 作为标签，而不是 Angular2 中使用的 <template>。<ng-template> 自 Angular 4 发布以来一直在使用，早期版本（即 Angular 2）也使用 <template> 来实现相同的目的。从 Angular 4 开始使用 <ng-template> 而不是 <template> 的原因是 <template> 标签和 html <template> 标准标签之间存在名称冲突。它将完全弃用。这是 Angular 4 版本中所做的主要更改之一。

现在让我们将模板与if else 条件一起使用并查看输出。

应用程序组件.html

<!--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，我们添加了带有else条件的if语句，并将调用模板条件 1 和 else 条件 2。

模板的调用方式如下 -

<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，则调用条件 1模板，否则调用条件 2。

应用程序组件.ts

import { Component } from '@angular/core';
@Component({ 
   selector: 'app-root', 
   templateUrl: './app.component.html', 
   styleUrls: ['./app.component.css'] 
}) 
export class AppComponent { 
   title = 'Angular 7'; 
   
   // declared array of months. 
   months = ["January", "February", "March", "April", "May", "June", "July", 
      "August", "September", "October", "November", "December"]; 
   isavailable = false; // variable is set to true
   
   myClickFunction(event) { 
      //just added console.log which will display the event details in browser on click of the button. 
      alert("Button is clicked"); 
      console.log(event); 
   }
   changemonths(event) { 
      alert("Changed month from the Dropdown"); 
   } 
}

浏览器中的输出如下 -

条件无效

变量isavailable为 false，因此打印了 condition2 模板。如果单击该按钮，将调用相应的模板。

应用程序组件.ts

import { Component } from '@angular/core';
@Component({ 
   selector: 'app-root', 
   templateUrl: './app.component.html', 
   styleUrls: ['./app.component.css'] 
}) 
export class AppComponent { 
   title = 'Angular 7'; 
   
   // declared array of months. 
   months = ["January", "Feburary", "March", "April", "May", "June", "July", 
      "August", "September", "October", "November", "December"];
   
   isavailable = false; //variable is set to true  
   myClickFunction(event) { 
      this.isavailable = !this.isavailable; 
      // variable is toggled onclick of the button 
   } 
   changemonths(event) {
      alert("Changed month from the Dropdown"); 
   }
}

单击按钮即可切换isavailable变量，如下所示 -

myClickFunction(event) { 
   this.isavailable = !this.isavailable; 
}

当您根据isavailable变量的值单击按钮时，将显示相应的模板 -

条件有效 模板无效

如果你检查浏览器，你会发现你永远不会在 dom 中获得 span 标签。下面的例子将帮助您理解这一点。

模板

尽管在app.component.html中我们添加了 span 标签和<ng-template>条件，如下所示 -

<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>

当我们在浏览器中检查时，我们在 dom 结构中看不到 span 标签和 <ng-template> 。

html 中的以下代码行将帮助我们获取 dom 中的 span 标签 -

<!--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，我们会在浏览器中收到“条件有效”消息，并且 span 标签在 dom 中也可用。例如，在app.component.ts中，我们将isavailable变量设置为 true。

可用的