Angular 4 - 事件绑定

在本章中，我们将讨论事件绑定在 Angular 4 中的工作原理。当用户以键盘移动、鼠标单击或鼠标悬停的形式与应用程序交互时，它会生成一个事件。 需要处理这些事件才能执行某种操作。 这就是事件绑定发挥作用的地方。

让我们考虑一个例子来更好地理解这一点。

app.component.html

<!--下面的内容只是占位符，可以替换。-->
<div style = "text-align:center">
   <h1>
      Welcome to {{title}}.
   </h1>
</div>

<div> Months :
   <select>
      <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</ng-template>
   <ng-template #condition2>Condition is invalid</ng-template>
</div>
<button (click)="myClickFunction($event)">
   Click Me
</button>

在app.component.html文件中，我们定义了一个按钮并使用点击事件向其添加了一个函数。

以下是定义按钮并向其添加功能的语法。

(click)="myClickFunction($event)"

该函数在.ts文件中定义: app.component.ts

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", "Feburary", "March", "April",
            "May", "June", "July", "August", "September",
            "October", "November", "December"];
   isavailable = true;
   myClickFunction(event) { 
      //刚刚添加了 console.log，单击按钮将在浏览器中显示事件详细信息。
      alert("Button is clicked");
      console.log(event);
   }
}

点击按钮后，控件会来到函数myClickFunction，并出现一个对话框，显示the Button is clicked（按钮已被点击），如下图所示 &minus ;

现在让我们将更改事件添加到下拉列表中。

以下代码行将帮助您将更改事件添加到下拉列表中 −

<!--以下内容只是占位符，可以替换。-->
<div style = "text-align:center">
   <h1>
      Welcome to {{title}}.
   </h1>
</div>

<div> Months :
   <select (change) = "changemonths($event)">
      <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</ng-template>
   <ng-template #condition2>Condition is invalid</ng-template>
</div>

<button (click) = "myClickFunction($event)">Click Me</button>

该函数在 app.component.ts 文件中声明 −

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", "Feburary", "March", "April",
            "May", "June", "July", "August", "September",
            "October", "November", "December"];
   isavailable = true;
   myClickFunction(event) {
      alert("Button is clicked");
      console.log(event);
   }
   changemonths(event) {
      console.log("Changed month from the Dropdown");
      console.log(event);
   }
}

控制台消息"Changed month from the Dropdown（从下拉菜单更改月份）"与事件一起显示在控制台中。

当下拉列表中的值发生更改时，让我们在 app.component.ts 中添加一条警报消息，如下所示 −

import { Component } from '@angular/core';

@Component({
   selector: 'app-root',
   templateUrl: './app.component.html',
   styleUrls: ['./app.component.css']
})

export class AppComponent {
   title = 'Angular 4 Project!';
   //月份数组。
   months = ["January", "February", "March", "April",
            "May", "June", "July", "August", "September",
            "October", "November", "December"];
   
   isavailable = true;
   myClickFunction(event) { 
      //刚刚添加了 console.log，单击按钮将在浏览器中显示事件详细信息。
      alert("Button is clicked");
      console.log(event);
   }
   changemonths(event) {
      alert("Changed month from the Dropdown");
   }
}

当下拉列表中的值更改时，将出现一个对话框，并显示以下消息 - "Changed month from the Dropdown(从下拉列表中更改月份)"。