Angular 4 - 事件繫結

在本章中，我們將討論事件繫結在 Angular 4 中是如何工作的。當用戶以鍵盤移動、滑鼠點選或滑鼠懸停的形式與應用程式互動時，會生成一個事件。這些事件需要被處理以執行某種操作。這就是事件繫結發揮作用的地方。

讓我們考慮一個例子來更好地理解這一點。

app.component.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>
      <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 檔案中，我們定義了一個按鈕並使用 click 事件向其添加了一個函式。

以下是定義按鈕並向其新增函式的語法。

(click)="myClickFunction($event)"

該函式在.ts 檔案中定義：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) { 
      //just added console.log which will display the event details in browser on click of the button.
      alert("Button is clicked");
      console.log(event);
   }
}

點選按鈕後，控制權將進入函式myClickFunction，並顯示一個對話方塊，其中顯示按鈕被點選，如下面的螢幕截圖所示：

現在讓我們將 change 事件新增到下拉列表中。

以下程式碼行將幫助您將 change 事件新增到下拉列表中：

<!--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)">
      <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>

該函式在app.component.ts 檔案中宣告：

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);
   }
}

控制檯訊息“從下拉列表中更改月份”與事件一起顯示在控制檯中。

讓我們在app.component.ts 中新增一個警報訊息，當下拉列表中的值更改時，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!';
   //array of months.
   months = ["January", "February", "March", "April",
            "May", "June", "July", "August", "September",
            "October", "November", "December"];
   
   isavailable = 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");
   }
}

當下拉列表中的值更改時，將出現一個對話方塊，並顯示以下訊息：“從下拉列表中更改月份”。