Angular2 でオブジェクト配列の長さをテンプレート内でチェックする *ngIf の使い方
例:
<div *ngIf="myItems.length > 0">
<ul>
<li *ngFor="let item of myItems">
{{ item.name }}
</li>
</ul>
</div>
<div *ngIf="myItems.length === 0">
<p>アイテムがありません。</p>
</div>
この例では、myItems という名前のオブジェクト配列が存在します。 *ngIf ディレクティブは、myItems.length 式を評価し、以下の条件に基づいて要素を表示/非表示を切り替えます。
myItems.length > 0: 配列の長さが 0 より大きい場合、ul要素と*ngForディレクティブを使用して、各アイテムを表示します。myItems.length === 0: 配列の長さが 0 の場合、「アイテムがありません。」というメッセージを表示します。
- オブジェクトの長さをチェックするには、
Object.keys()メソッドを使用して配列を取得し、その長さを取得することもできます。 *ngIfディレクティブは、プリミティブ値だけでなく、オブジェクトや配列などの複雑な式も評価できます。- より複雑なロジックが必要な場合は、
*ngForディレクティブと組み合わせて使用することができます。
- 上記の例は基本的な使用方法を示しています。より複雑なロジックが必要な場合は、ご自身のニーズに合わせて調整してください。
- Angular の最新情報については、公式ドキュメントを参照することをお勧めします。
<!DOCTYPE html>
<html>
<head>
<title>Angular2 *ngIf Example</title>
<script src="https://unpkg.com/@angular/core@latest/bundles/core.umd.js"></script>
<script src="app.component.js"></script>
</head>
<body>
<app-root></app-root>
</body>
</html>
import { Component } from '@angular/core';
@Component({
selector: 'app-root',
template: `
<h1>Items List</h1>
<ul>
<li *ngFor="let item of items">
{{ item.name }}
</li>
</ul>
<p *ngIf="items.length > 0">Total items: {{ items.length }}</p>
<p *ngIf="items.length === 0">No items found.</p>
`
})
export class AppComponent {
items = [
{ name: 'Item 1' },
{ name: 'Item 2' },
{ name: 'Item 3' }
];
}
Explanation:
HTML Template (
index.html):- Imports the Angular2 core library.
- References the
AppComponentcomponent.
AppComponent Component (
app.component.ts):- Defines an
AppComponentclass with the@Componentdecorator. - The
selectorproperty specifies the HTML element where the component will be rendered (<app-root>). - The
templateproperty defines the HTML markup for the component. - The
itemsproperty is an array of objects, each representing an item in the list.
- Defines an
Template Usage:
*ngIffor List Items:- Iterates over the
itemsarray using the*ngFordirective. - Displays the name of each item using
{{ item.name }}.
- Iterates over the
*ngIffor Total Items Count:- Checks if the length of the
itemsarray is greater than 0 using*ngIf="items.length > 0". - Displays the total number of items using
{{ items.length }}.
- Checks if the length of the
*ngIffor Empty List Message:- Displays a message indicating that no items were found.
Running the Example:
- Save the code as
index.htmlandapp.component.tsin the same directory. - Open a terminal in the directory and run the following command:
npx serve
- Open a web browser and navigate to
http://localhost:4200to view the application.
<div *ngIf="myItems">
<ul>
<li *ngFor="let item of myItems">
{{ item.name }}
</li>
</ul>
</div>
<div *ngIf="!myItems">
<p>アイテムがありません。</p>
</div>
In this approach, the *ngIf directive directly evaluates the myItems object. When the object is defined and not null or undefined, it is considered truthy, and the list is displayed. When the object is null or undefined, it is considered falsy, and the empty list message is displayed.
Using the trackBy function in *ngFor:
<ul *ngFor="let item of myItems; trackBy: trackByFn">
<li>{{ item.name }}</li>
</ul>
<p *ngIf="!myItems">
アイテムがありません。</p>
</div>
This approach utilizes the trackBy function in the *ngFor directive to improve change detection performance. The trackByFn function should return a unique identifier for each item in the array. When the array changes, Angular only updates the changed items instead of re-rendering the entire list. In this case, the *ngIf directive outside the *ngFor loop can still check the existence of the myItems array to display the empty list message.
Creating a custom directive:
For more complex scenarios, you can create a custom directive that encapsulates the logic for checking the array length and displaying the appropriate content. This allows you to reuse the functionality in multiple places without repeating the code.
Example:
import { Directive, Input } from '@angular/core';
@Directive({
selector: '[appHasItems]',
exportAs: 'appHasItems'
})
export class HasItemsDirective {
@Input() items: any[];
constructor() {}
ngOnInit() {
if (!this.items) {
throw new Error('appHasItems directive requires an "items" input');
}
}
}
<div *appHasItems="items">
</div>
<div *ngIf="!appHasItems">
<p>アイテムがありません。</p>
</div>
This custom directive HasItemsDirective checks the items input property and throws an error if it's not provided. It then evaluates the items array and only allows the content within the directive to be rendered if the array is not null or undefined. The *ngIf directive outside the directive can still check the existence of the items array to display the empty list message.
Choosing the Best Approach:
The most suitable approach depends on the specific requirements and complexity of your application. For simple cases, using the length property directly or the trackBy function in *ngFor might be sufficient. For more complex scenarios or reusable functionality, creating a custom directive can be a better choice.
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