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Version: v7 (beta)

Angularのパフォーマンス

*ngFor with Ionic Components

When using *ngFor with Ionic components, we recommend using Angular's trackBy option. This allows Angular to manage change propagation in a much more efficient way and only update the content inside of the component rather than re-create the component altogether.

By using trackBy you can provide a stable identity for each loop element so Angular can track insertions and deletions within the iterator. Below is an example of how to use trackBy:

home.page.html

<ion-item *ngFor="let item of items; trackBy:trackItems">
  <ion-label>{{ item.value }}</ion-label>
</ion-item>

home.component.ts


items = [
  { id: 0, value: 'Item 0' },
  { id: 1, value: 'Item 1' },
  ...
]

trackItems(index: number, itemObject: any) {
  return itemObject.id;
}

In this example, we have an array of objects called items. Each object contains a value and an id. Using trackBy, we pass a trackItems function which returns the id of each object. This id is used to provide a stable identity for each loop element.

For more information on how Angular manages change propagation with ngFor see https://angular.io/api/common/NgForOf#change-propagation.

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