Angular利用trackBy提升性能的方法

<ul>
 <li ngFor="let item of collection">{{item.id}}</li>
</ul>

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

@Component({
 selector: 'trackBy-test',
 template: `
 <ul><li ngFor="let item of items; trackBy: trackByIndex">{{item.name}}</li></ul>
 <button (click)="getItems()">Get Items</button>
 `
})
export class TrackByCmp{
 items: any[]=[];
 constructor(){
  this.items = [{name:'Tom'},{name:'Jerry'},{name:'Kitty'}];
 }
 getItems(){
  this.items = [{name:'Tom'},{name:'Jerry'},{name:'Mac'},{name:'John'}];
 }
 trackByIndex(index, item){
  return index;
 }
}