有角度 – 为什么ngOnInit叫两次？

父组件(MCQComponent)

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

@Component({
	selector: 'mcq-component',template: `
		<div *ngIf = 'isQuestionView'>
			.....
		</div>
		<result-comp *ngIf = '!isQuestionView' [answers] = 'ansArray'><result-comp>
	`,styles: [
		`
			....
		`
	],providers: [AppService],directives: [SelectableDirective,ResultComponent]
})
export class MCQComponent implements OnInit{
      private ansArray:Array<any> = [];
	....
	constructor(private appService: AppService){}
	....
}

子组件(result-comp)

import { Component,OnInit,Input } from '@angular/core';

@Component({
	selector:'result-comp',template: `
		<h2>Result page:</h2>

	`
})
export class ResultComponent implements OnInit{
	@Input('answers') ans:Array<any>;

	ngOnInit(){
		console.log('Ans array: '+this.ans);
	}
}

这里的控制台日志显示2次：第一次它显示正确的数组，但第二次它显示未定义，但我无法弄清楚，为什么ngOnInit从ResultComponent得到调用两次？