Angular 为什么 ngOnInit 被调用两次

我试图创建新组件ResultComponent，但它的ngOnInit()方法被调用了两次，我不知道为什么会发生这种情况.在代码中，ResultComponent从父组件mcq-component继承@Input.

以下是代码:

Parent Component (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){}
    ....
}

Child Component (result-comp)个

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);
    }
}

运行时，console.log显示两次，第一次显示正确的数组，但第二次显示undefined.我一直搞不明白:为什么ResultComponent人中有ngOnInit人被打了两次电话？