详解Angular 中 ngOnInit 和 constructor 使用场景

class AppComponent {
  public name: string;
  constructor(name) {
    console.log('Constructor initialization');
    this.name = name;
  }
}

let appCmp = new AppComponent('AppCmp');  // 这时候构造函数将被调用。
console.log(appCmp.name);

var AppComponent = (function () {
  function AppComponent(name) {
    console.log('Constructor initialization');
    this.name = name;
  }
  return AppComponent;  // 这里直接返回一个实例
}());
var appCmp = new AppComponent('AppCmp');
console.log(appCmp.name);

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

@Component({
 selector: 'my-app',
 template: `
  <h1>Welcome to Angular World</h1>
 `,
})
export class AppComponent implements OnInit {

 constructor() {
  console.log('Constructor initialization');
 }

 ngOnInit() {
  console.log('ngOnInit hook has been called');
 }
}

// parent.component.ts

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

@Component({
 selector: 'exe-parent',
 template: `
  <h1>Welcome to Angular World</h1>
  <p>Hello {{name}}</p>
  <exe-child [pname]="name"></exe-child>  <!-- 绑定到子组件的属性 -->
 `,
})
export class ParentComponent {
 name: string;

 constructor() {
  this.name = 'God eyes';
 }
}

// child.component.ts

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

@Component({
  selector: 'exe-child',
  template: `
   <p>父组件的名称：{{pname}} </p>
  `
})
export class ChildComponent implements OnInit {
  @Input()
  pname: string; // 父组件的输入属性

  constructor() {
    console.log('ChildComponent constructor', this.pname); // this.name=undefined
  }

  ngOnInit() {
    console.log('ChildComponent ngOnInit', this.pname); // this.name=God eyes
  }
}

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

@Component({
 selector: 'my-app',
 template: `
  <h1>Welcome to Angular World</h1>
  <p>Hello {{name}}</p>
 `,
})
export class AppComponent implements OnInt {
 name: string = '';

 constructor(public elementRef: ElementRef) { // 使用构造注入的方式注入依赖对象
  this.name = 'WXY';          // 执行初始化操作
 }

 ngOnInit() {
  this.gotId = this.activatedRoute.params.subscribe(params => this.articleId = params['id']);
 }
}