在本篇文章中，我们将创建：

用于显示代办项目列表的TodoListComponent
用于显示单个代办项目的TodoListItemComponent
用于创建新的代办项目的TodoListHeaderComponent
用于显示共有多少代办项目的TodoListFooterComponent

我们的应用程序架构会像这样：

本文将讨论标有红色边框的项目，以外的部分会在后续文章中说明。

通过这篇文章，我们将学习到：

基本Angular组件体系结构
如何利用属性绑定向组建传递数据
如何利用组件事件监听器监听事件源
为什么分解成可重用的组件是一个好的做法
smart和dumb组件的区别，为什么保持组件dumb(哑)是一个很好的实践。

更新并运行

安装最新版angular命令如下：

npm install -g @angular/cli@latest

如果需要移除过去版本重新安装：

npm uninstall -g @angular/cli angular-cli
npm cache clean --force
npm install -g @angular/cli@latest
＊win系统中需要管理者权限执行

已完成 AppComponent

让我们打开已完成的 src/app/app.component.html

<section class="todoapp">
  <header class="header">
    <h1>Todos</h1>
    <input class="new-todo" placeholder="What needs to be done?" autofocus="" [(ngModel)]="newTodo.title" (keyup.enter)="addTodo()">
  </header>
  <section class="main" *ngIf="todos.length > 0">
    <ul class="todo-list">
      <li *ngFor="let todo of todos" [class.completed]="todo.complete">
        <div class="view">
          <input class="toggle" type="checkbox" (click)="toggleTodoComplete(todo)" [checked]="todo.complete">
          <label>{{todo.title}}</label>
          <button class="destroy" (click)="removeTodo(todo)"></button>
        </div>
      </li>
    </ul>
  </section>
  <footer class="footer" *ngIf="todos.length > 0">
    <span class="todo-count"><strong>{{todos.length}}</strong> {{todos.length == 1 ? 'item' : 'items'}} left</span>
  </footer>
</section>

类 src/app/app.component.ts

import { Component } from '@angular/core';
import {Todo} from './todo';
import {TodoDataService} from './todo-data.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
  providers: [TodoDataService]
})
export class AppComponent {

  newTodo: Todo = new Todo();

  constructor(private todoDataService: TodoDataService) {
  }

  addTodo() {
    this.todoDataService.addTodo(this.newTodo);
    this.newTodo = new Todo();
  }

  toggleTodoComplete(todo) {
    this.todoDataService.toggleTodoComplete(todo);
  }

  removeTodo(todo) {
    this.todoDataService.deleteTodoById(todo.id);
  }

  get todos() {
    return this.todoDataService.getAllTodos();
  }
}

虽然我们的AppComponent能够正常的执行，但是并不推荐所有的代码都写在一个组件里面。

向Todo应用程序添加更多的功能会使得AppComponent更加复杂，庞大，难于理解。

因此我们推荐按照功能分割成较小的组件。理想情况下，较小的组件是可配置的，这个样我们就不必在业务逻辑更改时重写它们的代码。

例如，在以后的文章中我们将会更改TodoDataService为REST API，而当我们重构TodoDataService的时候，不必更改任何小的组件。

我门来看AppComponent的template，其底层结构如下：

<!-- header that lets us create new todo -->
<header></header>

<!-- list that displays todos -->
<ul class="todo-list">

    <!-- list item that displays single todo -->
    <li>Todo 1</li>

    <!-- list item that displays single todo -->
    <li>Todo 2</li>
</ul>

<!-- footer that displays statistics -->
<footer></footer>

我们将其变化为Angular组件结构：

<!-- TodoListHeaderComponent that lets us create new todo -->
<app-todo-list-header></app-todo-list-header>

<!-- TodoListComponent that displays todos -->
<app-todo-list>

    <!-- TodoListItemComponent that displays single todo -->
    <app-todo-list-item></app-todo-list-item>

    <!-- TodoListItemComponent that displays single todo -->
    <app-todo-list-item></app-todo-list-item>
</app-todo-list>

<!-- TodoListFooterComponent that displays statistics -->
<app-todo-list-footer></app-todo-list-footer>

让我们看看如何利用Angular的组件驱动开发来实现这一点。

创建TodoListHeaderComponent

进入工程根目录，执行如下命令

$ ng g component todo-list-header

将会生成如下文件：

CREATE src/app/todo-list-header/todo-list-header.component.html (35 bytes)
CREATE src/app/todo-list-header/todo-list-header.component.spec.ts (686 bytes)
CREATE src/app/todo-list-header/todo-list-header.component.ts (307 bytes)
CREATE src/app/todo-list-header/todo-list-header.component.css (0 bytes)

在AppModoule将会自动加入TodoListHeaderComponent

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';

import { AppRoutingModule } from './app-routing.module';
import { AppComponent } from './app.component';
import { TodoListHeaderComponent } from './todo-list-header/todo-list-header.component';

@NgModule({
  declarations: [
    AppComponent,
    TodoListHeaderComponent
  ],
  imports: [
    BrowserModule,
    FormsModule,
    AppRoutingModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

在AppModoule将会自动加入TodoListHeaderComponent组件声明是为了能够在所有view模板里面使用它。Angular CLI 方便的为我们添加了该组件所以我们无需手动添加它。

如果组件没有被声明就在view模板中使用，Angular将会抛出如下错误：

Error: Uncaught (in promise): Error: Template parse errors:
'app-todo-list-header' is not a known element:
1. If 'app-todo-list-header' is an Angular component, then verify that it is part of this module.
2. If 'app-todo-list-header' is a Web Component then add "CUSTOM_ELEMENTS_SCHEMA" to the '@NgModule.schemas' of this component to suppress this message.

现在我们生成了所有TodoListHeaderComponent所需文件，我们接下来移动src/app/app.component.html 的

到 src/app/todo-list-header/todo-list-header.component.html

<header class="header">
  <h1>Todos</h1>
  <input class="new-todo" placeholder="What needs to be done?" autofocus="" [(ngModel)]="newTodo.title" (keyup.enter)="addTodo()">
</header>

添加逻辑src/app/todo-list-header/todo-list-header.component.ts:

import { Component, OnInit, Output, EventEmitter } from '@angular/core';
import { Todo } from '../todo';

@Component({
  selector: 'app-todo-list-header',
  templateUrl: './todo-list-header.component.html',
  styleUrls: ['./todo-list-header.component.css']
})
export class TodoListHeaderComponent implements OnInit {
  newTodo: Todo = new Todo();

  @Output()
  add: EventEmitter<Todo> = new EventEmitter();

  constructor() { }

  addTodo() {
    this.add.emit(this.newTodo);
    this.newTodo = new Todo();
  }

  ngOnInit() {
  }
}

替换注入TodoDataService保存新的todo, 我们出发一个add事件，并且传递一个新的todo参数。

在Angular 模板中以如下方式捕捉事件：

这将在enter键按下的时候触发addTodo()方法。之所以能正常执行，是因为在angular framework 中 keyup.enter 被定义为一个有效的事件。

尽管如此，我们也经常通过 EventEmitter 和 @Output() 为一个组件定义自己的事件。

import { Component, Output, EventEmitter } from '@angular/core';
import { Todo } from '../todo';

@Component({
  // ...
})
export class TodoListHeaderComponent {

  // ...

  @Output()
  add: EventEmitter<Todo> = new EventEmitter();

  addTodo() {
    this.add.emit(this.newTodo);
    this.newTodo = new Todo();
  }

}

然后我们能够在模板中按照如下方式捕捉事件：

每当在 TodoListHeaderComponent 中调用 add.emit(value) 的时候，onAddTodo($event)事件将会被执行，参数$event将会被赋值为value。

这将解耦TodoListHeaderComponent 和 TodoDataService ，当创建一个新的to的时候，允许父组件处理想要做的事情。

修改 AppComponent 模板

<section class="todoapp">

  <!-- header is now replaced with app-todo-list-header -->
  <app-todo-list-header (add)="onAddTodo($event)"></app-todo-list-header>

  <section class="main" *ngIf="todos.length > 0">
    <ul class="todo-list">
      <li *ngFor="let todo of todos" [class.completed]="todo.complete">
        <div class="view">
          <input class="toggle" type="checkbox" (click)="toggleTodoComplete(todo)" [checked]="todo.complete">
          <label>{{todo.title}}</label>
          <button class="destroy" (click)="removeTodo(todo)"></button>
        </div>
      </li>
    </ul>
  </section>

  <footer class="footer" *ngIf="todos.length > 0">
    <span class="todo-count"><strong>{{todos.length}}</strong> {{todos.length == 1 ? 'item' : 'items'}} left</span>
  </footer>

</section>

在 AppComponent 中添加 onAddTodo()

import {Component} from '@angular/core';
import {Todo} from './todo';
import {TodoDataService} from './todo-data.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
  providers: [TodoDataService]
})
export class AppComponent {

  // No longer needed, now handled by TodoListHeaderComponent
  // newTodo: Todo = new Todo();

  constructor(private todoDataService: TodoDataService) {
  }

  // No longer needed, now handled by TodoListHeaderComponent
  // addTodo() {
  //   this.todoDataService.addTodo(this.newTodo);
  //   this.newTodo = new Todo();
  // }

  // Add new method to handle event emitted by TodoListHeaderComponent
  onAddTodo(todo: Todo) {
    this.todoDataService.addTodo(todo);
  }

  toggleTodoComplete(todo: Todo) {
    this.todoDataService.toggleTodoComplete(todo);
  }

  removeTodo(todo: Todo) {
    this.todoDataService.deleteTodoById(todo.id);
  }

  get todos() {
    return this.todoDataService.getAllTodos();
  }

}

创建TodoListComponent

首先生成 TodoListComponent:

ng g component todo-list

将会产生如下文件:

create src/app/todo-list/todo-list.component.css
create src/app/todo-list/todo-list.component.html
create src/app/todo-list/todo-list.component.spec.ts
create src/app/todo-list/todo-list.component.ts

并且在AppModule里面自动添加了如下声明

// ...
import { TodoListComponent } from './todo-list/todo-list.component';

@NgModule({
  declarations: [
    // ...
    TodoListComponent
  ],
  // ...
})
export class AppModule { }

打开 src/app/app.component.html:

<section class="main" *ngIf="todos.length > 0">
  <ul class="todo-list">
    <li *ngFor="let todo of todos" [class.completed]="todo.complete">
      <div class="view">
        <input class="toggle" type="checkbox" (click)="toggleTodoComplete(todo)" [checked]="todo.complete">
        <label>{{todo.title}}</label>
        <button class="destroy" (click)="removeTodo(todo)"></button>
      </div>
    </li>
  </ul>
</section>

将内容移动到 src/app/todo-list/todo-list.component.html:

<section class="main" *ngIf="todos.length > 0">
  <ul class="todo-list">
    <li *ngFor="let todo of todos" [class.completed]="todo.complete">
      <app-todo-list-item
        [todo]="todo"
        (toggleComplete)="onToggleTodoComplete($event)"
        (remove)="onRemoveTodo($event)"></app-todo-list-item>
    </li>
  </ul>
</section>

*TodoListItemComponent 将在后续做成，通过 todo 属性传递 todo 项目，并且处理在TodoListItemComponent中触发的事件。

打开 src/app/todo-list/todo-list.component.ts 添加如下处理：

import { Component, EventEmitter, Input, Output } from '@angular/core';
import { Todo } from '../todo';

@Component({
  selector: 'app-todo-list',
  templateUrl: './todo-list.component.html',
  styleUrls: ['./todo-list.component.css']
})
export class TodoListComponent {

  @Input()
  todos: Todo[];

  @Output()
  remove: EventEmitter<Todo> = new EventEmitter();

  @Output()
  toggleComplete: EventEmitter<Todo> = new EventEmitter();

  constructor() {
  }

  onToggleTodoComplete(todo: Todo) {
    this.toggleComplete.emit(todo);
  }

  onRemoveTodo(todo: Todo) {
    this.remove.emit(todo);
  }

}

我们定义了 onToggleTodoComplete(todo) 和 onRemoveTodo(todo) 来处理view的事件
(toggleComplete)=”onToggleTodoComplete($event)” and (remove)=”onRemoveTodo($event)”

注意我们在view中使用 $event 作为参数名字，在方法定义中使用todo作为参数名字。
为了能够在Angular模板中访问到事件的负载(emitted value)，我们必须使用 $event作为参数名。

事件在 TodoListItemComponent 产生，经由 TodoListComponent 最终在 AppComponent 中处理
修改 AppComponent

export class AppComponent {

  // rename from toggleTodoComplete
  onToggleTodoComplete(todo: Todo) {
    this.todoDataService.toggleTodoComplete(todo);
  }

  // rename from removeTodo
  onRemoveTodo(todo: Todo) {
    this.todoDataService.deleteTodoById(todo.id);
  }

}

创建TodoListItemComponent

利用Angular CLI 生成 TodoListItemComponent

ng g component todo-list-item

将生成如下文件：

create src/app/todo-list-item/todo-list-item.component.css
create src/app/todo-list-item/todo-list-item.component.html
create src/app/todo-list-item/todo-list-item.component.spec.ts
create src/app/todo-list-item/todo-list-item.component.ts

并且在AppModule里面自动添加了如下声明

// ...
import { TodoListItemComponent } from './todo-list-item/todo-list-item.component';

@NgModule({
  declarations: [
    // ...
    TodoListItemComponent
  ],
  // ...
})
export class AppModule {

移动原view中的\<li> 到 src/app/todo-list-item.component.html:

<div class="view">
  <input class="toggle" type="checkbox" (click)="toggleTodoComplete(todo)" [checked]="todo.complete">
  <label>{{todo.title}}</label>
  <button class="destroy" (click)="removeTodo(todo)"></button>
</div>

src/app/todo-list-item/todo-list-item.component.ts:

import { Component, Input, Output, EventEmitter } from '@angular/core';
import { Todo } from '../todo';

@Component({
  selector: 'app-todo-list-item',
  templateUrl: './todo-list-item.component.html',
  styleUrls: ['./todo-list-item.component.css']
})
export class TodoListItemComponent {

  @Input() todo: Todo;

  @Output()
  remove: EventEmitter<Todo> = new EventEmitter();

  @Output()
  toggleComplete: EventEmitter<Todo> = new EventEmitter();

  constructor() {
  }

  toggleTodoComplete(todo: Todo) {
    this.toggleComplete.emit(todo);
  }

  removeTodo(todo: Todo) {
    this.remove.emit(todo);
  }

}

更新AppComponent模板：

<section class="todoapp">

  <app-todo-list-header (add)="onAddTodo($event)"></app-todo-list-header>

  <!-- section is now replaced with app-todo-list -->
  <app-todo-list [todos]="todos" (toggleComplete)="onToggleTodoComplete($event)"
                 (remove)="onRemoveTodo($event)"></app-todo-list>

  <footer class="footer" *ngIf="todos.length > 0">
    <span class="todo-count"><strong>{{todos.length}}</strong> {{todos.length == 1 ? 'item' : 'items'}} left</span>
  </footer>

</section>

创建TodoListFooterComponent

生成TodoListFooterComponent

ng g component todo-list-footer

移动 \<footer> 元素 src/app/app.component.html 到 src/app/todo-list-footer/todo-list-footer.component.html:

1
2
3

<footer class="footer" *ngIf="todos.length > 0">
  <span class="todo-count"><strong>{{todos.length}}</strong> {{todos.length == 1 ? 'item' : 'items'}} left</span>
</footer>

修改 src/app/todo-list-footer/todo-list-footer.component.ts:

import { Component, Input } from '@angular/core';
import { Todo } from '../todo';

@Component({
  selector: 'app-todo-list-footer',
  templateUrl: './todo-list-footer.component.html',
  styleUrls: ['./todo-list-footer.component.css']
})
export class TodoListFooterComponent {

  @Input()
  todos: Todo[];

  constructor() {
  }

}

更新 AppComponent 模板：

<section class="todoapp">
  <app-todo-list-header (add)="onAddTodo($event)"></app-todo-list-header>
  <app-todo-list [todos]="todos" (toggleComplete)="onToggleTodoComplete($event)"
                 (remove)="onRemoveTodo($event)"></app-todo-list>
  <app-todo-list-footer [todos]="todos"></app-todo-list-footer>
</section>

这样我们就成功的重构了AppComponent，由下列组件构成，
TodoListHeaderComponent
TodoListComponent -> TodoListItemComponent
TodoListFooterComponent

以后的章节，我们会继续重构 TodoService 为 REST API。