angular--依赖注入

在share目录下创建product服务：ng g service share/product
angular的依赖注入，只能通过构造函数的方式注入。

应用模块中注入服务：

app.module.ts：
import { LoggerService } from './share/logger.service';
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppRoutingModule } from './app-routing.module';

import { AppComponent } from './app.component';
import { ProductComponent } from './product/product.component';
import { HomeComponent } from './home/home.component';
import { Code404Component } from './code404/code404.component';
import { ProductDetailComponent } from './product-detail/product-detail.component';
import { SellInfoComponent } from './sell-info/sell-info.component';
import { ChatComponent } from './chat/chat.component';
import { Product1Component } from './product1/product1.component';
import { ProductService } from './share/product.service';
import { Product2Component } from './product2/product2.component';
import { AnotherProductService } from './share/another-product.service';


@NgModule({
  declarations: [
    AppComponent,
    ProductComponent,
    HomeComponent,
    Code404Component,
    ProductDetailComponent,
    SellInfoComponent,
    ChatComponent,
    Product1Component,
    Product2Component
  ],
  imports: [
    BrowserModule,
    AppRoutingModule
  ],
  providers: [
    LoggerService,
    {
      provide: 'IS_DEV_ENV', useValue: false // 常量
    },
    {
      provide: 'IS_DEV_Obj', useValue: {isDev: true} // 对象
    },
    {provide: ProductService,
      useFactory: (logger: LoggerService, isDev, isObj) => {
        // const logger = new LoggerService();
        // const dev = Math.random() > 0.5;
        if (isObj.isDev) {
          return new ProductService(logger);
        }else {
          return new AnotherProductService(logger);
        }
      },
      deps: [LoggerService, 'IS_DEV_ENV', 'IS_DEV_Obj'] // deps表示ProductService需要依赖的其他服务（可以是变量、或者对象）
    }
  ],
  // providers: [ProductService, LoggerService],
  bootstrap: [AppComponent]
})
export class AppModule { }

如果在组件中创建和应用模块中，相同的服务，会使用组件中的自己创建的服务

import { Component, OnInit } from '@angular/core';
import { Product, ProductService } from '../share/product.service';
import { AnotherProductService } from '../share/another-product.service';

@Component({
  selector: 'app-product2',
  templateUrl: './product2.component.html',
  styleUrls: ['./product2.component.css']
  providers: [{
    provide: ProductService, useClass: AnotherProductService
  }]
})
export class Product2Component implements OnInit {

  private product: Product;
  constructor(private productService: ProductService) { }

  ngOnInit() {
    this.product = this.productService.getProduct();
  }

}