一直以來對 Angular Providers 這部份都還是有存在著半知半解的狀態，就趁著這次機會一次搞懂它吧！

Providers 提供了四種注入方式

• Class Provider : useClass
• Value Provider: useValue
• Factory Provider: useFactory
• Aliased Class Provider: useExisting

useClass

將 service 的服務注入進來

product.ts

export class Product {
  constructor(productID: number, name: string, price: number) {
    this.productID = productID;
    this.name = name;
    this.price = price;
  }

  productID: number;
  name: string;
  price: number;
}

product.service

import { Injectable } from "@angular/core";
import { Product } from "./product";

@Injectable()
export class ProductService {
  constructor() {}

  public getProducts() {
    let products: Product[];
    products = [new Product(1, "Memory Card", 100)];
    return products;
  }
}

component.ts

@Component({
  ...略
  providers: [ProductService],
})

export class AppComponent implements OnInit {
  constructor(
    private productService: ProductService,

  ) {
    cosnole.log(
      this.productService.getProducts()
      // {productID: 1, name: 'Memory Card', price: 100}
    )
  }
}

useValue

直接在 providers 裡的 useValue 設定想提供的值

const APP_CONFIG = Object.freeze({
  serviceURL: 'mtwmt.github.io',
  IsDevleomentMode: true,
});

@Component({
  ...略
  providers: [
    { provide: 'USE_FAKE', useValue: true },
    { provide: 'APP_CONFIG', useValue: APP_CONFIG },
    {
      provide: 'FUNC',
      useValue: () => {
        return 'hello world';
      },
    },
    {
      provide: TestService,
      useClass: TestService,
    },
  ],
})

export class AppComponent implements OnInit {
  constructor(
    @Inject('USE_FAKE') public useFake: string,
    @Inject('APP_CONFIG') public appConfig: any,
    @Inject('FUNC') public func: any
  ) {
    cosnole.log(
      useFake,  // true
      appConfig,
      // {
      //   serviceURL: 'mtwmt.github.io',
      //   IsDevleomentMode: true,
      // }
      func() // hello world
    )

  }
}

useFactory

有時後會需要拿 useValue 的值 注到其他服務裡做判斷，這時後就可以使用 useFactory，來處理一些邏輯上的事情

續上例：

這裡多做了一個 MoreProductService

more-product.service

import { Injectable } from "@angular/core";
import { Product } from "./product";

@Injectable()
export class MoreProductService {
  constructor() {}

  public getProducts() {
    let products: Product[];

    products = [
      new Product(1, "Memory Card", 500),
      new Product(2, "Pen Drive", 750),
      new Product(3, "Power Bank", 100),
      new Product(4, "Laptop", 10000),
      new Product(5, "Desktop", 100),
    ];
    return products;
  }
}

component.ts

import { Component, Inject, VERSION } from "@angular/core";
import { MoreProductService } from "./more-product.service";
import { ProductService } from "./product.service";

@Component({
  selector: "my-app",
  templateUrl: "./app.component.html",
  styleUrls: ["./app.component.css"],
  providers: [
    MoreProductService,
    {
      provide: "GET_MORE",
      useValue: false,
    },
    {
      provide: ProductService,
      // 取得 GET_MORE 的 value，判斷要產生的實體
      useFactory: (isMore) => {
        return isMore ? new MoreProductService() : new ProductService();
      },
      // 注入相依的 service
      deps: ["GET_MORE", MoreProductService],
    },
  ],
})
export class AppComponent {
  products = [];

  constructor(
    @Inject("GET_MORE") public getMore: string,
    private productService: ProductService
  ) {
    this.products = this.productService.getProducts();
  }
}

useExisting

當想有新的 provide 想取代舊的 provide 時，可以使用 useExisting 來做取代，而不用再將源有的重新注入

實作如下：https://stackblitz.com/edit/angular-ivy-zhtp2w

參考文章：
This Won’t Hurt a Bit — Dependency Injection Tokens in Angular
Angular Providers: useClass, useValue, useFactory & useExisting