为什么Angular中HttpClient的构造函数不需要参数，当我通过其他类的构造函数实例化它时，但需要通过new？

Question

I need to make a static method for instanstiating an object of class, but faced an issue.

import { HttpClient } from '@angular/common/http';

export MyClass {
  // Case 1
  public static init(): MyClass {
    return this(new HttpClient(***), ....); // Need to pass new HttpClient() instead ***
  }

  // Case 2
  public constructor(private http: HttpClient, ....) {} // Need to pass nothing
}

Why I have to pass argument in case 1, but in case 2 not? And how to solve my problem?

Answer 1

Answer

This happens thanks to dependency injection . Angular says:

Dependency injection, or DI, is a design pattern in which a class requests dependencies from external sources rather than creating them

In your case you don't need to "create" that resource but simply use it, and for that dependency injection comes to your aid

I recommend you take a look at the official documentation of Angular, always very clear and quite useful!

And how to solve my problem?

If you just need to use HttpClient, do as in // Case 2 and without the need to initialize anything ; otherwise, if you can please explain better what your goal is and how you can't achieve it so we can help you, thanks!

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Let me know if you have any doubts ;)