Angular 2 依赖注入 - 独立于构造函数注入对象

Question

假设我有以下课程：

export class Teacher {
    constructor( public name: String, private age: number ){}
    ...
}

Teachers 是这样创建的：

const firstGradeTeacher: Teacher = new Teacher("Hannah", 32);

我现在想给老师添加一个记录器。记录器是Injectable()。我的第一想法是这样做：

export class Teacher {
    constructor( public name: String, private age: number, private logger: Logger ){}
    ...
}

不幸的是，这意味着现在构建教师的每个人都需要：

1. 有一个记录器或

2. 知道如何构建记录器

我真正想做的是：

export class Teacher {
    private logger: Logger = Injector.get(); // <-- Does this exist?
    constructor( public name: String, private age: number){}
    ...
}

有没有办法可以做到这一点？

Answer 1

a hack 可以访问根注入器。

但是，这表示 XY 问题。该类应该是可注入的或不可注入的。混合这些概念并不是一个好主意，因为在 Angular 中没有惯用的方法来做到这一点。

不可注入类的工厂是处理此问题的好方法。

@Injectable()
class TeacherFactory {
  constructor(private logger: Logger) {}

  public createInstance(...args) {
    const teacher = new Teacher(...args);

    // or pass it as extra argument if logger is used in Teacher constructor
    teacher.logger = this.logger;

    return teacher;
  }
}

@Component({
  ...
  providers: [TeacherFactory]
})
class SomeComponent {
  constructor(private teacherFactory: TeacherFactory){
    const firstGradeTeacher: Teacher = teacherFactory.createInstance("Hannah", 32);
  }
}

或者

// non-injectable class
class TeacherFactory {}

@Component({
  ...
  providers: [{ 
    provide: TeacherFactory,
    deps: [Logger],
    useFactory: (logger: Logger) => (...args) => {
      const teacher = new Teacher(...args);
      teacher.logger = logger;

      return teacher;
    }
  }]
})
class SomeComponent {
  constructor(private teacherFactory: TeacherFactory){
    const firstGradeTeacher: Teacher = teacherFactory("Hannah", 32);
  }
}

Answer 2

我想你正在寻找这个：

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

export class Teacher {
   constructor(public name: String, private age: number, @Optional() private   logger: Logger) {
     if (this.logger) {
       this.logger.log(some_message);
    }
    ...
   }
 ...
}