在Typescript中作为函数参数类

Question

In Angular 2 testing utility I do something like this:

fixture = TestBed.createComponent(EditableValueComponent);

where EditableValueComponent is a normal component class.

I wonder how it works:

static createComponent<T>(component: Type<T>): ComponentFixture<T>;

Beceause I wanna do something similar (I want to simplify some testing stuff):

export class SuperFixture<T>
{  
    fixture: ComponentFixture<T>;
    component: T;

    constructor()
    {         
        this.fixture = TestBed.createComponent(T); // <--- problem here!
        this.component = this.fixture.componentInstance;   
    }
}

The problem is:

'T' only refers to a type, but is being used as a value here.'

EDIT #1

I solved the problem this way:

constructor(component)
{
    this.fixture = TestBed.createComponent<T>(component);

But I still don't know how it works..

Answer 1

You still need to pass actual class (constructor function that creates instances of a class) into constructor function of SuperFixture . Under the hood TestBed.createComponent calls provided constructor function with new to create instance of provided class. So SuperClass signature might look like this:

class SuperFixture<T>
{
  fixture: ComponentFixture<T>;
  component: T;

  // passing in the constructor for instances of T
  constructor(componentConstructor: new () => T)
  {
    this.fixture = TestBed.createComponent<T>(componentConstructor);
    this.component = this.fixture.componentInstance;
  }
}

Answer 2

Was working on that answer, but had to step out for coffee. ¯_(ツ)_/¯

The language feature you are using is called a Generic in TypeScript. It allows defining types at runtime with " type variables " (like <T> ), which are separate from function arguments.

Before, the type variable was being passed as a function argument , when the function expected an instance of type T. That's what the error means.

The change you made works because you are passing the type variable and the instance in their correct positions in the call.

The SuperFixture object gets the value of T when you create it, and then it will pass that type variable to createComponent in the constructor, along with the value of component .