feat(forms): add generic typings to AbstractControl - #20040

Stop me if i'm wrong but,
if i have an object with this interface :

interface MyList {
    name: string[],
    other: number,
}

I need to construct a form like this (without types) :

myform = new FormGroup({
    name: new FormArray([]),
    other: new FormControl(0)
)

If i use typed forms, the property 'name' need to be FormArray
And myform need to be FormGroup
Like,

myform = new FormGroup<MyList>({
    name: new FormArray<string>([]),
    other: new FormControl<number>(0)
)

But, if FormGroup contructor take
controls: {[key in keyof T]: AbstractControl<T[key]>}

Property 'name' will be of type AbstractControl<string[]>. And this is not compatible with FormArray<string>

@Trophalaxeur AbstractControl is the base class of all of the other 3 classes.
With FormArray you specify the type of a single item, but the type passed to AbstractControl is an array of that single type as you can see on the signature
FormArray<T = any> extends AbstractControl<T[]>

Therefore AbstractControl<string[]> and FormArray<string> are the same
here's a demo showing this to be true
https://stackblitz.com/edit/angular-gitter-zofzrc?file=app%2Ftyped-controls.ts

@Toxicable great job on implementing this.
But i see the PR is stuck at (final) documentation phase - are you accepting extra PRs, and if so maybe i can help ?

@c69 untill there is a final API on this there is no need to write docs since the API could change before it lands.

Nice work 👏

Nice one @Toxicable!

Nice work, any hope on merging this soon?

Hi @Toxicable! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

I really want this!!!! Can't wait to see this in action!!!!

+1

I’m actually surprised that this hasn’t made its way into the framework yet :) Seems like a no-brainer that people want strongly typed functionality these days.

Anyhow - me and my team is waiting eagerly on this one

Yes Angular team is aware of that but mentioned during angular Connect 2018 that they are waiting for some TS bug to be fixed 🤷‍♂️

In case you wonder, it's blocked on microsoft/TypeScript#16229.

Hello!
It is possible to resolve this with current TS version. Rewritten example from microsoft/TypeScript#16229:

class O<T=any> {
  constructor(public array: T[]) { }
}

type Unwrap<T> = T extends O<infer U> ? U : T

declare let val: O<number> | O<string>;
declare function f<V extends O<any>, T=Unwrap<V> >(x: V): T;

const res = f(val); // works

type returnType = typeof res // string | number

f(1); // Argument of type '1' is not assignable to parameter of type 'O<any>'.

playground link

This one also works.

type Unwrap<T> = T extends AbstractControl<infer U> ? U : never

class AbstractControl<T extends any = any> {
  value: T;
}
class FormGroup<T extends any> extends AbstractControl<T> {
  constructor(
    public controls: {[key in keyof T]: AbstractControl<T[key]>}
  ) {
    super()
  }
}
class FormArray<C extends AbstractControl<any> , T = Unwrap<C>> extends AbstractControl<T>{
  constructor(
    public controls: C[]
  ) {
    super()
  }
}
class FormControl<T extends any> extends AbstractControl<T>{
  constructor(value: T) {
    super()
    this.value = value
  }
}


let a = new FormArray([
  new FormGroup({'c2': new FormControl('v2'), 'c3': new FormControl('v3')}),
  new FormArray([new FormControl('v4'), new FormControl(5)])
]);

a.controls[0].value // string | number | { 'c2': string; 'c3': string; }
a.controls[1].value // string | number | { 'c2': string; 'c3': string; }

playground link

I can experiment a bit with typescript tuples to get
a.controls[0].value -> { 'c2': string; 'c3': string; }
and a.controls[1].value -> string | number if needed.

@IgorMinar
what are the priorities in Angular team? More powerful Typescript allows to create more type-safe scenarios, while Angular still focus in Ivy which will answer answer Life, the Universe and Everything question?

Can't we just ask the guys who created https://github.com/no0x9d/ngx-strongly-typed-forms to allow to implement their way of strongly typing into angular? Of course there can be some improvements made thanks to new features of current TS but it is already great implementation with over 1000+ daily downloads on npm.

In case you wonder, it's blocked on microsoft/TypeScript#16229.

Why is that a blocker?

@kara are there any news about this?

Possibly good news here...

#13721 (comment)

I cannot wait for strongly typed forms to be part of Angular.

Closing this PR in favor of more recent #38406 and #38906. Thank you.

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