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Parametrized Validators in Dynamic Forms

Mateo Tibaquirá
Senior Angular Developer, Library Author, Open Source Advocate, Dependency Maintainer by hobby.
Updated on ・3 min read

TL;DR
We are building the documentation of @myndpm/dyn-forms at mynd.dev and we've added support for a variety of custom functions like Validators, AsyncValidators, Matchers, Conditions and more.

The next crucial part of any form is validation, aka Validators and AsyncValidators, and we took some time to study a nice way to implement them and we picked the most declarative one:

createMatConfig('INPUT', {
  name: 'quantity',
  validators: ['required', ['min', 1] ],
  asyncValidators: ['myAsyncValidator'],
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Angular Validators

Angular provides default Validators that we're used to consume programatically in our Reactive Forms, some of them are Validator Functions (ValidatorFn) like Validators.required, and some others are Validator Factories ((args) => ValidatorFn) which builds a Validator based on a required parameter like Validators.minLength(4).

The definition of a Validator Function is:

(control: AbstractControl) => ValidationErrors | null
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it receives the control to be validated, and returns null if its value is valid, or an error object of the form { [error: string]: any }

Validator Factories are high-order functions that builds a Validator Function according some input parameters:

function minLength(minLength: number): ValidatorFn {
  return (control: AbstractControl) => {
    return (control.value && control.value.length < minLength)
      ? { minLength: true } // invalid
      : null; // valid
  }
}
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as you can see, this is a very nice way to parametrize our Functions, so we defined the provisioning of Validators (and all the other handlers) with an id and a factory fn:

export interface DynControlValidator {
  id: string;
  fn: (...args: any[]) => ValidatorFn;
}
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The id will be the string that we will use in our Configuration Object. By default, @myndpm/dyn-forms provide the default Angular Validators with the same name as we know them: required, requiredTrue, email, pattern, minLength, maxLength, min and max.

The notation to use them in the Config Object is as follows:

// without parameters
validators: ['required'],

// with parameters as array
validators: ['required', ['min', 1] ],

// with parameters as object
validators: { required: null, minLength: 4 },
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supporting these different notations is unexpensive and can be useful for different kind of systems or developer tastes.

Custom Validators

As mentioned, all we need is to provide our ValidatorFn Factory with an id and a fn. So we can easily provide them in our module with a code like this:

import { AbstractControl, ValidatorFn } from '@angular/forms';
import { DynFormsModule } from '@myndpm/dyn-forms';
import { DynControlValidator } from '@myndpm/dyn-forms/core';

const validators: DynControlValidator[] = [
  {
    id: 'email',
    fn: (): ValidatorFn => {
      return (control: AbstractControl) => {
        // implement my validator
        // to return { email: true } | null;
      }
    }
  }
];

@NgModule({
  imports: [
    DynFormsModule.forFeature({ validators, priority: 100 });
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note the priority parameter to override the default validators (which weight is 0); we will play with priorities in a further article.

AsyncValidators

Providing async validators works in the same way. You provide your fn with an id and use them in the Config Object:

createMatConfig('INPUT', {
  name: 'quantity',
  validators: ['required'],
  asyncValidators: ['myAsyncValidatorId'],
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and if you need to provide arguments to your AsyncValidator factory, you can use:

// single argument which can be an object
asyncValidators: [['myAsyncValidatorId', args]],
// your factory will receive fn(args)

// multiple arguments in array to be destructured
asyncValidators: [['myAsyncValidatorId', [arg1, arg2]]],
// your factory will receive fn(arg1, arg2)
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Custom Handlers

With this notation we added support for multiple kinds of functions that we require in the Dynamic Forms: Validators and AsyncValidators as we just saw, Matchers and Conditions to manipulate the controls under some special requirements, and also ParamFns to inject functions to the parameters of the DynControls too.

We will be digging into the conditional executions in the next chapter.
In the meantime, what do you think of this notation?

// PS. We are hiring!

Discussion (6)

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coffeetea profile image
Vitalii Baziuk • Edited

Liked it! Thanks for this!
Have a few questions here:

I. What if I wanna to provide a few params for one validator in array notation?
Will this be correct (range ex.):

options: { validators: ['required', ['range', [1, 5] ] ] },

II. For custom validator, if we have defined it in separate const, do we have some way to read value from other form controls somehow? Let's say I wanna validate current control value against another control value from this form. Or in that case we probably should declare custom validator in dyn form host component?


Important point: before reading this article it's nice to read this article first dev.to/myndpm/a-new-approach-to-ha...

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matheo profile image
Mateo Tibaquirá Author

I. Yes, ['range', [1, 5]] will be translated to its corresponding Factory call like fn(1, 5) and for more complex cases an object could be used like ['asyncUniqueEmail', { endpoint: '...', minLength: 4, ... }] and you will receive it in your Factory fn({ endpoint, minLength, ... }) :)

II. Let's try to implement that Validator with Matchers for the next chapter of this series: github.com/myndpm/open-source/issu...

Thank you for your feedback! :D

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dnmakarov profile image
dnmakarov • Edited

Hey, thanks for the great article.

My question is can we apply validators to the FormGroup e.g.

formGroup = new FormGroup(
  { ...fields }, 
  { validators: [ ...someCustomValidators ] }
);
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matheo profile image
Mateo Tibaquirá Author

Yes, every DynControl is able to have its Validators but in the Config type structure:

{
  options: { validators: [...someCustomValidators] },
  controls: [ ..fieldsConfigs ]
}
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I'm improving the typing of the Config right now to detect inline ValidatorFn in case we don't want to provide them via id and just want to build a fast prototype :)

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artem_shapilov profile image
Artem Shapilov

Is it required to register validators on the module level? Does this approach has advantages over providing validators inline when creating config? e.g. when doing createMatConfig()

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matheo profile image
Mateo Tibaquirá Author

while implementing matchers it was convenient to have inline functions, and in general they are nicer to implement a fast prototype without having to provide anything at module level.
I might update this article to include the inline functions in the notation. Thanks!

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