NgxSubForm

Utility library to improve the robustness of your Angular forms.

Whether you have simple and tiny forms or huge and complex ones, ngx-sub-form will help you build a solid base for them.

• ✅ Simple API: No angular module to setup, no ControlValueAccessor by hand, no inheritance, no boilerplate. Only one function to create all your forms!
• 🤖 Adds type safety to your forms
• ✂️ Lets you break down huge forms into smaller ones for simplicity and reusability

Please note one thing: If your goal is to generate forms dynamically (based on some JSON configuration for example) ngx-sub-form is not here for that!

Table of contents

• Basic API usage
• Setup
• Migration guide to the new API
• Principles
  • Root forms
  • Sub forms
  • Remap
  • Dealing with arrays
• Contribution
• Tell us about your experience with ngx-sub-form

Basic API usage

As a picture is often worth a 1000 words, let's take a quick overlook at the API before explaining in details further on:

Setup

ngx-sub-form is available on NPM:

npm i ngx-sub-form

Note about the versions:

@angular version	ngx-sub-form version
v <= 7	v <= 2.7.1
8.x	4.x
9.x <= v <= 12.x	5.1.2
13.x	5.2.0 (non breaking but new API available as well)
14.x	6.0.0 (Angular 14 upgrade only)
14.x	7.0.0 (deprecated API is now removed)
15.x	8.0.0
16.x	9.0.0
17.x	10.0.0

API

There's one function available to create all your forms: createForm.

This function takes as parameter a configuration object and returns an object ready to be used to use your form and all its new utilities. In this section we'll discover what configuration we can pass to createForm and what exactly we'll be getting back.

createForm configuration object:

Key	Type	Optional or required	Root form	Sub form	What is it for?
formType	FormType	Required	✅	✅	Defines the type of the form. Can either be FormType.ROOT or FormType.SUB
disabled$	Observable<boolean>	Required	✅	❌	When this observable emits true, the whole form (including the root form and all the sub forms) will be disabled
input$	Observable<ControlInterface | undefined>	Required	✅	❌	A root form is a component in between the parent passing raw data and the form itself. This property is an observable that you must provide which will be used behind the scenes to update for you the form values
output$	Subject<ControlInterface>	Required	✅	❌	A root form is a component in between the parent passing raw data and the form itself. This property is an observable that you must provide which will be used behind the scenes to broadcast the form value to the parent when it changes
manualSave$	Observable<void>	Optional	✅	❌	By default a root form will automatically broadcast all the form updates (through the output$) as soon as there's a change. If you wish to "save" the form only when you click on a save button for example, you can create a subject on your side and pass it here. Whenever you call next on your subject, assuming the form is valid, it'll broadcast te form value to the parent (through the output$)
outputFilterPredicate	(currentInputValue: FormInterface, outputValue: FormInterface) => boolean	Optional	✅	❌	The default behaviour is to compare the current transformed value of input$ with the current value of the form (deep check), and if these are equal, the value won't be passed to output$ in order to prevent the broadcast
handleEmissionRate	(obs$: Observable<FormInterface>) => Observable<FormInterface>	Optional	✅	❌	If you want to control how frequently the form emits on the output$, you can customise the emission rate with this. Example: handleEmissionRate: formValue$ => formValue$.pipe(debounceTime(300))

Principles

As simple as forms can look when they only have a few fields, their complexity can increase quite quickly. In order to keep your code as simple as possible and isolate the different concepts, we do recommend to write forms in complete isolation from the rest of your app.

In order to do so, you can create some top level forms that we call "root forms". As one form can become bigger and bigger over time, we also help by letting you create "sub forms" (without the pain of dealing manually with a ControlValueAccessor!). Let's dig into their specifics, how they differ and how to use them.

Root forms

Root forms let you isolate a form from the rest of your app.
You can encapsulate them and (pretty much) never have to deal with patchValue or setValue to update the form nor subscribe to valueChanges to listen to the updates.

Instead, you'll be able to create a dedicated form component and pass data using an input, receive updates using an output. Just like you would with a dumb component.

Let's have a look with a very simple workflow:

• Imagine an application with a list of people and when you click on one of them you can edit the person details
• A smart component is aware of the currently selected person (our "container component")
• A root form component lets us display the data we retrieved in a form and also edit them

In this scenario, the smart component could look like the following:

@Component({
  selector: 'person-container',
  template: `
    <person-form [person]="person$ | async" (personUpdate)="personUpdate($event)"></person-form>
  `,
})
export class PersonContainer {
  public person$: Observable<Person> = this.personService.person$;

  constructor(private personService: PersonService) {}

  public personUpdate(person: Person): void {
    this.personService.update(person);
  }
}

This component is only responsible to get the correct data and manage updates (if any). It completely delegates to the root form:

• How the data will be displayed to the user as a form
• How the user will interact with them

Now let's talk about the actual root form:

@Component({
  selector: 'person-form',
  template: `
    <form [formGroup]="form.formGroup">
      <input type="text" [formControlName]="form.formControlNames.firstName" />
      <input type="text" [formControlName]="form.formControlNames.lastName" />
      <address-control [formControlName]="form.formControlNames.address"></address-control>
    </form>
  `,
})
export class PersonForm {
  private input$: Subject<Person | undefined> = new Subject();
  @Input() set person(person: Person | undefined) {
    this.input$.next(person);
  }

  private disabled$: Subject<boolean> = new Subject();
  @Input() set disabled(value: boolean | undefined) {
    this.disabled$.next(!!value);
  }

  @Output() personUpdate: Subject<Person> = new Subject();

  public form = createForm<Person>(this, {
    formType: FormType.ROOT,
    disabled$: this.disabled$,
    input$: this.input$,
    output$: this.personUpdate,
    formControls: {
      id: new FormControl(null, Validators.required),
      firstName: new FormControl(null, Validators.required),
      lastName: new FormControl(null, Validators.required),
      address: new FormControl(null, Validators.required),
    },
  });
}

We'll go through the example above bit by bit.

public form = createForm<Person>(this, {
  formType: FormType.ROOT,
  disabled$: this.disabled$,
  input$: this.input$,
  output$: this.personUpdate,
  formControls: {
    id: new FormControl(null, Validators.required),
    firstName: new FormControl(null, Validators.required),
    lastName: new FormControl(null, Validators.required),
    address: new FormControl(null, Validators.required),
  },
});

This is what we provide to create a form with ngx-sub-form:

• A type (either FormType.ROOT or FormType.SUB)
• A disabled$ stream to know whether we should disable the whole form or not (including all the sub forms as well)
• An input$ stream which is the data we'll use to update the form
• An output$ stream, which would usually be our EventEmitter so that a parent component can listen to the form update through an output
• The formControls, which is exactly what you'd pass when creating a FormGroup

One thing to note: The createForm function takes a generic which will let you type our form. In this case, if you forgot to pass a property of the form in the formControls it'd be caught at build time by Typescript.

private input$: Subject<Person | undefined> = new Subject();
@Input() set person(person: Person | undefined) {
  this.input$.next(person);
}

private disabled$: Subject<boolean> = new Subject();
@Input() set disabled(value: boolean | undefined) {
  this.disabled$.next(!!value);
}

This is simply a way of binding an input to an observable. We do this because the createForm function requires us to pass an input$ stream and a disabled$ one. Hopefully Angular lets us one day access inputs as observables natively. In the meantime if you want to reduce this boilerplate even further, you can search on NPM for libraries which are doing this already. It's not as good as what Angular could do if it was built in, but it's still useful.

@Output() personUpdate: Subject<Person> = new Subject();

This is an Output. It could be an EventEmitter if you prefer a "classic" way of creating an output but really all we need is a Subject so that internally, the createForm function is able to push the form value whenever it's been updated.

Finally, our template:

<form [formGroup]="form.formGroup">
  <input type="text" [formControlName]="form.formControlNames.firstName" />
  <input type="text" [formControlName]="form.formControlNames.lastName" />
  <address-control [formControlName]="form.formControlNames.address"></address-control>
</form>

Our createForm function will return an object of type NgxRootForm. It means we'll then have access to the following properties:

• formGroup: The FormGroup instance with augmented capacity for type safety. While at runtime this object is really the form group itself, it is now defined as a TypedFormGroup<FormInterface> which provides type safety on a bunch of attributes and methods (value, valueChanges, controls, setValue, patchValue, getRawValue). If you want to know more about the TypedFormGroup interface, have a look in projects/ngx-sub-form/src/lib/shared/ngx-sub-form-utils.ts
• formControlNames: A typed object containing our form control names. The advantage of using this instead of a simple string is in case you ever update the type passed as the generic of the form (through a refactor or a change in the API upstream, etc). If you remove or update an existing property and forget to update the template, Typescript will catch the error (assuming you're using AoT which is the case by default)
• formGroupErrors: An object holding all the errors in the form. Bonus point: It also includes all the nested errors from the sub forms!
• controlValue$: If you want to listen to the form value, just use form.formGroup.valueChanges. But keep in mind that it will not be triggered when the form is being updated by the parent ⚠️. It'll only be triggered when the form is changed locally. If you want to know what's the latest form value from either the parent OR the local changes, you should use form.controlValue$ instead
• createFormArrayControl: We'll cover this one in the remap section, after the sub forms

Sub forms

When you've got a form represented by an object containing not one level of info but multiple ones (like a person which has an address, the address contains itself multiple fields), you should create a sub form to manage the address in isolation.

This is great for a couple of reasons:

• You can break down the complexity of your forms into smaller components
• You can reuse sub forms into other sub forms and root forms. It becomes easy to compose different bits of sub forms to create a bigger one

Here's a full example:

@Component({
  selector: 'address-control',
  template: `
    <div [formGroup]="form.formGroup">
      <input type="text" [formControlName]="form.formControlNames.street" />
      <input type="text" [formControlName]="form.formControlNames.city" />
      <input type="text" [formControlName]="form.formControlNames.state" />
      <input type="number" [formControlName]="form.formControlNames.zipCode" />
    </div>
  `,
  providers: subformComponentProviders(PersonForm),
})
export class PersonForm {
  public form = createForm<Address>(this, {
    formType: FormType.SUB,
    formControls: {
      street: new FormControl(null, Validators.required),
      city: new FormControl(null, Validators.required),
      state: new FormControl(null, Validators.required),
      zipCode: new FormControl(null, Validators.required),
    },
  });
}

A sub form looks very much like a root form but with an API that is even simpler.
When you call the createForm function, start by setting the formType to FormType.SUB and then define your formControls.

One important thing to note:

providers: subformComponentProviders(PersonForm);

subformComponentProviders is only here to help reduce the number of lines needed for each sub form component. It returns the following providers:

return [
  {
    provide: NG_VALUE_ACCESSOR,
    useExisting: component,
    multi: true,
  },
  {
    provide: NG_VALIDATORS,
    useExisting: component,
    multi: true,
  },
];

Behind the scenes those providers are allowing us to have a component considered as a ControlValueAccessor.
If you've ever created a ControlValueAccessor yourself, you can probably appreciate the amount of boilerplate ngx-sub-form is removing while adding features on top of it.

Just like the root form, the createForm function will return an object containing the following:

• formGroup
• formControlNames
• formGroupErrors
• createFormArrayControl
• controlValue$

As they're exactly the same as the ones in the root form we're not going to go over them again, feel free to check the previous section.

Remap

Sometimes a given data structure may not match the one you'd like to have internally for a form. When that's the case, ngx-sub-form offers 2 functions to:

• Take the input value and remap it to match the shape expected by the form
• Take the form value and remap it to match the shape expected as the output

Here are the 2 interfaces:

• toFormGroup: (obj: ControlInterface) => FormInterface;
• fromFormGroup: (formValue: FormInterface) => ControlInterface;

Example of a remap could be getting a date object that you want to convert to an ISO string date before passing that value to a date picker and before broadcasting that value back to the parent, convert it back to a date. Or vice versa.

A really interesting use case is to deal with polymorphic values. If we take the example of our live demo: https://cloudnc.github.io/ngx-sub-form we've got src/app/main/listing/listing-form/listing-form.component.ts. This form can receive either a vehicle or a droid. While polymorphism works great on typescript side, when it comes to templates... It's an other story! The best way is to have 2 sub components, which will handle 1 and 1 thing: Either a vehicle or a droid. And in the template use an ngIf or an ngSwitch to dynamically create the expected sub form.
That said, to be able to switch on a value, we need to know that value: A discriminator. It'll let us know what's the type of our current object really easily, without having to create a type guard for example. And a remap is a perfect candidate for this. If you want a full example please have a look to the listing-form.component.ts (path shown above).

Dealing with arrays

When your data structure contains one or more arrays, you may want to simply display the values in the view but chances are you want to bind them to the form.

In that case, working with a FormArray is the right way to go and for that, we will take advantage of the remap principles explained in the previous section.

If you have custom validations to set on the form controls, you can implement the createFormArrayControl function, which gives the library a hook with which to construct new form controls for the form array with the correct validators applied.

Its definition is the following:

createFormArrayControl(key, value) => FormControl;

Where key is a key of your main form and value, its associated value.

To see a complete example please refer to src/app/main/listing/listing-form/vehicle-listing/crew-members/crew-members.component.ts and its html part.

Contribution

Please, feel free to contribute to ngx-sub-form.
We've done our best to come up with a solution that helped us and our own needs when dealing with forms. But we might have forgotten some use cases that might be worth implementing in the core or the lib rather than on every project. Remember that contributing doesn't necessarily mean to make a pull request, you can raise an issue, edit the documentation (readme), etc.

Tell us about your experience with ngx-sub-form

We'd love to know more about who's using ngx-sub-form in production and on what kind of project! We've created an issue where everyone can share more about their experience.