Angular 2 - large scale application forms' handling

Your approach and Ovangle's one seem to be pretty good but even though this SO question is solved, I want to share my solution because it's a really different approach that I think you might like or might be useful to someone else.

what solutions there are for an app wide form where Components take care of different sub parts to the global form.

We've faced that exact same issue and after months of struggling with huge, nested and sometimes polymorphic forms, we've come up with a solution that pleases us, which is simple to use and which gives us "super powers" (like type safety within both TS and HTML), access to nested errors and others.

We've decided to extract that into a separated library and open source it.
Source code is available here: https://github.com/cloudnc/ngx-sub-form
And the npm package can be installed like that npm i ngx-sub-form

Behind the scenes, our library uses ControlValueAccessor and that allows us to use it on template forms AND reactive forms (you'll get the best out of it by using reactive forms though).

So what is it all about?

Before I start explaining, if you prefer to follow along with a proper editor I've made a Stackblitz example: https://stackblitz.com/edit/so-question-angular-2-large-scale-application-forms-handling

Well an example is worth a 1000 words I guess so let's redo one part of your form (the hardest one with nested data): personalDetailsForm$

First thing to do is make sure everything is going to be type safe. Let's create the interfaces for that:

export enum Gender {
  MALE = 'Male',
  FEMALE = 'Female',
  Other = 'Other',
}

export interface Name {
  firstname: string;
  lastname: string;
}

export interface Address {
  streetaddress: string;
  city: string;
  state: string;
  zip: string;
  country: string;
}

export interface Phone {
  phone: string;
  countrycode: string;
}

export interface PersonalDetails {
  name: Name;
  gender: Gender;
  address: Address;
  phone: Phone;
}

export interface MainForm {
  // this is one example out of what you posted
  personalDetails: PersonalDetails;

  // you'll probably want to add `parent` and `responsibilities` here too
  // which I'm not going to do because `personalDetails` covers it all :)
}

Then, we can create a component that extends NgxSubFormComponent.
Let's call it personal-details-form.component.

@Component({
  selector: 'app-personal-details-form',
  templateUrl: './personal-details-form.component.html',
  styleUrls: ['./personal-details-form.component.css'],
  providers: subformComponentProviders(PersonalDetailsFormComponent)
})
export class PersonalDetailsFormComponent extends NgxSubFormComponent<PersonalDetails> {
  protected getFormControls(): Controls<PersonalDetails> {
    return {
      name: new FormControl(null, { validators: [Validators.required] }),
      gender: new FormControl(null, { validators: [Validators.required] }),
      address: new FormControl(null, { validators: [Validators.required] }),
      phone: new FormControl(null, { validators: [Validators.required] }),
    };
  }
}

Few things to notice here:

• NgxSubFormComponent<PersonalDetails> is going to give us type safety
• We have to implements the getFormControls methods which expects a dictionary of the top level keys matching an abstract control (here name, gender, address, phone)
• We keep full control over the options to create the formControl (validators, async validators etc)
• providers: subformComponentProviders(PersonalDetailsFormComponent) is a small utility function to create the providers necessary to use a ControlValueAccessor (cf Angular doc), you just need to pass as argument the current component

Now, for every entry of name, gender, address, phone that is an object, we create a sub form for it (so in this case everything but gender).

Here's an example with phone:

@Component({
  selector: 'app-phone-form',
  templateUrl: './phone-form.component.html',
  styleUrls: ['./phone-form.component.css'],
  providers: subformComponentProviders(PhoneFormComponent)
})
export class PhoneFormComponent extends NgxSubFormComponent<Phone> {
  protected getFormControls(): Controls<Phone> {
    return {
      phone: new FormControl(null, { validators: [Validators.required] }),
      countrycode: new FormControl(null, { validators: [Validators.required] }),
    };
  }
}

Now, let's write the template for it:

<div [formGroup]="formGroup">
  <input type="text" placeholder="Phone" [formControlName]="formControlNames.phone">
  <input type="text" placeholder="Country code" [formControlName]="formControlNames.countrycode">
</div>

Notice that:

• We define <div [formGroup]="formGroup">, the formGroup here is provided by NgxSubFormComponent you don't have to create it yourself
• [formControlName]="formControlNames.phone" we use property binding to have a dynamic formControlName and then use formControlNames. This type safety mechanism is offered by NgxSubFormComponent too and if your interface changes at some point (we all know about refactors...), not only your TS will error for missing properties in the form but also the HTML (when you compile with AOT)!

Next step: Let's build the PersonalDetailsFormComponent template but first just add that line into the TS: public Gender: typeof Gender = Gender; so we can safely access the enum from the view

<div [formGroup]="formGroup">
    <app-name-form [formControlName]="formControlNames.name"></app-name-form>

    <select [formControlName]="formControlNames.gender">
    <option *ngFor="let gender of Gender | keyvalue" [value]="gender.value">{{ gender.value }}</option>
  </select>

  <app-address-form [formControlName]="formControlNames.address"></app-address-form>

  <app-phone-form [formControlName]="formControlNames.phone"></app-phone-form>
</div>

Notice how we delegate the responsibility to a sub component? <app-name-form [formControlName]="formControlNames.name"></app-name-form> that's the key point here!

Final step: built the top form component

Good news, we can also use NgxSubFormComponent to enjoy type safety!

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent extends NgxSubFormComponent<MainForm> {
  protected getFormControls(): Controls<MainForm> {
    return {
      personalDetails: new FormControl(null, { validators: [Validators.required] }),
    };
  }
}

And the template:

<form [formGroup]="formGroup">
  <app-personal-details-form [formControlName]="formControlNames.personalDetails"></app-personal-details-form>
</form>

<!-- let see how the form values looks like! -->
<h1>Values:</h1>
<pre>{{ formGroupValues | json }}</pre>

<!-- let see if there's any error (works with nested ones!) -->
<h1>Errors:</h1>
<pre>{{ formGroupErrors | json }}</pre>

Takeaway from all of that: - Type safe forms - Reusable! Needs to reuse the address one for the parents? Sure, no worries - Nice utilities to build nested forms, access form control names, form values, form errors (+nested!) - Have you noticed any complex logic at all? No observables, no service to inject... Just defining interfaces, extending a class, pass an object with the form controls and create the view. That's it

By the way, here's a live demo of everything I've been talking about:
https://stackblitz.com/edit/so-question-angular-2-large-scale-application-forms-handling

Also, it was not necessary in that case but for forms a little bit more complex, for example when you need to handle a polymorphic object like type Animal = Cat | Dog we've got another class for that which is NgxSubFormRemapComponent but you can read the README if you need more info.

Hope it helps you scale your forms!

Edit:

If you want to go further, I've just published a blog post to explain a lot of things about forms and ngx-sub-form here https://dev.to/maxime1992/building-scalable-robust-and-type-safe-forms-with-angular-3nf9

I commented elsewhere about @ngrx/store, and while I still recommend it, I believe I was misunderstanding your problem slightly.

Anyway, your FormsControlService is basically a global const. Seriously, replace the export class FormControlService ... with

export const formControlsDefinitions = {
   // ...
};

and what difference does it make? Instead of getting a service, you just import the object. And since we're now thinking of it as a typed const global, we can define the interfaces we use...

export interface ModelControl<T> {
    name: string;
    validators: ValidatorFn[];
}

export interface ModelGroup<T> {
   name: string;
   // Any subgroups of the group
   groups?: ModelGroup<any>[];
   // Any form controls of the group
   controls?: ModelControl<any>[];
}

and since we've done that, we can move the definitions of the individual form groups out of the single monolithic module and define the form group where we define the model. Much cleaner.

// personal_details.ts

export interface PersonalDetails {
  ...
}

export const personalDetailsFormGroup: ModelGroup<PersonalDetails> = {
   name: 'personalDetails$';
   groups: [...]
}

But now we have all these individual form group definitions scattered throughout our modules and no way to collect them all :( We need some way to know all the form groups in our application.

But we don't know how many modules we'll have in future, and we might want to lazy load them, so their model groups might not be registered at application start.

Inversion of control to the rescue! Let's make a service, with a single injected dependency -- a multi-provider which can be injected with all our scattered form groups when we distribute them throughout our modules.

export const MODEL_GROUP = new OpaqueToken('my_model_group');

/**
 * All the form controls for the application
 */
export class FormControlService {
    constructor(
        @Inject(MMODEL_GROUP) rootControls: ModelGroup<any>[]
    ) {}

    getControl(name: string): AbstractControl { /etc. }
}

then create a manifest module somewhere (which is injected into the "core" app module), building your FormService

@NgModule({
   providers : [
     {provide: MODEL_GROUP, useValue: personalDetailsFormGroup, multi: true}
     // and all your other form groups
     // finally inject our service, which knows about all the form controls
     // our app will ever use.
     FormControlService
   ]
})
export class CoreFormControlsModule {}

We've now got a solution which is:

• more local, the form controls are declared alongside the models
• more scalable, just need to add a form control and then add it to the manifest module; and
• less monolithic, no "god" config classes.

I did a similar application. The problem is that you are creating all your inputs at the same time, which is not likely scalable.

In my case, I did a FormManagerService who manages an array of FormGroup. Each step has a FormGroup that is initialized once in the execution on the ngOnInit of the step component by sending his FormGroup config to the FormManagerService. Something like that:

stepsForm: Array<FormGroup> = [];
getFormGroup(id:number, config: Object): FormGroup {
    let formGroup: FormGroup;
    if(this.stepsForm[id]){
        formGroup = this.stepsForm[id];
    } else {
        formGroup = this.createForm(config); // call function to create FormGroup
        this.stepsForm[id] = formGroup;
    }
    return formGroup;
}

You'll need an id to know which FormGroup corresponds to the step. But after that, you'll be able to split your Forms config in each step (so small config files that are easier for maintenance than a huge file). It will minimize the initial load time since the FormGroups are only create when needed.

Finally before submitting, you just need to map your FormGroup array and validate if they're all valid. Just make sure all the steps has been visited (otherwise some FormGroup won't be created).

This may not be the best solution but it was a good fit in my project since I'm forcing the user to follow my steps. Give me your feedback. :)