Angular 2 - formControlName inside component

You should not be adding formControlName attribute to the input field in the template of your custom component. You should be adding the formControlName on the custom-input element itself as per the best practice.

Here what you can use in your custom-input component is the controlValueAccessor interface to make your custom-input have the value updated whenever there is an event of input field in the template of your custom input changed or blurred.

It provides a connection (to update values or other needs) between the form control behavior of your custom input and the UI you are providing for that custom form control.

Below is the code of a custom input component in TypeScript.

import { Component, Input, forwardRef, AfterViewInit, trigger, state, animate, transition, style, HostListener, OnChanges, ViewEncapsulation, ViewChild, ElementRef } from '@angular/core';
import { NG_VALUE_ACCESSOR, ControlValueAccessor, FormControl } from '@angular/forms';

export const CUSTOM_INPUT_CONTROL_VALUE_ACCESSOR: any = {
    provide: NG_VALUE_ACCESSOR,
    useExisting: forwardRef(() => InputComponent),
    multi: true
};

@Component({
  selector: 'inv-input',
  templateUrl:'./input-text.component.html',
    styleUrls: ['./input-text.component.css'],
    encapsulation: ViewEncapsulation.None,
  providers: [CUSTOM_INPUT_CONTROL_VALUE_ACCESSOR],
    animations:[trigger(
        'visibilityChanged',[
            state('true',style({'height':'*','padding-top':'4px'})),
            state('false',style({height:'0px','padding-top':'0px'})),
            transition('*=>*',animate('200ms'))
        ]
    )]
})

export class InputComponent implements ControlValueAccessor, AfterViewInit, OnChanges {

    // Input field type eg:text,password
    @Input()  type = "text"; 

    // ID attribute for the field and for attribute for the label
    @Input()  idd = ""; 

    // The field name text . used to set placeholder also if no pH (placeholder) input is given
    @Input()  text = ""; 

    // placeholder input
    @Input()  pH:string; 

    //current form control input. helpful in validating and accessing form control
    @Input() c:FormControl = new FormControl(); 

    // set true if we need not show the asterisk in red color
    @Input() optional : boolean = false;

    //@Input() v:boolean = true; // validation input. if false we will not show error message.

    // errors for the form control will be stored in this array
    errors:Array<any> = ['This field is required']; 

    // get reference to the input element
    @ViewChild('input')  inputRef:ElementRef; 


    constructor() {

    }

    ngOnChanges(){

    }

    //Lifecycle hook. angular.io for more info
    ngAfterViewInit(){ 
        // set placeholder default value when no input given to pH property      
        if(this.pH === undefined){
            this.pH = "Enter "+this.text; 
        }

        // RESET the custom input form control UI when the form control is RESET
        this.c.valueChanges.subscribe(
            () => {
                // check condition if the form control is RESET
                if (this.c.value == "" || this.c.value == null || this.c.value == undefined) {
                    this.innerValue = "";      
                    this.inputRef.nativeElement.value = "";                 
                }
            }
        );
    }

   //The internal data model for form control value access
    private innerValue: any = '';

    // event fired when input value is changed . later propagated up to the form control using the custom value accessor interface
    onChange(e:Event, value:any){
        //set changed value
        this.innerValue = value;
        // propagate value into form control using control value accessor interface
        this.propagateChange(this.innerValue);

        //reset errors 
        this.errors = [];
        //setting, resetting error messages into an array (to loop) and adding the validation messages to show below the field area
        for (var key in this.c.errors) {
            if (this.c.errors.hasOwnProperty(key)) {
                if(key === "required"){
                    this.errors.push("This field is required");
                }else{
                    this.errors.push(this.c.errors[key]);
                }              
            }
        }
    }



    //get accessor
    get value(): any {
        return this.innerValue;
    };

    //set accessor including call the onchange callback
    set value(v: any) {
        if (v !== this.innerValue) {
            this.innerValue = v;
        }
    }

    //propagate changes into the custom form control
    propagateChange = (_: any) => { }

    //From ControlValueAccessor interface
    writeValue(value: any) {
        this.innerValue = value;
    }

    //From ControlValueAccessor interface
    registerOnChange(fn: any) {
        this.propagateChange = fn;
    }

    //From ControlValueAccessor interface
    registerOnTouched(fn: any) {

    }
}

Below is the template HTML for the custom input component

<div class="fg">
      <!--Label text-->
      <label [attr.for]="idd">{{text}}<sup *ngIf="!optional">*</sup></label>
      <!--Input form control element with on change event listener helpful to propagate changes -->
      <input type="{{type}}" #input id="{{idd}}" placeholder="{{pH}}" (blur)="onChange($event, input.value)">
      <!--Loop through errors-->
      <div style="height:0px;" [@visibilityChanged]="!c.pristine && !c.valid" class="error">
            <p *ngFor="let error of errors">{{error}}</p>
      </div>
</div>

Below is custom input component which can be used in a fromGroup or individually

<inv-input formControlName="title" [c]="newQueryForm.controls.title" [optional]="true" idd="title" placeholder="Type Title to search"
          text="Title"></inv-input>

In this fashion if you implement your custom form controls you can apply your custom validator directives easily and accumulate the errors on that form control to display your errors.

One can imitate the same style to develop custom select component, radio button group, checkbox, textarea, fileupload etc in the above fashion with minor changes as per what the form control's behavior demands.

Angular 8 and 9: Use viewProvider in you custom component. Working example:

@Component({
    selector: 'app-input',
    templateUrl: './input.component.html',
    styleUrls: ['./input.component.scss'],
    viewProviders: [
        {
            provide: ControlContainer,
            useExisting: FormGroupDirective
        }
    ]
})

Now, when you assign formControlName, your component will attach itself to the parent form.

<input matInput formControlName="{{name}}">

or

<input matInput [formControlName]='name'>

The main idea here is that you have to link the FormControl to the FormGroup, this can be done be passing the FormGroup to each input component...

So your input template might look something like the following:

<div [formGroup]="form">
    <label *ngIf="label">{{ label }}</label>
    <input [formControlName]="inputName" />
    <span *ngIf="message">{{ message }}</span>
</div>

Where the @Input's for the input component will be form, label, inputName and message.

It would be used like this:

<form [FormGroup]="yourFormGroup">
    <custom-input
        [form]="yourFormGroup"
        [inputName]="thisFormControlName"
        [message]="yourMessage"
        [label]="yourLabel">
    </custom-input>
</form>

For more info on custom Form Input Components I would recomend taking a look through Angular's Dynamic Forms. Also if you want more information on how to get the @Input and @Output working take a look through the Angular Docs Here

I'm solving this in a similar way like web-master-now. But instead of writing a complete own ControlValueAccessor I'm delegating everything to an inner <input> ControlValueAccessor. The result is a much shorter code and I don't have to handle the interaction with the <input> element on my own.

Here is my code

@Component({
  selector: 'form-field',
  template: `    
    <label>
      {{label}}
      <input ngDefaultControl type="text" >
    </label>
    `,
  providers: [{
    provide: NG_VALUE_ACCESSOR,
    useExisting: forwardRef(() => FormFieldComponent),
    multi: true
  }]
})
export class FormFieldComponent implements ControlValueAccessor, AfterViewInit {
  @Input() label: String;
  @Input() formControlName: String;
  @ViewChild(DefaultValueAccessor) valueAccessor: DefaultValueAccessor;

  delegatedMethodCalls = new ReplaySubject<(_: ControlValueAccessor) => void>();

  ngAfterViewInit(): void {
    this.delegatedMethodCalls.subscribe(fn => fn(this.valueAccessor));
  }

  registerOnChange(fn: (_: any) => void): void {
    this.delegatedMethodCalls.next(valueAccessor => valueAccessor.registerOnChange(fn));
  }
  registerOnTouched(fn: () => void): void {
    this.delegatedMethodCalls.next(valueAccessor => valueAccessor.registerOnTouched(fn));
  }

  setDisabledState(isDisabled: boolean): void {
    this.delegatedMethodCalls.next(valueAccessor => valueAccessor.setDisabledState(isDisabled));
  }

  writeValue(obj: any): void {
    this.delegatedMethodCalls.next(valueAccessor => valueAccessor.writeValue(obj));
  }
}

How does it work?

Generally this won't work, as a simpel <input> won't be a ControlValueAccessor without formControlName-directive, which is not allowed in the component due to missing [formGroup], as others already pointed out. However if we look at Angular's code for the DefaultValueAccessor implementation

@Directive({
    selector:
        'input:not([type=checkbox])[formControlName],textarea[formControlName],input:not([type=checkbox])[formControl],textarea[formControl],input:not([type=checkbox])[ngModel],textarea[ngModel],[ngDefaultControl]',

    //...
})
export class DefaultValueAccessor implements ControlValueAccessor {

... we can see that there is another attribute selector ngDefaultControl. It's available for a different purpose, but it seems to be supported officially.

A little disadvantage is that the @ViewChild query result with the value accessor will be available not before the ngAfterViewInit handler is called. (It will be available earlier depending on your template, but that's not supported officially .)

That's why I'm buffering all calls we want to delegate to our inner DefaultValueAccessor using a ReplaySubject. A ReplaySubject is an Observable, which buffers all events and emits them on subscription. A normal Subject would throw them away until subscription.

We emit lambda expressions representing the actual call that can be executed later. On ngAfterViewInit we subscribe to our ReplaySubject and simply call the received lambda functions.

I'm sharing two other ideas here, as they are very important for my own projects and it took me a while to work everything out. I see a lot of people having similar problems and use cases, so I hope this is useful for you:

Idea for improvement 1: Provide the FormControl for the view

I replaced the ngDefaultControl by formControl in my project, so we can pass the FormControl instance to the inner <input>. This is not useful by itself, however it is if you are using other directives which interact with FormControls, such as Angular Material's MatInput. E.g. if we replace our form-field template by...

<mat-form-field>
    <input [placeholder]="label" [formControl]="formControl>
    <mat-error>Error!</mat-error>
</mat-form-field> 

...Angular Material is able to automatically show errors set in the form control.

I have to adjust the component in order to pass the form control. I retrieve the form control from our FormControlName directive:

export class FormFieldComponent implements ControlValueAccessor, AfterContentInit {
  // ... see above

  @ContentChild(FormControlName) private formControlNameRef: FormControlName;
  formControl: FormControl;

  ngAfterContentInit(): void {
    this.formControl = <FormControl>this.formControlNameRef.control;
  }

  // ... see above
}

You should also adjust your selector to require the formControlName attribute: selector: 'form-field[formControlName]'.

Idea for improvement 2: Delegate to a more generic value accessor

I replaced the DefaultValueAccessor @ViewChild query by a query for all ControlValueAccessor implementations. This allows other HTML form controls than <input> like <select> and is useful if you want to make your form control type configurable.

@Component({
    selector: 'form-field',
    template: `    
    <label [ngSwitch]="controlType">
      {{label}}
      <input *ngSwitchCase="'text'" ngDefaultControl type="text" #valueAccessor>
      <select *ngSwitchCase="'dropdown'" ngModel #valueAccessor>
        <ng-content></ng-content>
      </select>
    </label>
    `,
    providers: [{
        provide: NG_VALUE_ACCESSOR,
        useExisting: forwardRef(() => FormFieldComponent),
        multi: true
    }]
})
export class FormFieldComponent implements ControlValueAccessor {
    // ... see above

    @Input() controlType: String = 'text';
    @ViewChild('valueAccessor', {read: NG_VALUE_ACCESSOR}) valueAccessor: ControlValueAccessor;

    // ... see above
}

Usage example:

<form [formGroup]="form">
  <form-field formControlName="firstName" label="First Name"></form-field>
  <form-field formControlName="lastName" label="Last Name" controlType="dropdown">
    <option>foo</option>
    <option>bar</option>
  </form-field>
  <p>Hello "{{form.get('firstName').value}} {{form.get('lastName').value}}"</p>
</form>

A problem with the select above is that ngModel is already deprecated together with reactive forms. Unfortunately there is nothing like ngDefaultControl for Angular's <select> control value accessor. Therefore I suggest to combine this with my first improvement idea.