angular - Angular2: validation for <input type="file"/> won't trigger when changing the file to upload

Question

Angular 2 seems to have troubles with running validation when a file input changes.

I made a plunk to illustrate this problem:

I make a formGroup like

this.frm = new FormGroup({
    file: new FormControl("", this.validateFile)
});

And in the validateFile function I throw an alert and log to the console:

public validateFile(formControl: FormControl): {[key: string]: any; } {
   alert('Validation ran');
   console.log('Validation ran');
}

Plunkr to illustrate the issue: https://plnkr.co/edit/Pgcg4IkejgaH5YgbY3Ar?p=preview

The validation will run when initializing the page but won't run each time you change the file to be uploaded.

Is there any solution to this problem?

Answer 1

I fixed it using kemsky answer and Sebastien's comment. I made a ngValueAccessor which registers itself on every input with type file.

Plunkr can be found here.

Most relevant code + explanation beneath:

This adds a ControlValueAccessor for file inputs which might be part of the angular framework itself someday(#7341). A file input works different than other controls. This piece of code makes sure the selected files are read as the value:

import {Directive} from "@angular/core";
import {NG_VALUE_ACCESSOR, ControlValueAccessor} from "@angular/forms";

@Directive({
    selector: "input[type=file]",
    host : {
        "(change)" : "onChange($event.target.files)",
        "(blur)": "onTouched()"
    },
    providers: [
        { provide: NG_VALUE_ACCESSOR, useExisting: FileValueAccessor, multi: true }
    ]
})
export class FileValueAccessor implements ControlValueAccessor {
    value: any;
    onChange = (_) => {};
    onTouched = () => {};

    writeValue(value) {}
    registerOnChange(fn: any) { this.onChange = fn; }
    registerOnTouched(fn: any) { this.onTouched = fn; }
}

And for the 'required' validation I made a validator which I use by adding the static validate method to the file FormControl for ReactiveForms. (or as a directive for template driven forms).

import {Directive} from "@angular/core";
import {NG_VALIDATORS, Validator, FormControl} from "@angular/forms";

@Directive({
    selector: "[requiredFile]",
    providers: [
        { provide: NG_VALIDATORS, useExisting: FileValidator, multi: true },
    ]
})
export class FileValidator implements Validator {
    static validate(c: FormControl): {[key: string]: any} {
        return c.value == null || c.value.length == 0 ? { "required" : true} : null;
    }

    validate(c: FormControl): {[key: string]: any} {
        return FileValidator.validate(c);
    }
}

Building my form looks like this:

private buildForm() {
    this.frm = new FormGroup({
        file: new FormControl("",    [FileValidator.validate])
    });
}

And for the html:

<input type="file" formControlName="file"/>