Subjects - apply changes on every next value [BehaviorSubject.onBeforeNext?]

Question

Say you have a simple BehaviorSubject

this.countryOfBirth$ = new BehaviorSubject<CountryEnum>(null);

get countryOfBirth() {
    return this.countryOfBirth$.value;
};

set countryOfBirth(val: CountryEnum) {
    this.countryOfBirth$.next(val);
};

So doing instance.countryOfBirth returns the last value and instance.countryOfBirth = CountryEnum.US pushes the new value to the Subject.

The problem I'm having is that I pass this subject instance to a custom model-driven form module which by default would wrongly pass a string instead of an instance of my CountryEnum class.

I can fix this in the forms module but I would like to keep that as decoupled as possible from any app-specific logic so it would make more sense to implement the fix in the BehaviorSubject instance itself.

My question is: is there hook or any other way to apply some changes to each and every "next" value before it triggers its subscribers? In other words, after my code does

instance.countryOfBirth = CountryEnum.US;

Before any of the subscribers are triggered I would like to check whether the value is a string (ex: US) and if so - I would like to get the corresponding CountryEnum instance and pass that to the "next" call instead of the original "US" string.

In code it would look something like

this.countryOfBirth$.onBeforeNext((val) => {
    if (typeof val == "string") {
        return CountryEnum.getInstance(val);
    } else if (val instanceof CountryEnum) {
        return val;
    } else {
        throw new Error("Unsupported value type");
    }
});

but obviously onBeforeNext doesn't exist and I can't seem to find anything in the dox that would do what I want.

Your help would be much appreciated!

Answer 1

Since apparently there is no readily available method to do what I needed this is how I implemented my solution using the approach that @estus mentioned in the comments:

// BehaviorSubjectWithValidation.ts

import { BehaviorSubject } from "rxjs/BehaviorSubject";

export class BehaviorSubjectWithValidation<T> extends BehaviorSubject<T> {

    private _validator: (val: T) => T;

    constructor(defaultValue: T, validator: (val: T) => T) {
        super(defaultValue);
        this._validator = validator;
    }

    next(value: T): void {
        if (this._validator) {
            value = this._validator(value);
        }
        super.next(value);
    }
}

Then in my CountryEnum class I added the following method

public static parse: (val: any) => CountryEnum = (val: any) => {
    if (val) {
        if (typeof val === "string") {
            return CountryEnum.getInstance(val);
        } else if (val instanceof CountryEnum) {
            return val;
        } else {
            throw new Error("Unsupported CountryEnum value");
        }
    } else {
        throw new Error("Invalid country");
    }
}

And then I use it in the following way in my main app's logic:

this.countryOfBirth$ = 
    new BehaviorSubjectWithValidation<CountryEnum>(null, CountryEnum.parse);

So now whatever part of my code adds a new value to this.countryOfBirth$ it will always going to filter it through CountryEnum.parse.

Hope this helps someone!