example.java

RxFormValidationHelper.subscribeValidateButton(butLogin,
        RxFormValidationHelper.validateText(etMail, mailLayout, RxFormValidationHelper.RuleIsEmail, getString(R.string.webservice_error_invalid_parameter_email)),
        RxFormValidationHelper.validateText(etPass, passLayout, RxFormValidationHelper.RuleMinChars.call(8), getString(R.string.webservice_error_invalid_parameter_password))
);

RxFormValidationHelper.java

package com.bja.whatever;

import android.support.design.widget.TextInputLayout;
import android.widget.Button;
import android.widget.EditText;

import com.jakewharton.rxbinding.view.RxView;
import com.jakewharton.rxbinding.widget.RxTextView;

import java.util.Arrays;

import rx.Observable;
import rx.functions.*;

public class RxFormValidationHelper {

    public static Func1<CharSequence, Boolean> RuleNotEmpty;
    public static Func1<Integer, Func1<CharSequence, Boolean>> RuleMinChars, RuleMaxChars;
    public static Func1<CharSequence, Boolean> RuleIsEmail;

    static {
        RuleMaxChars = len -> txt -> txt.length() <= len;
        RuleMinChars = len -> txt -> txt.length() >= len;
        RuleNotEmpty = RuleMinChars.call(1);
        RuleIsEmail = txt -> RuleNotEmpty.call(txt) && android.util.Patterns.EMAIL_ADDRESS.matcher(txt).matches();
    }

    public static Observable<Boolean> validateText(EditText etEmail, TextInputLayout emailLayout, Func1<CharSequence, Boolean> rule, String errorString) {
        return RxTextView.textChanges(etEmail)
                .map(rule)
                .distinctUntilChanged()
                .doOnNext(b -> emailLayout.setError(!b ? errorString : null));
    }

    public static void subscribeValidateButton(Button butLogin, Observable<Boolean>... booleanObservables) {
        RxView.clicks(butLogin).map(aVoid -> true) // avoids triggering the rest if the button hasn't been clicked once
            .flatMap(b ->
                Observable.combineLatest(Arrays.asList(booleanObservables), args -> {
                    for (Object arg : args) {
                        if (!((Boolean) arg))
                            return false;
                    }
                    return true;
                })
                .distinctUntilChanged()
            )
            .subscribe(RxView.clickable(butLogin));
    }
}