EmailValidation.java

// Return true jika email user sudah terpakai
        Observable<Boolean> emailStream = RxTextView.textChanges(etEmail)
                .map(new Func1<CharSequence, String>() {
                    @Override
                    public String call(CharSequence charSequence) {
                        return charSequence.toString();
                    }
                })
                .filter(new Func1<String, Boolean>() {
                    @Override
                    public Boolean call(String input) {
                        return input.length() > 3;
                    }
                })
                .debounce(100,TimeUnit.MILLISECONDS)
                .flatMap(new Func1<String, Observable<Boolean>>() {
                    @Override
                    public Observable<Boolean> call(String input) {
                        return checkIfEmailExistFromAPI(input);
                    }
                });

    // Ambil daftar email dari APi, kemudian cek apakah email user sudah dipakai
    public Observable<Boolean> checkIfEmailExistFromAPI(final String input){
        return service.getEmails()
                .flatMap(new Func1<List<String>, Observable<String>>() { // Mengubah stream of List<String> menjadi stream of String
                    @Override
                    public Observable<String> call(List<String> strings) {
                        return Observable.from(strings);
                    }
                }).contains(input); // Cek apakah email di emit oleh stream sebelumnya
    }