private boolean isEmailValid(String email) {
  //TODO: Replace this with your own logic
  return email.contains("@");
 }
private boolean isPasswordValid(String password) {
 //TODO: Replace this with your own logic
 return password.length() > 4;
}

Observable<String> ObservableEmail = Observable.create(new Observable.OnSubscribe<String>() {
   @Override
   public void call(final Subscriber<? super String> subscriber) {
    mEmailView.addTextChangedListener(new TextWatcher() {
     @Override
     public void beforeTextChanged(CharSequence s, int start, int count, int after) {
     }
     @Override
     public void onTextChanged(CharSequence s, int start, int before, int count) {
     }
     @Override
     public void afterTextChanged(Editable s) {
      subscriber.onNext(s.toString());
     }
    });
   }
  });
Observable<String> ObservablePassword = Observable.create(new Observable.OnSubscribe<String>() {
 @Override
 public void call(final Subscriber<? super String> subscriber) {
  mPasswordView.addTextChangedListener(new TextWatcher() {
   @Override
   public void beforeTextChanged(CharSequence s, int start, int count, int after) {
   }
   @Override
   public void onTextChanged(CharSequence s, int start, int before, int count) {
   }
   @Override
   public void afterTextChanged(Editable s) {
    subscriber.onNext(s.toString());
   }
  });
 }
});

Observable.combineLatest(ObservableEmail, ObservablePassword, new Func2<String, String, Boolean>() {
   @Override
   public Boolean call(String email, String password) {
    return isEmailValid(email) && isPasswordValid(password);
   }
  }).subscribe(new Subscriber<Boolean>() {
   @Override
   public void onCompleted() {
   }
   @Override
   public void onError(Throwable e) {
   }
   @Override
   public void onNext(Boolean verify) {
    if (verify) {
     mEmailSignInButton.setEnabled(true);
    } else {
     mEmailSignInButton.setEnabled(false);
    }
   }
  });

return isEmailValid(email) && isPasswordValid(password);

Observable<CharSequence> ObservableEmail = RxTextView.textChanges(mEmailView);
Observable<CharSequence> ObservablePassword = RxTextView.textChanges(mPasswordView);
Observable.combineLatest(ObservableEmail, ObservablePassword, new Func2<CharSequence, CharSequence, Boolean>() {
 @Override
 public Boolean call(CharSequence email, CharSequence password) {
  return isEmailValid(email.toString()) && isPasswordValid(password.toString());
 }
}).subscribe(new Subscriber<Boolean>() {
 @Override
 public void onCompleted() {
 }
 @Override
 public void onError(Throwable e) {
 }
 @Override
 public void onNext(Boolean verify) {
  if (verify) {
   mEmailSignInButton.setEnabled(true);
  } else {
   mEmailSignInButton.setEnabled(false);
  }
 }
});