Xlythe

  void foo(Callback callback) {
    if (error1) {
      callback.onError();
      return;
    }
    if (error2) {
      callback.onError();
      return;
    }
    if (error3) {
      callback.onError();
      return;
    }

    doWork();
    callback.onSuccess();
  }

  void foo(Callback callback) {
    if (bar()) {
      callback.onSuccess();
    } else {
      callback.onError();
    }
  }

  boolean bar() {
    if (error1) {
      return false;
    }
    if (error2) {
      return false;
    }
    if (error3) {
      return false;
    }

    doWork();
    return true;
  }

  * Verifies the caller is allowed to call that method
  * Verifies the method can be called at this point in time (eg. hackerNewsApi.postComment(threadId, msg) only works if the threadId is valid)
  * Verifies that the arguments make sense (eg. 'msg' is not empty/null/above the max comment size).