java - 使用全局变量从内部函数获取空字符串

Question

请帮我解决一些小问题，我确信你能做到:D

我正在尝试在 firestore 文档“user_cases_information”上设置一个字段，其中包含“case_number”字段

1. 首先我声明这个全局变量

private String case_number_consecutive = new String("");

在.setOnClickListener中我有这个:(从其他文档读取旧的连续编号或案例编号)

if (service_type.equals("support")){
    DocumentReference docRef = firestore_popup.collection("param_data_app").document("support_case_numbers");
    docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
     @Override
     public void onComplete(@NonNull Task<DocumentSnapshot> task) {
            if (task.isSuccessful()) {
                 DocumentSnapshot document = task.getResult();
                 String consecutive = document.get("consecutive").toString();
                    if (consecutive.equals(null)) {
                         int random = new Random().nextInt(117) + 4;
                         case_number_consecutive = "IOC-09"+random;
                    } else {
                        case_number_consecutive = consecutive;                                        UpdateCaseNumbersConsecutive("support");
                    }
             } else {
                  int random = new Random().nextInt(117) + 4;
                  case_number_consecutive = "IOC-09"+random;
              }
      }
     });
} 


Map<String, Object> user_cases_information = new HashMap<>();
user_cases_information.put("case_number", case_number_consecutive);
... (then i use a firestore reference to put the new case number)

假设当我检查 firestore 时，正是我设置“user_cases_information”文档的文档，我应该看到数字或新的 case_number，但我总是得到一个 null 引用，该字段连续总是 = null

为什么我得到这个空值？我用了一个 toast 并将其放在这之前:

Toast.makeText(...."Value: "+case_number_consecutive
Map<String, Object> user_cases_information = new HashMap<>();
user_cases_information.put("case_number", case_number_consecutive);

并且它显示null，因此上述函数中的值或数据正在丢失

我该如何解决这个问题？？？万分感谢。

Answer 1

这样做的原因是因为 firebase 是异步执行的，而您在编码时就好像这一切都是同步发生的。这是什么意思 ？这意味着 firebase 调用不会立即从服务器返回值，需要一些时间才能获取该值，但其余代码在响应返回之前正在执行，这就是您遇到问题的原因。因此，即使此代码执行:

if (service_type.equals("support")){
    DocumentReference docRef = firestore_popup.collection("param_data_app").document("support_case_numbers");
    docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
     @Override
     public void onComplete(@NonNull Task<DocumentSnapshot> task) {
            if (task.isSuccessful()) {
                 DocumentSnapshot document = task.getResult();
                 String consecutive = document.get("consecutive").toString();
                    if (consecutive.equals(null)) {
                         int random = new Random().nextInt(117) + 4;
                         case_number_consecutive = "IOC-09"+random;
                    } else {
                        case_number_consecutive = consecutive;                                        UpdateCaseNumbersConsecutive("support");
                    }
             } else {
                  int random = new Random().nextInt(117) + 4;
                  case_number_consecutive = "IOC-09"+random;
              }
      }
     });
} 

当响应返回时，您的其他代码:

Map<String, Object> user_cases_information = new HashMap<>();
user_cases_information.put("case_number", case_number_consecutive);

已经完成并调用，然后你永远不会从 firebase 获得实际值。

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您可以执行以下操作:

interface Callback{
        void firebaseResponseCallback(String result);//whatever your return type is.
    }

更改此方法的签名，如下所示:

public void getCaseNumber(Callback callback) { 

将您的代码更改为:

if (service_type.equals("support")){
    DocumentReference docRef = firestore_popup.collection("param_data_app").document("support_case_numbers");
    docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
     @Override
     public void onComplete(@NonNull Task<DocumentSnapshot> task) {
            if (task.isSuccessful()) {
                 DocumentSnapshot document = task.getResult();
                 String consecutive = document.get("consecutive").toString();
                    if (consecutive.equals(null)) {
                         int random = new Random().nextInt(117) + 4;
                         case_number_consecutive = "IOC-09"+random;
                    } else {
                        case_number_consecutive = consecutive;                                        UpdateCaseNumbersConsecutive("support");
                    }
             } else {
                  int random = new Random().nextInt(117) + 4;
                  case_number_consecutive = "IOC-09"+random;
              }

       callback.firebaseResponseCallback(case_number_consecutive);
      }
     });
} 

然后，当您调用 getCaseNumber 时:

getCaseNumber(new Callback() {
        @Override
        public void firebaseResponseCallback(String result) {
            here, this result parameter that comes through is your api call result to use, so result will be your case number, so make your objects inside this method, not outside
        }
    });
}

<小时/>

编辑

我专门针对这些类型的问题撰写了这篇文章: How to get data from any asynchronous operation in android