Java 为什么调用API的函数返回空值或空值？

（免责声明：当通过facebook、firebase等请求使用异步操作时，人们会提出大量关于数据为空/不正确的问题。我提出这个问题的目的是为每个在android中开始异步操作的人提供一个简单的答案）

我试图从我的一个操作中获取数据，当我使用断点或日志调试它时，值在那里，但是当我运行它时，它们总是空的，我如何解决这个问题

Firebase

firebaseFirestore.collection("some collection").get()
            .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                @Override
                public void onSuccess(QuerySnapshot documentSnapshots) {
                     //I want to return these values I receive here? 
            })

---

等等。

什么是同步/异步操作？

那么，同步将等待任务完成。在这种情况下，代码执行“自上而下”

异步在后台完成任务，并在任务完成时通知您

如果希望通过方法/函数从异步操作返回值，可以在方法/函数中定义自己的回调，以便在从这些操作返回值时使用这些值

以下是Java的使用方法

首先定义一个接口：

interface Callback {
 void myResponseCallback(YourReturnType result);//whatever your return type is: string, integer, etc.
}

接下来，将方法签名更改为如下所示：

public void foo(final Callback callback) { // make your method, which was previously returning something, return void, and add in the new callback interface.

接下来，无论您以前想在哪里使用这些值，请添加以下行：

   callback.myResponseCallback(yourResponseObject);

例如：

 @Override
 public void onSuccess(QuerySnapshot documentSnapshots) {
 // create your object you want to return here
 String bar = document.get("something").toString();
 callback.myResponseCallback(bar);
 })

现在，在前面调用名为

foo

的方法的地方：

foo(new Callback() {
        @Override
        public void myResponseCallback(YourReturnType result) {
            //here, this result parameter that comes through is your api call result to use, so use this result right here to do any operation you previously wanted to do. 
        }
    });
}

您如何为Kotlin做到这一点？ （作为一个基本示例，您只关心一个结果）

首先，将方法签名更改为以下内容：

fun foo(callback:(YourReturnType) -> Unit) {
.....

然后，在异步操作的结果中：

firestore.collection("something").document("document").get().addOnSuccessListener { 
                    val bar = it.get("something").toString()
                    callback.invoke(bar)
                }

然后，在以前调用名为

foo

的方法的地方，现在执行以下操作：

foo { result->
here, this result parameter that comes through is your api call result to use, so use this result right here to do any operation you previously wanted to do. 
}

如果您的

foo

方法以前接受了参数：

fun foo(value:SomeType, callback:(YourType) -> Unit)

您只需将其更改为：

foo(yourValueHere) { result ->
here, this result parameter that comes through is your api call result to use, so use this result right here to do any operation you previously wanted to do. 
    }

这些解决方案展示了如何创建一个方法/函数，从通过使用回调执行的异步操作中返回值

---

但是，重要的是要了解，如果您不想为以下内容创建方法/函数：

@Override
 public void onSuccess(SomeApiObjectType someApiResult) {
//here, this `onSuccess` callback provided by the api already has the data you're looking for (in this example, that data would be `someApiResult`).
//you can simply add all your relevant code which would be using this result inside this block here, this will include any manipulation of data, populating adapters, etc. 
//this is the only place where you will have access to the data returned by the api call, assuming your api follows this pattern
     })

@Override
 public void onSuccess(SomeApiObjectType someApiResult) {
//here, this `onSuccess` callback provided by the api already has the data you're looking for (in this example, that data would be `someApiResult`).
//you can simply add all your relevant code which would be using this result inside this block here, this will include any manipulation of data, populating adapters, etc. 
//this is the only place where you will have access to the data returned by the api call, assuming your api follows this pattern
     })