(免责声明:当人们通过 facebook、firebase 等请求使用异步操作时，会出现很多关于数据为空/不正确的问题。我提出这个问题的目的是提供一个简单的为所有开始在 android 中使用异步操作的人回答这个问题)

我正在尝试从我的一项操作中获取数据，当我使用断点或日志对其进行调试时，值在那里，但是当我运行它时，它们始终为空，我该如何解决这个问题？

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... 
        });
//...and use the returned value here.

Facebook

GraphRequest request = GraphRequest.newGraphPathRequest(
    accessToken,
    "some path",
    new GraphRequest.Callback() {
        @Override
        public void onCompleted(GraphResponse response) {
            //I want to return these values I receive here...
        }
    });
request.executeAsync();
//...and use the returned value here.

Kotlin 协程

var result: SomeResultType? = null
someScope.launch {
    result = someSuspendFunctionToRetrieveSomething()
    //I want to return the value I received here... 
}
Log.d("result", result.toString()) //...but it is still null here.

等等

最佳答案

什么是同步/异步操作？

好吧，同步等到任务完成。在这种情况下，您的代码会“自上而下”执行。

异步在后台完成一项任务，并在完成时通知您。

如果您想通过方法/函数从异步操作返回值，您可以在方法/函数中定义自己的回调，以便在这些操作返回时使用这些值。

Java 的方法如下

首先定义一个接口(interface):

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

接下来，将您的方法签名更改为:

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);

举个例子:

@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，您之前调用了名为 foo 的方法:

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，您只关心单个结果)

首先将您的方法签名更改为以下内容:

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

然后，在您的异步操作结果中:

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

然后，您之前会调用名为 foo 的方法，现在您可以这样做:

foo() { result->
    // here, this result parameter that comes through is 
    // whatever you passed to the callback in the code aboce, 
    // so use this result right here to do any operation 
    // you previously wanted to do. 
}
// Be aware that code outside the callback here will run
// BEFORE the code above, and cannot rely on any data that may
// be set inside the callback.

如果你的 foo 方法之前接受了参数:

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 
    // whatever you passed to the callback in the code aboce, 
    // 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/函数来从您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，如果您对为这些创建方法/函数不感兴趣:

@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
})

关于java - 为什么我调用 API 或启动协程的函数返回空值或 null 值？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/57330766/