为什么我调用 API 的函数返回空值或 null 值? [英] Why does my function that calls an API return an empty or null value?

问题描述

(免责声明:人们在通过 facebook、firebase 等请求使用异步操作时询问数据是否为空/不正确时出现了很多问题.我对这个问题的意图是提供一个简单的为每个开始在 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，值就在那里，但是当我运行它时，它们始终为空，我该如何解决这个问题?

I'm trying to get data from one of my operations, when I debug it using breakpoints or logs, the values are there, but when I run it they are always null, how can I solve this ?

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? 
            })

脸书

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();

等

推荐答案

什么是同步/异步操作?

好吧，同步等待任务完成.在这种情况下，您的代码会自上而下"执行.

Well, Synchronous waits until the task has completed. Your code executes "top-down" in this situation.

异步在后台完成一项任务，并在完成时通知您.

Asynchronous completes a task in the background and can notify you when it is complete.

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，以使用这些操作返回的值.

If you want to return the values from an async operation through a method/function, you can define your own callbacks in your method/function to use these values as they are returned from these operations.

Java 的方法

从定义一个接口开始:

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如下所示:

next, change your method signature to be like this :

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，无论您以前想在何处使用这些值，请添加以下行:

next up, wherever you previously wanted to use those values, add this line :

   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，您之前调用名为 foo 的方法的位置:

now, where you were previously calling your method called foo:

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 执行此操作?(作为您只关心单个结果的基本示例)

How do you do this for Kotlin ? (as a basic example where you only care for a single result)

首先将您的方法签名更改为如下所示:

start off by changing your method signature to something like this:

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

然后，在异步操作的结果中:

then, inside your asynchronous operation's result :

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

然后，在您之前调用名为 foo 的方法的地方，您现在这样做:

then, where you would have previously called your method called foo, you now do this :

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 方法之前接受了参数:

if your foo method previously took in parameters :

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/函数来从您通过使用回调执行的异步操作中返回值.

these solutions show how you can create a method/function to return values from async operations you've performed through the use of callbacks.

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:

However, it is important to understand that, should you not be interested in creating a method/function for these:

@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
     })

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