为什么我在android中放入MutableLiveData后会得到Null

Question

我尝试使用 OpenWeatherAPI 获取天气信息。

所以我使用了 Retrofit 库。

API 连接成功。

但问题是我从 OpenWeatherAPI 获得的天气数据 将数据放入 MutableLiveData 后突然变为 null。

这是代码

package wook.co.weather.models.repository;

import android.util.Log;

import androidx.lifecycle.MutableLiveData;

import java.util.List;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;
import wook.co.weather.interfaces.OpenWeatherAPI;
import wook.co.weather.models.dto.OpenWeather;
import wook.co.weather.models.retrofit.RetrofitService;

public class OpenWeatherRepos {
    private final String TAG = "OpenWeatherRepository";
    private final static String BASE_URL = "https://api.openweathermap.org/data/2.5/"; 
    private static OpenWeatherRepos instance;
    private Retrofit retrofit;
    private OpenWeatherAPI opwAPI;
    private MutableLiveData<OpenWeather> data;

    public static OpenWeatherRepos getInStance() {
        if(instance == null){
            instance = new OpenWeatherRepos();
        }
        return instance;
    }

    public MutableLiveData<OpenWeather> getWeather() {

        retrofit = new RetrofitService().getRetroInstance(BASE_URL);

        opwAPI = retrofit.create(OpenWeatherAPI.class);

        data = new MutableLiveData<OpenWeather>();
        callWeatherAPI();
        Log.i(TAG,data.getValue().toString()); //error occurs here
        return data;
    }

    private void callWeatherAPI() {

        Call<OpenWeather> call = opwAPI.getWeather("seoul","this is my id","kr");

        call.enqueue(new Callback<OpenWeather>() {
            @Override
            public void onResponse(Call<OpenWeather> call, Response<OpenWeather> response) {
                OpenWeather temp = response.body();
                Log.i(TAG,"API CONNECT SUCCESS");
                if(temp != null){
                    Log.i(TAG,temp.toString());
                    data.setValue(temp);
                }
            }

            @Override
            public void onFailure(Call<OpenWeather> call, Throwable t) {
                Log.d(TAG,"onFailure : "+t.getMessage());
            }
        });
    }

}

这是这段代码的结果

D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: wook.co.weather, PID: 10063
    java.lang.RuntimeException: Unable to start activity ComponentInfo{wook.co.weather/wook.co.weather.view.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String wook.co.weather.models.dto.OpenWeather.toString()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String wook.co.weather.models.dto.OpenWeather.toString()' on a null object reference
        at wook.co.weather.models.repository.OpenWeatherRepos.getWeather(OpenWeatherRepos.java:46)
        at wook.co.weather.viewmodels.MainActivityViewModel.init(MainActivityViewModel.java:25)
        at wook.co.weather.view.MainActivity.onCreate(MainActivity.java:26)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 
I/Process: Sending signal. PID: 10063 SIG: 9

我该如何解决这个问题？？

Answer 1

我相信问题出在这里：

if(temp != null){
    Log.i(TAG,temp.toString());
    data.postValue(temp);
    Log.i(TAG,data.getValue().toString());
}

postValue() 方法不会立即更新 LiveData 的值；而是将任务发布到最终将更新值的主线程。此处的文档：https://developer.android.com/reference/androidx/lifecycle/MutableLiveData#postValue(T)

这意味着调用postValue() 然后立即调用getValue() 将导致您收到“旧”值（在您发布新值之前），因为任务尚未完成并更新值的 LiveData。

在您的情况下，解决方法是简单地删除第二个 Log.i() 调用。您可以对其进行修改以使其正常工作，如下所示：

data.postValue(temp);
OpenWeather value = data.getValue();
if (value != null) {
    Log.i(TAG, value.toString());
}

但这确实没有任何好处；您已经知道 data.getValue() 此时将过时。