Transformations.map()返回空值
Transformations.map()返回空值
提问于 2020-07-18 12:12:44
我正在编写一个程序，其中我有两个LiveData对象，如下所示：
public LiveData<placeImages> chosenImages;
public LiveData<List<Bitmap>> chosenBitmaps= Transformations.map(chosenImages, (placeimags) ->
{
ArrayList<Bitmap> bitmaps=new ArrayList<Bitmap>();
for(PhotoI photoI:placeimags.images)
{
try {
Bitmap b=getThumbnail(app.getContentResolver(),photoI.imageURI);
if(b!=null) {
bitmaps.add(b);
}
} catch (Exception e) {
e.printStackTrace();
}
}
return bitmaps;
});问题是，每当我观察到chosenImages LiveData时，它都能正常工作，但是当我观察chosenBitmaps LiveData时，它会返回以下错误：
2020-07-18 17:40:39.790 32164-32164/com.nisarg.locsav E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.nisarg.locsav, PID: 32164
java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.lifecycle.LiveData.observeForever(androidx.lifecycle.Observer)' on a null object reference
at androidx.lifecycle.MediatorLiveData$Source.plug(MediatorLiveData.java:141)
at androidx.lifecycle.MediatorLiveData.onActive(MediatorLiveData.java:118)
at androidx.lifecycle.LiveData$ObserverWrapper.activeStateChanged(LiveData.java:437)
at androidx.lifecycle.LiveData$LifecycleBoundObserver.onStateChanged(LiveData.java:395)
at androidx.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:361)
at androidx.lifecycle.LifecycleRegistry.forwardPass(LifecycleRegistry.java:300)
at androidx.lifecycle.LifecycleRegistry.sync(LifecycleRegistry.java:339)
at androidx.lifecycle.LifecycleRegistry.moveToState(LifecycleRegistry.java:145)
at androidx.lifecycle.LifecycleRegistry.handleLifecycleEvent(LifecycleRegistry.java:131)
at androidx.fragment.app.FragmentViewLifecycleOwner.handleLifecycleEvent(FragmentViewLifecycleOwner.java:51)
at androidx.fragment.app.Fragment.performStart(Fragment.java:2737)
at androidx.fragment.app.FragmentStateManager.start(FragmentStateManager.java:365)
at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1194)
at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2224)
at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1997)
at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1953)
at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:1849)
at androidx.fragment.app.FragmentManager$4.run(FragmentManager.java:413)
at android.os.Handler.handleCallback(Handler.java:794)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:176)
at android.app.ActivityThread.main(ActivityThread.java:6635)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)我认为Transformations.map()正在返回空值。我以前在一个项目中做过类似的事情，在我使用Transformations.map()时也发生了同样的事情。有人知道为什么会这样吗？
回答 2
Stack Overflow用户
发布于 2021-12-24 17:31:58
想必在MutableLiveData后面有一个chosenImages
当该MutableLiveData未通过使用noargs构造函数new MutableLiveData<placeImages>()初始化时发生了什么，而Transformations.map()派生的LiveData也没有初始化，因此产生了null。
解决方案是在创建时初始化MutableLiveData，即使它被初始化为null：new MutableLiveData<placeImages>(null)，以便派生也被初始化。
Stack Overflow用户
发布于 2021-02-23 18:51:05
将public LiveData<placeImages> chosenImages;更改为
public LiveData<placeImages> chosenImages = MutableLiveData()
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/62968431
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