BottomSheetListener 即使在下面的 onAttach 中初始化后也会抛出空指针异常

Question

我正在创建一个模态底部 sheet，它将从用户那里获取过滤条件。我想将它与 Fragment (HomeFragment) 一起使用，并通过在我的 BottomSheetFragment 接口中的 onButtonClicked 方法中传递字符串值并在我的 HomeFragment 中获取这些值来获取从我的 HomeFragment 中的 BottomSheet 选择的按钮。即使在 onAttach 中初始化它之后，当我在 Log.i.

中显示它的状态时，它也会显示 null

这是我的模态底部sheet class:

public class StartupSearchBottomSheet extends BottomSheetDialogFragment {
    private StartupSearchBottomSheetListener sbsListener;
    Button show;
    String checkedCatButton, checkedSortButton;

    public View onCreateView(LayoutInflater inflater,  @Nullable ViewGroup container, @Nullable  Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.startupsearch_bottomsheet, container, false);

        show = v.findViewById(R.id.show);

        checkedCatButton = "Technology";
        checkedSortButton = "companyName";

        show.setOnClickListener(v -> {
            Log.i(TAG, "432: " + sbsListener); //shows me null here
            sbsListener.onButtonClicked(checkedCatButton, checkedSortButton);
            dismiss();
        });
        return v;
    }
    
    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        try{
            Log.i(TAG,  "431: onAttach invoked");
            sbsListener = (StartupSearchBottomSheetListener) getParentFragment();//initialized my listener here
        }catch(ClassCastException e){
            throw new ClassCastException(context + " must implement StartupSearchBottomSheetListener.");
        }
    }
}

这是我遇到的错误：

2022-04-03 21:38:25.330 4465-4465/com.phtlearning.nivesh E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.phtlearning.nivesh, PID: 4465
    java.lang.NullPointerException: Attempt to invoke interface method 'void com.phtlearning.nivesh.StartupSearchBottomSheet$StartupSearchBottomSheetListener.onButtonClicked(java.lang.String, java.lang.String)' on a null object reference
        at com.phtlearning.nivesh.StartupSearchBottomSheet.onClick(StartupSearchBottomSheet.java:183)
        at android.view.View.performClick(View.java:7575)
        at android.view.View.performClickInternal(View.java:7548)
        at android.view.View.access00(View.java:837)
        at android.view.View$PerformClick.run(View.java:28933)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:236)
        at android.app.ActivityThread.main(ActivityThread.java:8057)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:620)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1011)

Answer 1

听起来您的片段嵌套不正确，因此没有父片段。确保在添加时使用 getChildFragmentManager 而不是 getSupportFragmentManager，否则 getParentFragment() 将 return null.

像这样添加空检查也是一个好主意

sbsListener = (StartupSearchBottomSheetListener) getParentFragment();
if( sbsListener == null ) {
    throw new RuntimeException("Could not get listener");
}

因此，如果您期望 non-null 的变量为空，您会立即知道。

BottomSheetListener 即使在下面的 onAttach 中初始化后也会抛出空指针异常

BottomSheetListener throws null pointer exception even after initializing it below in onAttach

java

android

android-fragments

android-bottomsheetdialog