java - 当 Android fragment onCallBack 带有数据时出现 NullPointerException

Question

我正在尝试在回调时在 fragment 之间传递数据。在我的第一个 fragment 中:

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View v = ...

    categoryList = (ArrayList<Category>) getArguments().getSerializable("categorylist");
    return v;
}

@Override
public void callbackCall(ArrayList<Category> list)
{
    Toast.makeText(getActivity(), "call back",
            Toast.LENGTH_SHORT).show();
}

我的界面:

public interface CategoryListCallBack {
void callbackCall(ArrayList<Category> list);
}

在我成功更新数据库后的第二个 fragment 中，我再次拉取数据以传递回第一个 fragment :

private CategoryListCallBack mcallback;

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = ...

        mcallback = (CategoryListCallBack) getFragmentManager().findFragmentByTag("CategoryListActivity");

return v;
}

final CategoryListActivity categoryFragment = new CategoryListActivity();
    new GetAllCategoriesAsyncTask(
            new GetAllCategoriesAsyncTask.OnRoutineFinished() {
                public void onFinish() {
                    getFragmentManager().popBackStack();

                    /*Bundle bundle = new Bundle();
                    FragmentTransaction it = getFragmentManager().beginTransaction();

                    bundle.putSerializable("categorylist", GetAllCategoriesAsyncTask.categoryList);
                    categoryFragment.setArguments(bundle);
                    it.replace(R.id.frame,categoryFragment);
                    it.addToBackStack("tag");
                    it.commit();*/
                    mcallback.callbackCall(GetAllCategoriesAsyncTask.categoryList);
                }
            }).execute();

但是，我收到此错误消息:

java.lang.NullPointerException: Attempt to invoke interface method 'void Controller.CategoryListCallBack.callbackCall(java.util.ArrayList)' on a null object reference

有什么想法吗？提前致谢。

编辑

当我的 ListView 项 onclick 时，我正在调用第二个 fragment :

Bundle bundle = new Bundle();
            EditCategoryActivity editFragment = new EditCategoryActivity();
            FragmentTransaction et = getFragmentManager().beginTransaction();

            bundle.putSerializable("category", model);
            editFragment.setArguments(bundle);
            et.replace(R.id.frame,editFragment);
            et.addToBackStack("tag");
            et.commit();

在我的第二个 fragment onCreate() 中:

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

    mcallback = (CategoryListCallBack) getFragmentManager().findFragmentByTag("CategoryListActivity");
    //v.setCategoryListCallBack(mcallback);

    // get category details pass from previous intent
    category = (Category) getArguments().getSerializable("category");

    // button onclick insert data to database, then call the async task and call back to first fragment
    return v;
 }

Answer 1

在你的第二个 fragment 中

mcallback = (CategoryListCallBack) getFragmentManager().findFragmentByTag("CategoryListActivity");

此行必须返回 null ，尝试此代码

public void setCategoryListCallBack(CategoryListCallBack categoryListCallBack){
    this.mcallback = categoryListCallBack;
}

从此方法设置接口(interface)引用，并始终对接口(interface)引用进行空检查

final CategoryListActivity categoryFragment = new CategoryListActivity();
new GetAllCategoriesAsyncTask(
    new GetAllCategoriesAsyncTask.OnRoutineFinished() {
    public void onFinish() {
        if(mcallback != null){
            mcallback.callbackCall(getContext());
        }
        getFragmentManager().popBackStack();

            /*Bundle bundle = new Bundle();
            FragmentTransaction it = getFragmentManager().beginTransaction();

            bundle.putSerializable("categorylist", GetAllCategoriesAsyncTask.categoryList);
            categoryFragment.setArguments(bundle);
            it.replace(R.id.frame,categoryFragment);
            it.addToBackStack("tag");
            it.commit();*/

    }
}).execute();

为第二个 fragment 添加监听器，例如:

Bundle bundle = new Bundle();
EditCategoryActivity editFragment = new EditCategoryActivity();
FragmentTransaction et = getFragmentManager().beginTransaction();
editFragment.setCategoryListCallBack(this);
        bundle.putSerializable("category", model);
        editFragment.setArguments(bundle);
        et.replace(R.id.frame,editFragment);
        et.addToBackStack("tag");
        et.commit();

并从onCreatView()

中删除这一行

mcallback = (CategoryListCallBack) getFragmentManager().findFragmentByTag("CategoryListActivity");