android - Interface Null object Reference 通过在 Activity 中实现接口(interface)来与 fragment 通信？

Question

尝试在空对象引用上调用接口(interface)方法。在 Activity 中声明接口(interface)并在 fragment 中实现。当通过单击产生错误的选项卡调用实现的方法时。

Activity

    private  HistoryListener listener;

    public interface HistoryListener{

        public void onSelect();
    }

TabselectedListener Of Activity

    tabLayout.addOnTabSelectedListener(new TabLayout.BaseOnTabSelectedListener() {
                @Override
                public void onTabSelected(TabLayout.Tab tab) {
                    switch(tab.getPosition()) {
                        case 0:
                           break;
                        case 1:
                            break;
                        case 2:
                            Log.d("Tab", "onTabSelected: " +tab.getPosition());
                            listener.onSelect();
                            break;

                    }
                }

                @Override
                public void onTabUnselected(TabLayout.Tab tab) {

                }

                @Override
                public void onTabReselected(TabLayout.Tab tab) {

                }

Fragment

    class Fragment implements Activity. HistoryListener{

          @Override
            public void onSelect() {


            }

        }

Answer 1

你需要在 fragment 中将监听器初始化到 onAttach(Context context) 方法中

 @Override
    public void onAttach(Context context) {
        super.onAttach(activity);
       try{
      listener = (HistoryListener) context;
          } catch(Exception e){
               throw new ClassCastException(context.toString()
                    + " must implemenet HistoryListener.onSelect()");
          }

        } 
    }