FragmentPagerAdapter加载fragment并使用setUserVisibleHint()处理预加载时遇到的坑,给textview赋值时出现的空指针异常

Posted 2020-08-08 西瓜皮不甜

FragmentPagerAdapter加载fragment并使用setUserVisibleHint()处理预加载时,给textview赋值时出现的空指针异常

public class BaseFragment extends Fragment {
    /*布局*/
    private View view;
    /**/
    boolean isVisible = false;
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        view = setRootView(inflater,container);
        Log.i("包名："+getClass().getName(),"onCreateView");
        return view;

    }
    protected View setRootView(LayoutInflater inflater, @Nullable ViewGroup container){
        return null;
    }
    protected void initWidght(){};
    protected void initData(){};
    /**
     * 解决预加载
     * */
    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        Log.i("包名："+getClass().getName()," setUserVisibleHint  显示"+getUserVisibleHint());
        if (getUserVisibleHint()){
            onVisible();
            isVisible = true;
        }else {
//            onInvisible();
        }
    }
    /**
     *  预加载--显示-加载数据
     * */
    private void onVisible() {
        if (!isVisible){
            initWidght();
        }
    }
    /**
     * 预加载--不显示时
     * */
    private void onInvisible() {
        isVisible = false;
    }
    protected <T extends View>T getView(int resourcesId){
        return (T) view.findViewById(resourcesId);
    }
    /**
     * Toast提示;
     * @param content 提示内容;
     */
    protected void showToast(String content) {
        Toast.makeText(getActivity(), content, Toast.LENGTH_SHORT).show();
    }
}

public class IndexChatFragment extends BaseFragment {
    TextView mTextView;
    @Override
    protected View setRootView(LayoutInflater inflater, @Nullable ViewGroup container) {
        return inflater.inflate(R.layout.fragment_index_chat,container,false);
    }

    @Override
    protected void initWidght() {
        mTextView = getView(R.id.ceshi);
        mTextView.setText("赋值测试");
    }

出现了这个错误

 

测试发现具体问题是：只有加载的第一个Fragment会出现这种状况，后边则可以正常显示

最后打印了生命周期才发现是这样的，setUserVisibleHint是比oncreateView先加载，所以第一个调用setUserVisibleHint时是空

 

onCreateView和setUserVisibleHint都调用onvisible()，两个条件同事满足则加载布局或者数据

view也可以单独换成一个标记，在oncreateView里赋值true