Question

我有FragmentA和FragmentB，当我回到FragmentA时我在FragmentB中我需要加载最后一个值，但我的savedInstanceState返回总是为null：

public static class PlaceholderFragment extends Fragment {

        private boolean active=false;



        @Override
        public void onSaveInstanceState(Bundle outState) {
            super.onSaveInstanceState(outState);
            outState.putBoolean("active",active);

        }

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);



        }

        @Override
        public void onActivityCreated(Bundle savedInstanceState) {
            super.onActivityCreated(savedInstanceState);

            if(savedInstanceState!=null)
            {

                active=savedInstanceState.getBoolean("active");
                if(attivo==true)
                {
                    Toast.makeText(getActivity(),"True",Toast.LENGTH_SHORT).show();
                }
                else
                {
                   Toast.makeText(getActivity(),"False",Toast.LENGTH_SHORT).show();

                }
            }

        }

传入FragmentB我使用：

getFragmentManager().beginTransaction().replace(R.id.container,new PrefFragment()).addToBackStack("back").commit();

Answer 1

Bundle savedInstanceState将始终在此特定情况下返回null，因为只有在您的片段被android系统销毁并重新创建时才会传入（例如，当屏幕旋转或您的应用因系统资源不足而被销毁时））。 savedInstanceState不用于在应用程序中持久存储数据，如果你想这样做，我建议使用SharedPreferences：

private void saveBooleanToPreferences(String key, boolean bool){
     SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
     SharedPreferences.Editor editor = preferences.edit();
     editor.putBoolean(key, bool);
     editor.apply();
}

private boolean getBooleanFromPreferences(String key){
    SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
    boolean bool = preferences.getBoolean(key, false);
    return bool;
}

因此，您的代码将类似于：

 public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putBoolean("active",active);
        saveBooleanToPreferences("active",active);
    }

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
        }

        @Override
        public void onActivityCreated(Bundle savedInstanceState) {
            super.onActivityCreated(savedInstanceState);

            if(savedInstanceState!=null)
            {

                active=getBooleanFromPreferences("active")
                if(attivo==true)
                {
                    Toast.makeText(getActivity(),"True",Toast.LENGTH_SHORT).show();
                }
                else
                {
                   Toast.makeText(getActivity(),"False",Toast.LENGTH_SHORT).show();

                }
            }

Answer 2

确保包含Fragments的活动不会覆盖onSaveInstanceState（或至少调用super方法）。

Answer 3

使用sharedPreferences将数据从片段B传递到片段A.

Answer 4

错误，如果您在没有处理onBackPressed（）的情况下按下，那么您的应用程序将关闭而不保存任何实例，但如果您按回家，则会发生保存。记得在backPressed上处理，删除它的超级

Android savedInstanceState返回始终为null

4 个答案: