Android，如何从 FragmentActivty 重新启动/刷新片段？

Android, How to restart/refresh a fragment from FragmentActivty?

提问by Hesam

In my FragmentActivity I have a fragment. This fragment loads data from server. I want we user clicks on refresh button then I call a method in fragment and do refreshing process.

在我的 FragmentActivity 我有一个片段。此片段从服务器加载数据。我希望我们用户点击刷新按钮，然后我在片段中调用一个方法并进行刷新过程。

I created an interface in FragmentActivity and set its variable when user clicks on refresh button. The code is like this:

我在 FragmentActivity 中创建了一个界面，并在用户单击刷新按钮时设置其变量。代码是这样的：

public class MainScreen extends FragmentActivity {
    public interface OnRefreshSelected {
        public void refreshFragment(boolean flag);
    }

    private OnRefreshSelected onRefreshSelected;

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

            Log.i(TAG, "Try to create MainScreen...");

            setContentView(R.layout.activity_main);

        onRefreshSelected = (OnRefreshSelected) MainScreen.this;
    }

     @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            getMenuInflater().inflate(R.menu.activity_main, menu);
            return true;
        }

        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            switch (item.getItemId()) {
                    case R.id.menu_refresh:
                        onRefreshSelected.refreshFragment(true);
                        return true;
                    default:
                        return super.onOptionsItemSelected(item);
        }
    }
}

And in Fragment class i implemented this interface:

在 Fragment 类中，我实现了这个接口：

public class CategoryFragment extends Fragment implements OnRefreshSelected {
    @Override
    public void refreshFragment(boolean flag) {
        Log.i(TAG, "refresh requested. Try to reload data for this fragment...");

        getData();
    }
} 

When I run, the application crashes and logcat shows this message:

当我运行时，应用程序崩溃并且 logcat 显示此消息：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.astro.recipe.activities/com.astro.recipe.activities.MainScreen}: java.lang.ClassCastException: com.astro.recipe.activities.MainScreen

and points to this line:

并指向这一行：

onRefreshSelected = (OnRefreshSelected) MainScreen.this;

What is the best way to have access to a method of fragment from its host? any suggestion would be appreciated. Thanks

从其主机访问片段方法的最佳方法是什么？任何建议将不胜感激。谢谢

回答by biegleux

Executing this

执行这个

onRefreshSelected = (OnRefreshSelected) MainScreen.this;

you are assigning your activity in onRefreshSelectedfield, trying to cast it to your interface, but your activity doesn't implement the interface, that's why ClassCastExceptionraised.

您正在onRefreshSelected现场分配您的活动，试图将其投射到您的界面，但您的活动没有实现该界面，这就是为什么ClassCastException提出。

Instead use something like this

而是使用这样的东西

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
                case R.id.menu_refresh:
                    CategoryFragment fragment = (CategoryFragment) getFragmentManager()
                        .findFragmentById(R.id.category_fragment);
                    if (fragment != null) {
                        fragment.getData();
                    }
                    return true;
                default:
                    return super.onOptionsItemSelected(item);
    }

Your activity can call methods in the fragment by acquiring a reference to the Fragment from FragmentManager, using findFragmentById()or findFragmentByTag().

您的活动可以通过使用findFragmentById()或findFragmentByTag()从 FragmentManager 获取对 Fragment 的引用来调用 Fragment 中的方法。

回答by Pradeep

I agree with biegleux... But my suggestion is

我同意biegleux...但我的建议是

public class CategoryFragment extends Fragment{

   private static CategoryFragment categoryFrag;

    private CategoryFragment(){}

    public static CategoryFragment getInstance(){
        if(null == categoryFrag){
            categoryFrag = new CategoryFragment();
        }

        return categoryFrag;
    }

    public void  getData(){
       /*** Screen Refresh code****/
    }
} 

I your active when he selects option

当他选择选项时，我是活跃的

@Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
                case R.id.menu_refresh:
                    CategoryFragment fragment = CategoryFragment.getInstance();
                     fragment.getData();

                    return true;
                default:
                    return super.onOptionsItemSelected(item);
    }