java - 如何从 ViewPager 的 onPageSelected 异步任务到 fragment ？

Question

我是 Android 编程新手。
我用谷歌搜索和 stackoverflow，阅读了 android 文档，但没有任何帮助，或者我可能不太理解。

我正在尝试通过 ViewPager 的 onPageSelected 对 AsyncTask UI 进行分段。

像 AndroMoney 应用程序 ViewPager(在 onPageSelected 之前，它显示“正在加载...”，在 onPageSelected 时，它 AsyncTasked 并加载。):

AndroMoney ViewPager Fragment Screenshoot

但我的代码是:它在滑动时异步执行，有时它会在两个 fragment 之间停止，如下所示:

my code viewpager screenshot

与 AndroMoney 应用程序不同，我的代码不会异步处理 onPageSelected。

这里是 MyFragment.java:

public class MyFragment extends Fragment {
    public static final String EXTRA_MESSAGE = "EXTRA_MESSAGE";
    final StringBuilder buffer=new StringBuilder();
    View v;
    public static final MyFragment newInstance(int message) {
        MyFragment f = new MyFragment();
        Bundle bdl = new Bundle(1);
        bdl.putInt(EXTRA_MESSAGE, message);
        f.setArguments(bdl);
        return f;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        v = inflater.inflate(R.layout.myfragment_layout, container, false);
        new AsyncDatabase().execute();
        return v;
    }

    public class AsyncDatabase extends AsyncTask<Void,Void,Void>{
        @Override
        protected Void doInBackground(Void... params){
            int message = getArguments().getInt(EXTRA_MESSAGE);
            BibleService bService=new BibleServiceImpl(getActivity().getApplication());
            List<Bible> bible=bService.getBibleByChapterNumber("Salm", message);
            for(Bible b: bible){
                for(int i=0;i<10;i++){
                    buffer.append(b);
                }
            }
            return null;
        }
        @Override
        protected void onPostExecute(Void result){
            TextView txt1=(TextView)v.findViewById(R.id.textView1);
            TextView txt2=(TextView)v.findViewById(R.id.textView2);
            txt1.setText("Done");
            txt2.setText(buffer.toString());
        }
        @Override
        protected void onPreExecute(){
            TextView txt1=(TextView)v.findViewById(R.id.textView1);
            txt1.setText("Loading...");
        }
        @Override
        protected void onProgressUpdate(Void... values){

        }
    }


}

这里是 PageViewActivity.java:

public class PageViewActivity extends FragmentActivity{
    MyPageAdapter pageAdapter;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_page_view);

        List<Fragment> fragments = getFragments();

        pageAdapter = new MyPageAdapter(getSupportFragmentManager(), fragments);

        ViewPager pager = (ViewPager) findViewById(R.id.viewpager);
        pager.setOffscreenPageLimit(1);
        pager.setAdapter(pageAdapter);
        pager.setCurrentItem(11);

        pager.setOnPageChangeListener(new OnPageChangeListener() {
            public void onPageScrollStateChanged(int state) {

            }

            public void onPageScrolled(int position, float positionOffset,
                    int positionOffsetPixels) {
            }

            public void onPageSelected(int position) {

            }
        });

    }

    private List<Fragment> getFragments() {
        List<Fragment> fList = new ArrayList<Fragment>();

        for (int j = 2; j < 150; j++) {
            fList.add(MyFragment.newInstance(j));
        }

        return fList;
    }

    private class MyPageAdapter extends FragmentStatePagerAdapter {
        private List<Fragment> fragments;

        public MyPageAdapter(FragmentManager fm, List<Fragment> fragments) {
            super(fm);
            this.fragments = fragments;
        }

        @Override
        public Fragment getItem(int position) {
            return this.fragments.get(position);
        }

        @Override
        public int getCount() {
            return this.fragments.size();
        }

    }

我傻傻的测试了一下，添加了如下代码:

public void onPageSelected(int position) {
                new MyFragment().new AsyncDatabase().execute();
            }

没用。:-P

那么，如何像 AndroMoney 应用那样从 PageViewActivity 的 onPageSelected 异步任务到 fragment 。

Answer 1

在 fragment 中公开一个新方法:

public void executeAsyncTask() {
   new AsyncDatabase().execute();
}

在 Activity 中，使您的 fragment 列表成为最终 fragment ，以便您可以在适配器中访问它:

final List<MyFragment> fragments = getFragments();

然后将您的 onPageSelected() 方法更改为:

public void onPageSelected(int position) {
    fragments.get(position).executeAsyncTask();
}

您还需要更新 MyPagerAdapter 以使用 MyFragment 而不是 Fragment:

private class MyPageAdapter extends FragmentStatePagerAdapter {
    private List<MyFragment> fragments;

    public MyPageAdapter(FragmentManager fm, List<MyFragment> fragments) {
        super(fm);
        this.fragments = fragments;
    }

    @Override
    public Fragment getItem(int position) {
        return this.fragments.get(position);
    }

    @Override
    public int getCount() {
        return this.fragments.size();
    }

}

private List<MyFragment> getFragments() {
    List<MyFragment> fList = new ArrayList<MyFragment>();

    for (int j = 2; j < 150; j++) {
        fList.add(MyFragment.newInstance(j));
    }

    return fList;
}