使用ViewPager,PagerAdapter和AsyncTask的活动会导致空白视图

Question

我正在尝试构建一个将作为日历日视图的活动.当用户向左或向右滑动时,它们将在明天或昨天进入,依此类推.

我决定使用ViewPager/PagerAdapter处理视图并控制几天的分页.

作为设置日视图的一部分,应用程序将转到我的API并请求当天的任何约会.约会将在当天返回并显示.为了从API中检索数据,我使用的是AsyncTask.所以基本上,instantiateItem()调用API并设置空listView.然后BroadcastReceiver捕获响应,解析数据并显示它.

我遇到的问题是显示的第一个视图始终为空白. 左侧或右侧的视图已填充,如果我在任一方向上移动两个位置,足以使原始视图被销毁,则返回原始视图,它具有数据.为什么？如何在不必移动2次滑动的情况下获取第一个要填充的视图？

这是我的活动.我目前还没有解析从API返回的数据,只是在平均时间内使用字符串列表进行模拟.

public class MyPagerActivity extends Activity {

    private ViewPager myPager;
    private static int viewCount = 1000;
    private Context ctx;
    private MyPagerAdapter myAdapter;

    private static final String tag = "MyPagerActivity";

    private static final String apiAction = "getAppointmentsForDate"; 
    private static final String apiUri = "https://myAPI.com/api.php";
    private static final String resource = "appointments";
    private static final String action = "getappointmentsfordate";
    private static final String date = "20120124";
    private ProgressDialog progress;
    private SharedPreferences loginPreferences;
    private SharedPreferences.Editor loginPreferencesEditor;
    private ListView v;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        ctx = this;

        myAdapter = new MyPagerAdapter();
        myPager = (ViewPager) findViewById(R.id.myPager);
        myPager.setAdapter(myAdapter);
        myPager.setCurrentItem(500);
    }

    private class MyPagerAdapter extends PagerAdapter {


        @Override
        public int getCount() {
            return viewCount;
        }

        @Override
        public Object instantiateItem(View collection, int position) {

            try {
                Log.d(tag, "trying http connection");
                loginPreferences = getSharedPreferences("loginPrefs", MODE_PRIVATE);
                loginPreferencesEditor = loginPreferences.edit();
                String authToken = loginPreferences.getString("authToken", "");
                String staffOrRoomsId = loginPreferences.getString("staffOrRoomsId", "");
                String staffOrRoomsIdName = loginPreferences.getString("staffOrRoomsIdName", "");

                HttpPost apiRequest = new HttpPost(new URI(apiUri));
                List<NameValuePair> parameters = new ArrayList<NameValuePair>();
                parameters.add(new BasicNameValuePair("authToken", authToken));
                parameters.add(new BasicNameValuePair("resource", resource));
                parameters.add(new BasicNameValuePair("action", action));
                parameters.add(new BasicNameValuePair("date", date));
                parameters.add(new BasicNameValuePair(staffOrRoomsIdName, staffOrRoomsId));
                apiRequest.setEntity(new UrlEncodedFormEntity(parameters));

                RestTask task = new RestTask(ctx, apiAction); 
                task.execute(apiRequest); 
                //Display progress to the user 
    //            progress = ProgressDialog.show(ctx, "Searching", "Waiting For Results...", true); 
            } catch (Exception e) { 
                e.printStackTrace(); 
            } 

            Log.d(tag, "Creating another view! Position: " + position);

            myPager.setTag(collection);

            v = new ListView(ctx);
            ((ViewPager) collection).addView(v, 0);
            return v;
        }

        @Override
        public void destroyItem(View collection, int position, Object view) {
            Log.d(tag, "Destroying position: " + position + "!");
            ((ViewPager) collection).removeView((ListView) view);
        }

        @Override
        public boolean isViewFromObject(View view, Object object) {
            return view==((ListView)object);
        }

        @Override
        public void finishUpdate(View arg0) {}

        @Override
        public void restoreState(Parcelable arg0, ClassLoader arg1) {}

        @Override
        public Parcelable saveState() {
            return null;
        }

        @Override
        public void startUpdate(View arg0) {}
    }

    @Override 
    public void onResume() { 
        super.onResume(); 
        registerReceiver(receiver, new IntentFilter(apiAction));  
    } 

    @Override 
    public void onPause() { 
        super.onPause(); 
        unregisterReceiver(receiver); 
    }

    private BroadcastReceiver receiver = new BroadcastReceiver() { 
        @Override 
        public void onReceive(Context context, Intent intent) { 
            //Clear progress indicator 
            if (progress != null) { 
                progress.dismiss(); 
            }

            Log.d(tag, "Broadcast received");

            String[] from = new String[] { "str" };
            int[] to = new int[] { android.R.id.text1 };
            List<Map<String, String>> items = new ArrayList<Map<String, String>>();
            for (int i = 0; i < 20; i++)
            {
                Map<String, String> map = new HashMap<String, String>();
                map.put("str", String.format("Item %d", i + 1));
                items.add(map);
            }
            SimpleAdapter adapter = new SimpleAdapter(ctx, items, android.R.layout.simple_list_item_1, from, to);
            v.setAdapter(adapter);
        } 
    };
}

感谢您抽出宝贵时间来回顾这个问题!

Answer 1

默认情况下，aViewPager不仅加载当前可见的页面，还加载两侧的页面。这是offscreenPageLimit. 最小值为 1。

instantiateItem将为它想要创建的每个页面调用。在 的实现中instantiateItem，您正在启动AsyncTask并为其赋值v。只有最后一次调用instantiateItem（即直到 的最后一页offscreenPageLimit）才会正确分配v，因此最初不会填充第一页。

您不能依赖最后一次调用instantiateItem将您指向当前活动页面。我会切换到为每个页面使用一个 Fragment，它维护自己的 HTTP 请求和ListView.