使用ViewPager，PagerAdapter和AsyncTask的活动引起空白视图 [英] Activity using ViewPager, PagerAdapter and AsyncTask causes a blank view

问题描述

我试图建立，将作为一个日历日视图的活动。当向左或向右用户挥笔，他们会通过日历到明天或者昨天等等。

I'm trying to build an Activity that will serve as a Calendar Day View. When the user swipes left or right, they'll go to tomorrow or yesterday and so on through the calendar.

我决定用ViewPager / PagerAdapter处理意见，并通过天控制分页。

I decided to use ViewPager/PagerAdapter to handle the views and control the paging through days.

由于设立日视图的一部分，该应用程序会去我的API，并要求任何约会的那一天。该任命将返回并显示出来的那一天。对于从API获取数据，我使用的是AsyncTask的。所以基本上， instantiateItem（）调用API，并设置了空的ListView。然后的BroadcastReceiver 捕获响应，分析数据并显示出来。

As part of setting up the Day View, the app will go to my API and request any appointments for that day. The appointments will be returned and displayed for that day. For retrieving data from the API, I'm using an AsyncTask. So basically, instantiateItem() calls the API and sets up the empty listView. Then BroadcastReceiver catches the response, parses the data and displays it.

我遇到的问题是，所显示的第一个视图总是空。的意见向左或向右，填充，如果我走在两个方向上的两个位置，足以让原以被摧毁，然后再回到原来的观点，有数据。 为什么？怎么办得到的第一个视图进行填充，而不必移动超过2挥笔？

The problem I'm having is that the first view displayed is always blank. The views to the left or right, are populated and if I go two positions in either direction, enough for the original view to get destroyed, then go back to the original view, it has data. Why? How to do get the first view to be populated without having to move over 2 swipes?

下面是我的活动。我目前没有解析数据从API回来呢，只需用字符串的平均时间列表模拟。

Here's my activity. I'm not currently parsing the data coming back from the API yet, just simulating with a list of strings in the mean time.

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);
        } 
    };
}

感谢您抽出宝贵的时间来审查这个问题！

Thanks for taking the time to review this question!

推荐答案

默认情况下， ViewPager 不仅只加载当前可见的页面，而且页面要么侧。这是 offscreenPageLimit 。的最小值为1

By default, a ViewPager not only loads only the currently visible page, but also the page to either side. This is the offscreenPageLimit. The minimum value is 1.

instantiateItem 将呼吁每一页就是了创建。在您的实现 instantiateItem ，你已经开始了的AsyncTask 并分配一个值 v 。只有到了最后一个电话 instantiateItem （即高达最后一页 offscreenPageLimit ）将<$ C的正确分配$ C> v ，所以在第一页就不会被最初填充。

instantiateItem will be called for each page it wants to create. In your implementation of instantiateItem, you are starting an AsyncTask and assigning a value to v. Only the last call to instantiateItem (i.e. the last page upto the offscreenPageLimit) will have the correct assignment of v, and so the first page will not be populated initially.

您不能依靠 instantiateItem 最后一次给你打电话指向当前活动的页面。我会切换到使用一个片段的每一页，维护自己的HTTP请求和的ListView 。

You cannot rely on the last call to instantiateItem to point you to the currently active page. I would switch to using a Fragment for each page, which maintains its own HTTP request and ListView.

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