如何在ViewPager中使用cursorLoader？

Question

I have a `cursorLoader` which is working fine.

问题是我没有像我应该那样使用它，我将数据从cursorLoader加载到arrayLists然后使用列表。

我发现本教程展示了如何将cursorLoader与viewPager一起使用，但我不了解如何实现这一目标。

  

http://tumble.mlcastle.net/post/25875136857/bridging-cursorloaders-and-viewpagers-on-android

我有一个看起来像这样的片段：

public class FirstFragment extends Fragment {
    MapView mapView;
    GoogleMap map;
    // Store instance variables
    private String email,about,imagepath,latitude,longitude;
    Button getDirections;

    // newInstance constructor for creating fragment with arguments
    public static FirstFragment newInstance(String email,String about,String imagepath, String latitude, String longitude) {
        FirstFragment fragmentFirst = new FirstFragment();
        Bundle args = new Bundle();
        args.putString("email", email);
        args.putString("about", about);
        args.putString("imagepath", imagepath);
        args.putString("latitude", latitude);
        args.putString("longitude", longitude);
        fragmentFirst.setArguments(args);
        return fragmentFirst;
    }

    // Store instance variables based on arguments passed
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        email = getArguments().getString("email");
        about = getArguments().getString("about");
        imagepath = getArguments().getString("imagepath");
        latitude = getArguments().getString("latitude");
        longitude = getArguments().getString("longitude");
    }

    // Inflate the view for the fragment based on layout XML
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             final Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.zzfragment_pager_items, container, false);
        ImageView imageView = (ImageView) view.findViewById(R.id.listpager_imageView);
        TextView about = (TextView) view.findViewById(R.id.listpager_text);
        TextView emaill = (TextView) view.findViewById(R.id.listpager_title);
        about.setText(this.about);
        emaill.setText(this.email);

        ImageLoader imageLoader = ImageLoader.getInstance();
        DisplayImageOptions options = new DisplayImageOptions.Builder().cacheInMemory(true)
                .cacheOnDisc(true).resetViewBeforeLoading(true)
                .considerExifParams(true)
                .build();
        imageLoader.getInstance().displayImage(imagepath, imageView, options);

        getDirections = (Button) view.findViewById(R.id.getdirections);

        getDirections.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                String strUri = "http://maps.google.com/maps?q=loc:" + latitude + "," + longitude + " (" + email + ")";
                Intent mapIntent = new Intent(android.content.Intent.ACTION_VIEW, Uri.parse(strUri));

                mapIntent.setClassName("com.google.android.apps.maps", "com.google.android.maps.MapsActivity");

                getActivity().startActivity(mapIntent);

            }
        });

//        View v = inflater.inflate(R.layout.listviewtopager, container, false);
        // Gets the MapView from the XML layout and creates it
        mapView = (MapView) view.findViewById(R.id.mapview);
        mapView.onCreate(savedInstanceState);

        // Gets to GoogleMap from the MapView and does initialization stuff
        map = mapView.getMap();
        map.getUiSettings().setMyLocationButtonEnabled(false);
        map.setMyLocationEnabled(true);

        // Needs to call MapsInitializer before doing any CameraUpdateFactory calls
        MapsInitializer.initialize(this.getActivity());

        // Updates the location and zoom of the MapView
        CameraUpdate cameraUpdate = CameraUpdateFactory.newLatLngZoom(new LatLng(Double.parseDouble(latitude), Double.parseDouble(longitude)), 10);
        map.animateCamera(cameraUpdate);

        return view;
    }

    @Override
    public void onResume() {
        mapView.onResume();
        super.onResume();
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        mapView.onDestroy();
    }

    @Override
    public void onLowMemory() {
        super.onLowMemory();
        mapView.onLowMemory();
    }
}

我在这堂课中称呼它：

public class ViewPagerFragment extends FragmentActivity implements
        LoaderManager.LoaderCallbacks<Cursor>{

    ArrayList<String> e

mail = new ArrayList<String>();
ArrayList<String> about = new ArrayList<String>();

ArrayList<String> imagepath = new ArrayList<String>();

ArrayList<String> latitude = new ArrayList<String>();

ArrayList<String> longitude = new ArrayList<String>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_view_pager_fragment);
    getLoaderManager().initLoader(1, null, this);
}

private SmartFragmentStatePagerAdapter adapterViewPager;

// Extend from SmartFragmentStatePagerAdapter now instead for more dynamic ViewPager items
public static class MyPagerAdapter extends SmartFragmentStatePagerAdapter {
    private final ArrayList<String> email;
    private final ArrayList<String> about;
    private final ArrayList<String> imagepath;
    private final ArrayList<String> latitude;
    private final ArrayList<String> longitude;
    private int listPosition;

    public MyPagerAdapter(FragmentManager fragmentManager,ArrayList<String> email,ArrayList<String> about, ArrayList<String> imagepath,ArrayList<String> latitude,ArrayList<String> longitude,int lPosition) {
        super(fragmentManager);
        this.imagepath=imagepath;
        this.email=email;
        this.about = about;
        this.latitude= latitude;
        this.longitude = longitude;
        listPosition = lPosition;
    }

// Returns total number of pages
@Override
public int getCount() {
    return email.size();
}



 // Returns the fragment to display for that page
    @Override
        public Fragment getItem(int position) {
//            return FirstFragment.newInstance(listPosition+position, email.get(listPosition+position));
            return FirstFragment.newInstance(email.get(position), about.get(position),imagepath.get(position),latitude.get(position),longitude.get(position));
    }

    // Returns the page title for the top indicator
    @Override
    public CharSequence getPageTitle(int position) {
        return "Page " + position;
    }

}

@Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
    String[] projection =
            {
                    ActivitiesTable.KEY_EMAIL,
                    ActivitiesTable.KEY_ABOUT,
                    ActivitiesTable.KEY_IMAGEPATH,
                    ActivitiesTable.KEY_LATITUDE,
                    ActivitiesTable.KEY_LONGITUTDE

            };
    CursorLoader cursorLoader = new CursorLoader(this, NameContentProvider.NAME_ACTIVITIES_URI, projection,
            null, null, null);
    return cursorLoader;
}

@Override
public void onLoadFinished(android.content.Loader<Cursor> loader, Cursor cursor) {
    if (cursor != null && cursor.getCount() > 0) {
        cursor.moveToFirst();
        do {
            email.add(cursor.getString(cursor
                    .getColumnIndexOrThrow("email")));
            about.add(cursor.getString(cursor
                    .getColumnIndexOrThrow("about")));
            imagepath.add(cursor.getString(cursor
                    .getColumnIndexOrThrow("imagepath")));
            latitude.add(cursor.getString(cursor
                    .getColumnIndexOrThrow("serverLatitude")));
            longitude.add(cursor.getString(cursor
                    .getColumnIndexOrThrow("serverLongitude")));
        } while (cursor.moveToNext());
    }

    int listPosition = getIntent().getExtras().getInt("position");


    ViewPager vpPager = (ViewPager) findViewById(R.id.vpPager);
    adapterViewPager = new MyPagerAdapter(getSupportFragmentManager(),email,about,imagepath,latitude,longitude,listPosition);
    vpPager.setAdapter(adapterViewPager);
    vpPager.setCurrentItem(listPosition);

}

@Override
public void onLoaderReset(android.content.Loader<Cursor> loader) {



   }
}

如何通过使用该教程修改代码以直接将viewPager与加载器一起使用，而不是将所有内容存储在列表中然后使用列表？

Answer 1

我无法评论，所以我正在写答案..

您有一个实施LoaderCallbacks<Cursor>的活动。加载数据后，您的活动会收到onLoadFinished回调。在此方法中，您有一个Cursor应该显示在ViewPager。

中

要显示Cursor中的数据，请在适配器上调用swapCursor方法。因此，每次加载数据时都不要创建适配器。创建一次，然后只需在其上调用swapCursor即可。

另外，每次都找不到ViewPager - findViewById是一个繁重的操作，应该在创建视图层次结构后执行。

因此，您的onLoadFinished将如下所示：

@Override
public void onLoadFinished(android.content.Loader<Cursor> loader, Cursor cursor) {
    if (adapterViewPager == null) {
        adapterViewPager = new MyPagerAdapter(getSupportFragmentManager(), cursor);
        vpPager.setAdapter(adapterViewPager);
    } else {
        adapterViewPager.swapCursor(cursor);
    }
}

Answer 2

如果我在你身边，我会做的第一件事就是创建一个模型类，它将保存光标中包含的信息。

public class Information implements Parcelable {

   public String imagepath;
   public String email;
   public String about;
   public String latitude;
   public String longitude;


protected Information(Parcel in) {
    imagepath = in.readString();
    email = in.readString();
    about = in.readString();
    latitude = in.readString();
    longitude = in.readString();
}

@Override
public int describeContents() {
    return 0;
}

@Override
public void writeToParcel(Parcel dest, int flags) {
    dest.writeString(imagepath);
    dest.writeString(email);
    dest.writeString(about);
    dest.writeString(latitude);
    dest.writeString(longitude);
}

@SuppressWarnings("unused")
public static final Parcelable.Creator<Information> CREATOR = new Parcelable.Creator<Information>() {
    @Override
    public Information createFromParcel(Parcel in) {
        return new Information(in);
    }

    @Override
    public Information[] newArray(int size) {
        return new Information[size];
    }
};
} 

这已经是其中的大部分了。我要做的第二件事是创建两个辅助方法，一个用于创建此类的实例，从Cursor读取数据，另一个用于创建集合。

 public static Information createInfoFromCursor(Cursor c) {
         Information info = new Information();
         info.email = cursor.getString(cursor
                    .getColumnIndexOrThrow("email")));
         info.about = cursor.getString(cursor
                    .getColumnIndexOrThrow("about")));
         info.imagepath =(cursor.getString(cursor
                    .getColumnIndexOrThrow("imagepath")));
         info.latitude = (cursor.getString(cursor
                    .getColumnIndexOrThrow("serverLatitude")));
         info.longitude = (cursor.getString(cursor
                    .getColumnIndexOrThrow("serverLongitude")));
         return info;
  } 

和

 public static ArrayList<Information> createInfoListFromCursor(Cursor c) {
    ArrayList<Information> info = new ArrayList<>();
    while(c.moveToNext()) {
        info.add(createInfoFromCursor());
    }
    return info;
 }

现在您可以决定向适配器提供Cursor或ArrayList<Information>。如果您决定通过Cursor，请在适配器中使用

Cursor cursor;

 public MyPagerAdapter(FragmentManager fragmentManager,Cursor c) {
    super(fragmentManager);
    cursor = c;
}


@Override
public Fragment getItem(int position) {
   cursor.moveAtPosition(position);
   Information info = createInfoFromCursor(cursor);
   // create a version of `newInstance` that takes an Information object
   return FirstFragment.newInstance(info);
}

@Override
public int getCount() {
  return cursor == null ? 0 : cursor.getCount();
}      

public Cursor swapCursor(Cursor newCursor) {
    if (newCursor == cursor) {
        return null;
    }
    cursor = newCursor;
    if (newCursor != null) {
        notifyDataSetChanged();
    } 
    return oldCursor;
}

在中，创建一个Adapter实例，传递一个空游标，并将其保存为成员classe。调用onLoadFinished时，使用该引用调用swapCursor，刷新适配器的数据集

Answer 3

不是保留5个数据数组，而是将光标保留在视图寻呼机适配器中，并在需要时从光标加载数据。这样，如果光标中有很多项目，UI就不会阻止。

此外，您还可以添加一个交换光标的方法，并通知数据已更改，并且每次都会刷新viewpager，而不是重新创建和附加适配器（与您提到的链接完全相同）

MyPagerAdapter应如下所示：

Cursor cursor;

// Returns total number of pages 
@Override 
public int getCount() { 
    return cursor==null?0:cursor.getCount();
} 

// Returns the fragment to display for that page 
@Override 
public Fragment getItem(int position) {
    // position the cursor first
    cursor.moveToPosition(position);

    // read each field
    String email = cursor.getString(cursor
                    .getColumnIndexOrThrow("email"));
    ...
    return FirstFragment.newInstance(email, ...);
} 

public void swapCursor(Cursor cursor) {
    this.cursor = cursor;
    // notify that data has changed and viewpager needs to be refreshed
    notifyDataSetChanged();
}

重置装载机后不要忘记清洁

@Override 
public void onLoaderReset(android.content.Loader<Cursor> loader) {
    adapterViewPager.swapCursor(null) 
}

Answer 4

如果您正在寻找一个如何使用Cursor中的Fragments填充ViewPager的简单示例，请查看我为FOSDEM Companion应用程序编写的this Activity代码。

注意：您应该避免在ViewPager中放置地图，因为MapView会拦截触摸事件，用户将无法滑动到下一页或上一页。