基于ListItem专用变量的ListItem充气布局

Question

我使用一个SimpleCursorAdapter和一个ListView来显示用Loader加载的一些数据。在cursor中，我有一个int从0到3的项目。

我希望这个int等于0-1的项目有一个布局(右对齐，一种颜色)，2-3的项目有另一个布局(左对齐，另一种颜色)。much like a chat app, where sent messages are on the right and received ones are on the left.

有没有一种简单的方法呢？这就像一个开关，0-1我膨胀layout_1，2-3我膨胀layout_2。

编辑:我已经添加了我试图填充的ListFragment的代码。用作开关的int是MyContentProvider.Data.E_TYPE。我不能理解它的诀窍，但也许有人能解释清楚我要写的东西！

   import com.actionbarsherlock.view.Menu;
   import com.actionbarsherlock.view.MenuInflater;
   import com.corsalini.survcontr.MyContentProvider.Data;

   import android.content.ContentResolver;
   import android.content.ContentValues;
   import android.database.Cursor;
   import android.os.Bundle;
   import android.support.v4.app.ListFragment;
   import android.support.v4.content.CursorLoader;
   import android.support.v4.app.LoaderManager;
   import android.support.v4.content.Loader;
   import android.support.v4.widget.CursorAdapter;
   import android.support.v4.widget.SimpleCursorAdapter;
   import android.util.Log;
   import android.view.View;
   import android.widget.ListView;



  public class FragEvents extends ListFragment implements  LoaderManager.LoaderCallbacks<Cursor>{
@Override
public void onPause() {
    allRead();
    super.onPause();

}

private static final int EVENTS_LOADER = 0x02;

// This is the Adapter being used to display the list's data.
SimpleCursorAdapter mAdapter;

// If non-null, this is the current filter the user has provided.
String mCurFilter;

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

    // Give some text to display if there is no data.  In a real
    // application this would come from a resource.
    setEmptyText(this.getString(R.string.perform_event)); 

    // We have a menu item to show in action bar.
    setHasOptionsMenu(true);

    // Create an empty adapter we will use to display the loaded data.
    mAdapter = new SimpleCursorAdapter(getActivity(),
            android.R.layout.simple_list_item_2, null,
            new String[] { MyContentProvider.Data.E_TEXT, MyContentProvider.Data.E_DATE, 
        MyContentProvider.Data.E_NUMBER, MyContentProvider.Data.E_TYPE  },
        new int[] { android.R.id.text1, android.R.id.text2 },
        CursorAdapter.FLAG_REGISTER_CONTENT_OBSERVER);
    setListAdapter(mAdapter);

    // Start out with a progress indicator.
    setListShown(false);

    // Prepare the loader.  Either re-connect with an existing one,
    // or start a new one.
    getActivity().getSupportLoaderManager().initLoader(EVENTS_LOADER, null, this);


}

public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
     inflater.inflate(R.menu.menu_events, menu);  
}



@Override public void onListItemClick(ListView l, View v, int position, long id) {
    //TODO Insert desired behavior here.
    Log.i("FragmentComplexList", "Item clicked: " + id);
}

// These are the Contacts rows that we will retrieve.
static final String[] SUMMARY_PROJECTION = new String[] {
    MyContentProvider.Data.E_ID,
    MyContentProvider.Data.E_DATE,
    MyContentProvider.Data.E_NUMBER,
    MyContentProvider.Data.E_TEXT,
    MyContentProvider.Data.E_TYPE,

};

public Loader<Cursor> onCreateLoader(int id, Bundle args) {


    return new CursorLoader(getActivity(),  MyContentProvider.Data.CONTENT_URI_EVENTS,
            SUMMARY_PROJECTION, null, null,
            Data.E_ID + " DESC");
}

public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
    // Swap the new cursor in.  (The framework will take care of closing the
    // old cursor once we return.)
    mAdapter.swapCursor(data);

    // The list should now be shown.
    if (isResumed()) {
        setListShown(true);
    } else {
        setListShownNoAnimation(true);
    }
}

public void onLoaderReset(Loader<Cursor> loader) {
    // This is called when the last Cursor provided to onLoadFinished()
    // above is about to be closed.  We need to make sure we are no
    // longer using it.
    mAdapter.swapCursor(null);
}

public void deleteEvent(ContentResolver contentResolver,
        long id){
    String selection = Data.E_ID + "=";
    String[] args = {String.valueOf(id)};
    contentResolver.delete(Data.CONTENT_URI_EVENTS, selection, args);
}

public void allRead(){
    ContentResolver contentResolver = getActivity().getContentResolver();
    ContentValues contentValue = new ContentValues();
    contentValue.put(Data.E_NUMBER, Data.RECEIVED_READ);
    String selection= Data.E_TYPE+"=";
    String[] args= {String.valueOf(Data.RECEIVED_UNREAD)};
    contentResolver.update(Data.CONTENT_URI_EVENTS, contentValue, selection, args);
}



   }

编辑:如果我没有弄错，我的最终EventsAdapter (扩展SimpleCursorAdapter)应该如下所示：

    import android.content.Context;
    import android.database.Cursor;
    import android.support.v4.widget.SimpleCursorAdapter;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.TextView;

    public class EventsAdapter extends SimpleCursorAdapter {

private Context localContext;

public EventsAdapter(Context context, int layout, Cursor c, String[] from,
        int[] to, int flags) {
    super(context, layout, c, from, to, flags);

    localContext = context;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {

    Cursor c= getCursor();
    c.moveToPosition(position);
    if(convertView == null)
    {
        LayoutInflater layoutInflator = (LayoutInflater)localContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        switch (getItemViewType(position)){
        case 0:
            convertView = layoutInflator.inflate(R.layout.item_event_0, null);
            break;
        case 1:
            convertView = layoutInflator.inflate(R.layout.item_event_1, null);
            break;
        case 2:
            convertView = layoutInflator.inflate(R.layout.item_event_2, null);
            break;
        case 3:
            convertView = layoutInflator.inflate(R.layout.item_event_3, null);
            break;
        }

    }
    switch (getItemViewType(position)){
    case 0:
        TextView date0=(TextView)convertView.findViewById(R.id.date0);
        TextView text0=(TextView)convertView.findViewById(R.id.text0);
        date0.setText(""+c.getString(c.getColumnIndex(Data.E_DATE)));
        text0.setText(""+c.getString(c.getColumnIndex(Data.E_TEXT)));
        break;
    case 1:
        TextView date1=(TextView)convertView.findViewById(R.id.date1);
        TextView text1=(TextView)convertView.findViewById(R.id.text1);
        date1.setText(""+c.getString(c.getColumnIndex(Data.E_DATE)));
        text1.setText(""+c.getString(c.getColumnIndex(Data.E_TEXT)));
    case 2:
        TextView date2=(TextView)convertView.findViewById(R.id.date2);
        TextView text2=(TextView)convertView.findViewById(R.id.text2);
        date2.setText(""+c.getString(c.getColumnIndex(Data.E_DATE)));
        text2.setText(""+c.getString(c.getColumnIndex(Data.E_TEXT)));
    case 3:
        TextView date3=(TextView)convertView.findViewById(R.id.date3);
        TextView text3=(TextView)convertView.findViewById(R.id.text3);
        date3.setText(""+c.getString(c.getColumnIndex(Data.E_DATE)));
        text3.setText(""+c.getString(c.getColumnIndex(Data.E_TEXT)));
    }
    return convertView;
}

@Override
public int getItemViewType(int position) {
    int type = 0;
    int returnInt = 0;
    Cursor c= getCursor();
    c.moveToPosition(position);
    type= c.getInt(c.getColumnIndex(Data.E_TYPE));
    switch (type){
    case Data.RECEIVED_READ:
        returnInt=3;
    case Data.RECEIVED_UNREAD: 
        returnInt= 2;
    case Data.SENT_COMPLETED:
        returnInt= 1;
    case Data.SENT_PROGRESS:
        returnInt= 0;
    default:
        returnInt=0;
    }
    return returnInt;
}

@Override
public int getViewTypeCount() {
    return 4;
}

    }

Answer 1

在使用ListViews时，特别是像您所描述的那样复杂的应用程序，正确处理视图回收是很重要的。SimpleCursorAdapter的超类BaseAdapter类有几个方法，您可以覆盖这些方法，以便在使用最少资源的情况下达到所需的效果。我以前从未使用过SimpleCursorAdatper，所以在编写本文时考虑到了一个普通的CursorAdapter，但是您可以将其用于任何覆盖BaseAdapter的Adapter类。

Android中的ListViews以一种非常特殊的方式来降低内存成本。当您在ListView中滚动时，移出屏幕的项目的视图被放置在一个小的视图池中。convertView参数取自此池。之所以这样做，是因为将每个列表项视图保留在内存中不能很好地扩展，并且可能很快导致OutOfMemory异常。getView()方法是获取这些视图并为当前列表项配置它们的位置。通常你会有一行代码，看起来像这样：

if(convertView == null)
    convertView = layoutInflator.inflate(R.layout.list_item, null);

在这种情况下，如果convertView不是null，我们知道它之前是膨胀的。我们不想重新膨胀它，因为这是一个代价高昂的操作，并且getView应该在视图显示之前用数据快速填充它们。

现在，在您的案例中，convertView有两个潜在的通货膨胀。我们可以覆盖基类中的两个方法，以确保convertView始终是正确的类型，而不是每次都重新膨胀视图(非常糟糕)，或者使用某种技巧为每个视图使用唯一的资源is (更好，但不理想)。这两种方法是getItemViewCount()和getItemViewType(int position)。

适配器使用getItemViewCount()来确定它应该为列表维护多少视图池。覆盖它很简单，在您的例子中应该是这样的。

@Override
public int getViewTypeCount()
{
    return 2; //Even though you have four cases, there are only 2 view types.
}

在调用getView之前，适配器使用getItemViewType(int position)来决定convertView应该来自哪个池。在这里，您需要一个switch或if/else语句来检查底层数据源是哪种视图类型并返回它。(请注意，根据安卓文档，此处的返回值必须介于0和getViewTypeCount() -1之间，因此在本例中为0或1。)

@Override
public int getItemViewType(int position)
{
    Item item = getItem(position)  //Or however you're getting the data associated with a particular list position
    switch(item.myInt)
    {
         //I simplified this a bit, basically, check your int, if it's the first type, return 0 for your first layout type, else return 1 for your second.
         case(0):
         case(1):
             return 0;
         case(2):
         case(3):
             return 1;
    }
}

现在，最后，我们将修改getView以执行初始layoutInflation，以便您在池中拥有正确的视图。

@Override
public View getView(int position, View convertView, ViewGroup viewParent)
{
    //if convertView is not null, we got a view from the pool, just go on
    if(convertView == null)
    {
        //This means we didn't have a view in the pool to match this view type.  Inflate it and it will be placed in the proper pool when this list item is scrolled off the screen
        if(getItemViewType(position) == 0)
            convertView = layoutInflator.inflate(R.layout.list_item_type1, null);
        else if(getItemViewType(position) == 1)
            convertView = layoutInflator.inflate(R.layout.list_item_type2, null);
    }

    //Populate the view with whatever data you need here

    //And finally....
    return convertView;
}

ListViews和他们的适配器是我在Android中遇到的最复杂的东西之一，但花点时间做好它会极大地提高你的应用程序的性能和用户体验。祝好运!

Answer 2

我认为你只需要在你的getView(int position, View convertView, ViewGroup parent)方法上膨胀正确的布局：

MyItem item = getItem(position);
View vi = convertView;
if(vi == null){
    switch(item.getStatus())
    {
        case 0:
            vi = mInflater.inflate(R.layout.item1, null);
            break;
        case 1:
            vi = mInflater.inflate(R.layout.item2, null);
            break;
        case 2:
            vi = mInflater.inflate(R.layout.item3, null);
            break;
    }
    //set viewholder ...
}else{
    //get viewholder ...
}
// set values to views ...

这是你需要的吗？