android - Android:自定义SimpleCursorAdapter中的newView和bindView问题

我从the only example I found中的一个创建了一个自定义的SimpleCursorAdapter。

调用ListActivity时，将为我的每个数据库条目调用newView和bindView，并为每个条目再次调用newView和bindView。我有几个问题:

-示例是否正确(如果没有，我可以在哪里找到)？

-如果bindView调用始终在newView调用之前，为什么在两个函数中都这样做呢？

-为什么序列newView-bindView为每个项目调用两次？

-为什么某些CursorAdapter示例使用getView而不是newView和bindView？

基本上，应该如何使用SimpleCursorAdapter，我的代码有什么问题？

谢谢

ListActivity

public class ContactSelection extends ListActivity {

    private WhipemDBAdapter mDbHelper;
    private FriendAdapter friendAdapter;

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

        mDbHelper = new WhipemDBAdapter(this);
        mDbHelper.open();

        setContentView(R.layout.contact_list);

        Cursor c = mDbHelper.fetchAllFriends();
        startManagingCursor(c);
        String[] from = new String[] {};
        int[] to = new int[] {};

        this.friendAdapter = new FriendAdapter(this, R.layout.contact_row, c, from, to);
        setListAdapter(this.friendAdapter);

        getListView().setItemsCanFocus(false);
        getListView().setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
    }

    @Override
    protected void onResume() {
        super.onResume();
        mDbHelper.open();
    }

    @Override
    protected void onPause() {
        super.onPause();
        mDbHelper.close();
    }
}

自定义SimpleCursorAdapter

public class FriendAdapter extends SimpleCursorAdapter implements OnClickListener {

    private Context mContext;
    private int mLayout;

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

        this.mContext = context;
        this.mLayout = layout;
    }

    @Override
    public View newView(Context context, Cursor cursor, ViewGroup parent) {

        Cursor c = getCursor();

        final LayoutInflater inflater = LayoutInflater.from(context);
        View v = inflater.inflate(mLayout, parent, false);

        String name = c.getString(c.getColumnIndex(WhipemDBAdapter.KEY_NAME));
        String fb_id = c.getString(c.getColumnIndex(WhipemDBAdapter.KEY_FB_ID));

        TextView name_text = (TextView) v.findViewById(R.id.contact_name);
        if (name_text != null) {
            name_text.setText(name);
        }

        ImageView im = (ImageView) v.findViewById(R.id.contact_pic);
        Drawable drawable = LoadImageFromWebOperations("http://graph.facebook.com/"+fb_id+"/picture");
        if (im != null) {
            im.setImageDrawable(drawable);
        }

        CheckBox bCheck = (CheckBox) v.findViewById(R.id.checkbox);
        if (im != null) {
            bCheck.setTag(fb_id);
        }

        if (((GlobalVars) mContext.getApplicationContext()).isFriendSelected(fb_id))
            bCheck.setChecked(true);

        bCheck.setOnClickListener(this);

        return v;
    }

    @Override
    public void bindView(View v, Context context, Cursor c) {

         String name = c.getString(c.getColumnIndex(WhipemDBAdapter.KEY_NAME));
         String fb_id = c.getString(c.getColumnIndex(WhipemDBAdapter.KEY_FB_ID));


         TextView name_text = (TextView) v.findViewById(R.id.contact_name);
         if (name_text != null) {
             name_text.setText(name);
         }

        ImageView im = (ImageView) v.findViewById(R.id.contact_pic);
        Drawable drawable = LoadImageFromWebOperations("http://graph.facebook.com/"+fb_id+"/picture");
        if (im != null) {
            im.setImageDrawable(drawable);
        }

        CheckBox bCheck = (CheckBox) v.findViewById(R.id.checkbox);
        if (im != null) {
            bCheck.setTag(fb_id);
        }

        ArrayList<String> dude = ((GlobalVars) mContext.getApplicationContext()).getSelectedFriendList();

        if (((GlobalVars) mContext.getApplicationContext()).isFriendSelected(fb_id))
            bCheck.setChecked(true);

        bCheck.setOnClickListener(this);
    }


    @Override
    public void onClick(View v) {
        CheckBox cBox = (CheckBox) v;
        String fb_id = (String) cBox.getTag();

        if (cBox.isChecked()) {
            if (!((GlobalVars) mContext.getApplicationContext()).isFriendSelected(fb_id))
                ((GlobalVars) mContext.getApplicationContext()).addSelectedFriend(fb_id);
        } else {
            if (((GlobalVars) mContext.getApplicationContext()).isFriendSelected(fb_id))
                ((GlobalVars) mContext.getApplicationContext()).removeSelectedFriend(fb_id);
        }

    }

    private Drawable LoadImageFromWebOperations(String url)
    {
        try
        {
            InputStream is = (InputStream) new URL(url).getContent();
            Drawable d = Drawable.createFromStream(is, "src name");
            return d;
        }catch (Exception e) {
            System.out.println("Exc="+e);
            return null;
        }
    }

}

最佳答案

覆盖getView()函数使您可以“重新使用”已经膨胀的列表项(当您前后滚动列表时，这些列表项是从当前 View 端口“滚动”出来的)。

这样做可以节省大量内存资源和处理器运行时间，因为膨胀是一项非常耗时的操作。对于您重复使用的每个convertView，还可以节省GC运行时间(因为垃圾收集器不必收集特定的列表项)。

您还可以创建一个“ View 集合”类(在下面的示例中为ViewHolder类)，该类将在膨胀列表项中保存每个 View 的引用。这样，您不必每次用新值更新列表项时(通常在滚动列表时)都找到它们。 findViewById()也是一个相当耗时的操作。

另外，我认为您可以缓存更多变量，例如布局膨胀器和列索引。一切都可以节省时间:-)

private final Context mContext;
private final int mLayout;
private final Cursor mCursor;
private final int mNameIndex;
private final int mIdIndex;
private final LayoutInflater mLayoutInflater;

private final class ViewHolder {
    public TextView name;
    public ImageView image;
    public CheckBox checkBox;
}

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

    this.mContext = context;
    this.mLayout = layout;
    this.mCursor = c;
    this.mNameIndex = mCursor.getColumnIndex(WhipemDBAdapter.KEY_NAME);
    this.mIdIndex = mCursor.getColumnIndex(WhipemDBAdapter.KEY_FB_ID);
    this.mLayoutInflater = LayoutInflater.from(mContext);
}

public View getView(int position, View convertView, ViewGroup parent) {
    if (mCursor.moveToPosition(position)) {
        ViewHolder viewHolder;

        if (convertView == null) {
            convertView = mLayoutInflater.inflate(mLayout, null);

            viewHolder = new ViewHolder();
            viewHolder.name = (TextView) convertView.findViewById(R.id.contact_name);
            viewHolder.image = (ImageView) convertView.findViewById(R.id.contact_pic);
            viewHolder.checkBox = (CheckBox) convertView.findViewById(R.id.checkbox);

            convertView.setTag(viewHolder);
        }
        else {
            viewHolder = (ViewHolder) convertView.getTag();
        }

        String name = mCursor.getString(mNameIndex);
        String fb_id = mCursor.getString(mIdIndex);
        Drawable drawable = LoadImageFromWebOperations("http://graph.facebook.com/"+fb_id+"/picture");
        boolean isChecked = ((GlobalVars) mContext.getApplicationContext()).isFriendSelected(fb_id);

        viewHolder.name.setText(name);
        viewHolder.image.setImageDrawable(drawable);
        viewHolder.checkBox.setTag(fb_id);
        viewHolder.checkBox.setChecked(isChecked);
    }

    return convertView;
}