我开始学习android并且遇到了问题。在“OnFling”方法中的手势监听器上，我有一个带有 e1 参数的 NullPointerException，但 e2 arg 指向正常。请帮忙。
我的活动代码如下：
public class MainActivity extends Activity {
public dbWorker d = null;
public ArrayList<Row> newsContainer = new ArrayList<Row>();
private static ArrayList<Row> cached = new ArrayList<Row>();
public static int cache_guid = 0;
public static Row selectedItem = null;
private ListView newsList = null;
ListArrayAdapter aa = null;
private static final int SWIPE_MIN_DISTANCE = 120;
private static final int SWIPE_MAX_OFF_PATH = 250;
private static final int SWIPE_THRESHOLD_VELOCITY = 200;
private GestureDetector gestureDetector;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
d = new dbWorker(this);
cache_guid = d.getMaxGuid();
newsList = (ListView) findViewById(R.id.listView1);
aa = new ListArrayAdapter(this, newsContainer);
newsList.setAdapter(aa);
newsList.setClickable(true);
newsList.setBackgroundColor(color.holo_blue_dark);
refreshRssList();
setTitle("Rss Reader/ Main page");
gestureDetector = new GestureDetector(this,new MyGestureDetector(), null, false);
// Set the touch listener for the main view to be our custom gesture listener
newsList.setOnTouchListener(new View.OnTouchListener() {
public boolean onTouch(View v, MotionEvent event) {
if (gestureDetector.onTouchEvent(event)) {
return true;
}
return false;
}
});
newsList.setOnItemClickListener(new OnItemClickListener(){
@Override
public void onItemClick(AdapterView<?> arg0, View arg1, int arg2,
long arg3) {
}
});
}
class MyGestureDetector extends SimpleOnGestureListener {
@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
Intent intent = new Intent(MainActivity.this.getBaseContext(), FavActivity.class);
if (Math.abs(e1.getY() - e2.getY()) > SWIPE_MAX_OFF_PATH) {
return false;
}
/* right to left swipe
if(e1.getX() - e2.getX() > SWIPE_MIN_DISTANCE && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY) {
startActivity(intent);
MainActivity.this.overridePendingTransition(
R.anim.slide_in_right,
R.anim.slide_out_left
);
// right to left swipe
} else */
if (e2.getX() - e1.getX() > SWIPE_MIN_DISTANCE && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY) {
startActivity(intent);
MainActivity.this.overridePendingTransition(
R.anim.slide_in_left,
R.anim.slide_out_right
);
}
return false;
}
// It is necessary to return true from onDown for the onFling event to register
@Override
public boolean onDown(MotionEvent e) {
return true;
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
private void refreshRssList() {
ArrayList<Row> newItems = rssWorker.getNews(this);
ArrayList<Row> for_save = new ArrayList<Row>();
if (dbWorker.readAll() != null)
cached = dbWorker.readAll();
if (newItems == null)
{
Toast.makeText(this, "Connection error!", 3).show();
return;
}
for (int i = 0; i < newItems.size(); i++)
{
Row tmp = newItems.get(i);
cached.add(tmp);
for_save.add(tmp);
}
newsContainer.clear();
if (cached != null)
newsContainer.addAll(cached);
d.SaveAll(for_save);
aa.notifyDataSetChanged();
}
}
和 ArrayAdapter：
包 com.example.rssreader;
public final class ListArrayAdapter extends ArrayAdapter<Row> {
private final Context c;
private final List<Row> items;
public ListArrayAdapter(Context context, List<Row> objects) {
super(context, R.layout.list, objects);
c = context;
items = objects;
}
@Override
public View getView(final int position, View convertView, ViewGroup parent) {
final Row selectedItem = items.get(position);
LayoutInflater inflater = (LayoutInflater) c.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View rowView = inflater.inflate(R.layout.list, parent, false);
TextView newsTitle = (TextView) rowView.findViewById(R.id.titleView);
final ImageView favIcon = (ImageView) rowView.findViewById(R.id.imageView1);
LinearLayout newsLay = (LinearLayout) rowView.findViewById(R.id.listLayout);
Date d = new Date();
d.setTime(selectedItem.pubdate);
SimpleDateFormat sdf = new SimpleDateFormat("MM/dd - hh:mm:ss");
newsTitle.setText(selectedItem.name + " " + sdf.format(d));
if (selectedItem.isRead == 0)
newsLay.setBackgroundColor(Color.BLUE);
else
newsLay.setBackgroundColor(Color.GRAY);
if (selectedItem.isFav == 1)
{
favIcon.setImageResource(R.drawable.ic_tab_name_selected);
} else {
favIcon.setImageResource(R.drawable.ic_tab_name_unselected);
}
favIcon.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v) {
if (selectedItem.isFav == 1){
favIcon.setImageResource(R.drawable.ic_tab_name_unselected);
selectedItem.isFav = 0;
}
else
{
favIcon.setImageResource(R.drawable.ic_tab_name_selected);
selectedItem.isFav = 1;
}
dbWorker.SaveState(selectedItem);
}
});
newsTitle.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v) {
MainActivity.selectedItem = selectedItem;
c.startActivity(new Intent(v.getContext(), ItemActivity.class));
if (selectedItem.isRead != 1)
{
selectedItem.isRead = 1;
dbWorker.SaveState(selectedItem);
}
}
});
return rowView;
}
}