这是这个任务的一个实现。
EndlessScrollBaseAdapter.java
package com.example.endlessscrollinbothdirections;
import java.util.Map;
import android.content.Context;
import android.view.Gravity;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AbsListView;
import android.widget.AbsListView.OnScrollListener;
import android.widget.BaseAdapter;
import android.widget.TextView;
/** A child class shall subclass this Adapter and implement method getDataRow(int position,
* View convertView, ViewGroup parent), which supplies a View present data in a ListRow.
* This parent Adapter takes care of displaying ProgressBar in a row or indicating that it
* has reached the last row. */
public abstract class EndlessScrollBaseAdapter<T> extends BaseAdapter implements
OnScrollListener {
private int mVisibleThreshold = 5;
// the main data structure to save loaded data
protected Map<Integer, T> mItems;
protected Context mContext;
// the serverListSize is the total number of items on the server side,
// which should be returned from the web request results
protected int mServerListSize = -1;
// Two view types which will be used to determine whether a row should be displaying
// data or a Progressbar
public static final int VIEW_TYPE_LOADING = 0;
public static final int VIEW_TYPE_ACTIVITY = 1;
public static final int VIRTUAL_MIDDLE_OFFSET = Integer.MAX_VALUE / 2;
public EndlessScrollBaseAdapter(Context context, Map<Integer, T> items) {
mContext = context;
mItems = items;
}
public void setServerListSize(int serverListSize) {
this.mServerListSize = serverListSize;
}
/** disable click events on indicating rows */
@Override
public boolean isEnabled(int position) {
return getItemViewType(position) == EndlessScrollBaseAdapter.VIEW_TYPE_ACTIVITY;
}
/** One type is normal data row, the other type is Progressbar */
@Override
public int getViewTypeCount() {
return 2;
}
/** the size of the List plus one, the one is the last row, which displays a
* Progressbar */
@Override
public int getCount() {
return Integer.MAX_VALUE;
}
/** return the type of the row, the last row indicates the user that the ListView is
* loading more data */
@Override
public int getItemViewType(int position) {
return mItems.containsKey(position
- EndlessScrollBaseAdapter.VIRTUAL_MIDDLE_OFFSET) ? EndlessScrollBaseAdapter.VIEW_TYPE_ACTIVITY
: EndlessScrollBaseAdapter.VIEW_TYPE_LOADING;
}
@Override
public T getItem(int position) {
return mItems.get(position - EndlessScrollBaseAdapter.VIRTUAL_MIDDLE_OFFSET);
}
@Override
public long getItemId(int position) {
return position;
}
/** returns the correct view */
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if (getItemViewType(position) == EndlessScrollBaseAdapter.VIEW_TYPE_LOADING) {
return getFooterView(position, convertView, parent);
}
return getDataRow(position, convertView, parent);
};
/** A subclass should override this method to supply the data row.
*
* @param position
* @param convertView
* @param parent
* @return */
public abstract View getDataRow(int position, View convertView, ViewGroup parent);
/** returns a View to be displayed in the last row.
*
* @param position
* @param convertView
* @param parent
* @return */
public View getFooterView(int position, View convertView, ViewGroup parent) {
if (position >= mServerListSize && mServerListSize > 0) {
// the ListView has reached the last row
TextView tvLastRow = new TextView(mContext);
tvLastRow.setHint("Reached the last row.");
tvLastRow.setGravity(Gravity.CENTER);
return tvLastRow;
} else {
TextView tvLastRow = new TextView(mContext);
tvLastRow.setHint("Loading...\n position: " + position);
tvLastRow.setGravity(Gravity.CENTER);
return tvLastRow;
}
}
// Defines the process for actually loading more data based on page
public abstract void onLoadMore(int virtualPosition);
@Override
public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount,
int totalItemCount) {
for (int i = -mVisibleThreshold; i < visibleItemCount + mVisibleThreshold; i++) {
int virtualPosition = firstVisibleItem
- EndlessScrollBaseAdapter.VIRTUAL_MIDDLE_OFFSET + i;
onLoadMore(virtualPosition);
}
}
@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
}
}
EndlessScrollAdapter.java
package com.example.endlessscrollinbothdirections;
import java.util.Map;
import android.app.Activity;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
public class EndlessScrollAdapter extends EndlessScrollBaseAdapter<Integer> {
public EndlessScrollAdapter(Activity activity, Map<Integer, Integer> list) {
super(activity, list);
}
@Override
public View getDataRow(int position, View convertView, ViewGroup parent) {
TextView TextView;
if (convertView == null) {
TextView = new TextView(mContext);
} else {
TextView = (TextView) convertView;
}
TextView.setText("virtualPosition: "
+ (position - EndlessScrollBaseAdapter.VIRTUAL_MIDDLE_OFFSET) + "\n"
+ "row data: "
+ mItems.get(position - EndlessScrollBaseAdapter.VIRTUAL_MIDDLE_OFFSET));
return TextView;
}
@Override
public void onLoadMore(int virtualPosition) {
// here you might launch an AsyncTask instead
if (!mItems.containsKey(virtualPosition)) {
mItems.put(virtualPosition, virtualPosition);
notifyDataSetChanged();
}
}
}
MainActivity.java
package com.example.endlessscrollinbothdirections;
import java.util.HashMap;
import java.util.Map;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.widget.ListView;
public class MainActivity extends ActionBarActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ListView listView = (ListView) findViewById(R.id.lvItems);
Map<Integer, Integer> items = new HashMap<Integer, Integer>();
EndlessScrollAdapter endlessScrollAdapter = new EndlessScrollAdapter(this, items);
listView.setAdapter(endlessScrollAdapter);
listView.setSelection(EndlessScrollBaseAdapter.VIRTUAL_MIDDLE_OFFSET);
listView.setOnScrollListener(endlessScrollAdapter);
}
}
activity_main.xml
<ListView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/lvItems"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
</ListView>