使用ListView的事件滚动加载更多数据时出错

Question

I have problem with scroll of listview. I have a class like:

private int itemsPerPage = 6;
private int count = 0;
private boolean loadingMore = false;
private ArrayList<Item> item;
private ListItemAdapter adapter;
View footerView;
private JSONArray array;
private String json;

protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.menu);
    lvListItem = (ListView) findViewById(R.id.lvListItem);
    item = new ArrayList<Item>();
    footerView = ((LayoutInflater)this.getSystemService(Context.LAYOUT_INFLATER_SERVICE)).inflate(R.layout.list_footer, null, false);

    json = new JSONParse()
                    .getData("....");
    array = new JSONArray(json);

    load();
    adapter = new ListItemAdapter(this, 0, item);
    lvListItem.addFooterView(footerView);
    lvListItem.setAdapter(adapter);

    lvListItem.setOnScrollListener(new OnScrollListener() {

        public void onScrollStateChanged(AbsListView view, int scrollState) {
            // TODO Auto-generated method stub

        }

        public void onScroll(AbsListView view, int firstVisibleItem,
                int visibleItemCount, int totalItemCount) {
            // TODO Auto-generated method stub
            int lastInScreen = firstVisibleItem + visibleItemCount;

            // is the bottom item visible & not loading more already ? Load
            // more !
            if ((lastInScreen == totalItemCount) && !(loadingMore)) {
                Thread thread = new Thread(null, loadData);
                thread.start();
            }
        }
    });
    }

public void load(){
for (int i = 0; i < itemsPerPage; i++) {
    if (count == array.length()) {
        lvListItem.removeFooterView(footerView);
        break;
    }
    /*****Code to load data*******/
}
}


private Runnable loadData = new Runnable() {
    @Override
    public void run() {

        loadingMore = true;


        item = new ArrayList<Item>();


        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
        }

        load();


        runOnUiThread(returnRes);

    }
};


private Runnable returnRes = new Runnable() {
    @Override
    public void run() {


        if (item != null && item.size() > 0) {
            for (int i = 0; i < item.size(); i++)
                adapter.add(item.get(i));
        }


        if (count == array.length())
            lvListItem.removeFooterView(footerView);
        adapter.notifyDataSetChanged();

        // Done loading more.
        loadingMore = false;
    }
};

if i just using load() , it run and listview show data ok. But when I scrolling listview, it throw error:

  D/AndroidRuntime(1635): Shutting down VM W/dalvikvm(1635): threadid=1: thread exiting with uncaught exception (group=0x40a13300) E/AndroidRuntime(1635): FATAL EXCEPTION: main E/AndroidRuntime(1635): java.lang.IndexOutOfBoundsException: Invalid index 6, size is 6 E/AndroidRuntime(1635): at java.util.ArrayList.throwIndexOutOfBoundsException(ArrayList.java:251) E/AndroidRuntime(1635): at java.util.ArrayList.get(ArrayList.java:304) 

I need a help

Answer 1

It seems like your array has only 6 elements and you are trying to access the element with index 6 which is effectively the 7th element so

Change

itemsPerPage = 5;` 

Answer 2

Your code is not threadsafe at all. You are accessing shared data from multiple threads without any kind of locking. It is possible that your item array is being reinitialized in your worker thread while it is being read from in your resultRes runnable (running on the main thread), which would result in seemingly impossible out of bounds exceptions.

Create an object to use as your mutex:

private final Object monitor = new Object(); // an ivar in your Activity

Then when accessing shared data (like item and loadingMore ), create a critical section, for example:

synchronized(monitor) {
    item = new ArrayList<Item>();
}

synchronized(monitor) {
    if (item != null && item.size() > 0) {
        for (int i = 0; i < item.size(); i++)
            adapter.add(item.get(i));
    }
}

synchronized(monitor) {
    if ((lastInScreen == totalItemCount) && !(loadingMore)) {
        Thread thread = new Thread(null, loadData);
        thread.start(); // will exit critical section before new thread execution begins
    }
}

And so on. I'll leave it as an exercise for you to figure out where the rest of the locking should be done.