使用列表视图的事件滚动加载更多数据时出错

Question

列表视图的滚动有问题。我有一个像这样的课程：

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;
    }
};

如果我只是使用 load()，它会运行并且 listview 显示数据正常。但是当我滚动列表视图时，它会抛出错误：

 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)

我需要帮助

Answer 1

您的数组似乎只有 6 个元素，而您正试图访问索引为 6 的元素，这实际上是第 7 个元素，所以

改变

itemsPerPage = 5;` 

Answer 2

您的代码根本不是线程安全的。您正在从多个线程访问共享数据而没有任何类型的锁定。您的 item 数组可能在您的工作线程中被重新初始化，而它正在从您的 resultRes 可运行（在主线程上运行）中读取，这将导致看似不可能的越界异常。

创建一个对象以用作您的互斥锁：

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

然后在访问共享数据（如item和loadingMore）时，创建一个临界区，例如：

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
    }
}

等等。我将把它作为练习留给你，让你弄清楚应该在哪里完成其余的锁定。