在项目中，遇到不能ListView及时更新的问题。写了一个demo，其中也遇到一些问题，一并写出来。好吧，上代码：
public class PersonAdapter extends BaseAdapter { private ArrayList<PersonBean> mList; private Context mContext; public PersonAdapter(ArrayList<PersonBean> list, Context context) { mList = list; mContext = context; } public void refresh(ArrayList<PersonBean> list) { mList = list; notifyDataSetChanged(); } @Override public int getCount() { return mList.size(); } @Override public Object getItem(int position) { return mList.get(position); } @Override public long getItemId(int position) { return position; } @Override public View getView(int position, View convertView, ViewGroup parent) { Holder holder = null; if (convertView == null) { LayoutInflater inflater = LayoutInflater.from(mContext); convertView = inflater.inflate(R.layout.list_item, null); holder = new Holder(); holder.mNameText = (TextView)convertView.findViewById(R.id.name_text); holder.mIDText = (TextView)convertView.findViewById(R.id.id_text); convertView.setTag(holder); } else { holder = (Holder) convertView.getTag(); } holder.mNameText.setText(mList.get(getCount() - position - 1).getName()); holder.mIDText.setText(mList.get(getCount() - position - 1).getID()); return convertView; } class Holder { private TextView mNameText, mIDText; } }
PersonAdapter继承自BaseAdapter，里面的代码都应该比较熟悉。里面注意这点代码：
public void refresh(ArrayList<PersonBean> list) {
mList = list;
notifyDataSetChanged();
}
在初始化PersonAdapter的时候，需要外部导入一个mList。
public PersonAdapter(ArrayList<PersonBean> list, Context context) {
mList = list;
mContext = context;
}
在使用这种类型时，在Activity使用mAdapter.notifyDataSetChanged()时候，有时候会发现数据不能够及时的更新。这个时候，就比较需要调用refresh()这个方法了。
下面看一下主Activity：
public class ListViewRefreshActivity extends Activity { private ListView mListView; private ArrayList<PersonBean> mList; private PersonAdapter mAdapter; private Handler mHandler; private String mName, mID; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); mListView = (ListView)findViewById(R.id.listView); mList = new ArrayList<PersonBean>(); mAdapter = new PersonAdapter(mList, ListViewRefreshActivity.this); mListView.setAdapter(mAdapter); mHandler = new Handler() { @Override public void handleMessage(Message msg) { super.handleMessage(msg); mList.add((PersonBean) msg.obj); Log.v("@@@@@@", "this is get message"); mAdapter.refresh(mList); // mAdapter.notifyDataSetChanged(); } }; // final Message message = new Message(); new Thread(new Runnable() { @Override public void run() { try { for (int i = 0; i < 10; i++) { mName = "hao :" + i; mID = "" + i; PersonBean bean = new PersonBean(); bean.setID(mID); bean.setName(mName); Message message = new Message(); message.obj = bean; Thread.sleep(3000); mHandler.sendMessage(message); // mHandler.sendMessageDelayed(message, 10000); }}catch (Exception e) { e.printStackTrace(); } } }).start(); } }
如果用这个message，注释run方法体内的message
，运行程序，在我机子上，发送第四个消息时，就会报android.util.AndroidRuntimeException:This message is already in use这个错，message已经被使用。所以，每一次发送，都要重新创建一个新的message。也可以使用一下语句：
message = mHandler.obtainMessage();
里面主要看一下handler中重写handlerMessage这个方法：
|
1 2 3 4 5 6 7 8 |
@Override public void handleMessage(Message msg) { super.handleMessage(msg); mList.add((PersonBean) msg.obj); Log.v("@@@@@@", "this is get message"); mAdapter.refresh(mList); // mAdapter.notifyDataSetChanged(); } |
当然，在这个小例子中，使用mAdapter.refresh这个方法更麻烦点，直接调用notifyDataSetChange就可以达到效果，如果你的代码里面不能达到效果，就可以使用mAdapter.refresh试一下。
notifyDataSetChanged这个方法的设计是典型观察者模式。看一下源代码：
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 |
public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter { private final DataSetObservable mDataSetObservable = new DataSetObservable();
public boolean hasStableIds() { return false; }
public void registerDataSetObserver(DataSetObserver observer) { mDataSetObservable.registerObserver(observer); }
public void unregisterDataSetObserver(DataSetObserver observer) { mDataSetObservable.unregisterObserver(observer); }
/** * Notifies the attached observers that the underlying data has been changed * and any View reflecting the data set should refresh itself. */ public void notifyDataSetChanged() { mDataSetObservable.notifyChanged(); }
/** * Notifies the attached observers that the underlying data is no longer valid * or available. Once invoked this adapter is no longer valid and should * not report further data set changes. */ public void notifyDataSetInvalidated() { mDataSetObservable.notifyInvalidated(); } |
有一个数据被观察者：mDataSetObservable。当被观察者数据发生改变时，通知观察者。我们使用registerDataSetObserver这个方法注册观察者。都是调用notifyDataSetChanged方法。就是告诉观察者，数据有所改变。在这个方法中，又调用了DataSetObserveable的notifyChanged方法：
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 |
/** * Invokes onChanged on each observer. Called when the data set being observed has * changed, and which when read contains the new state of the data. */ public void notifyChanged() { synchronized(mObservers) { // since onChanged() is implemented by the app, it could do anything, including // removing itself from {@link mObservers} - and that could cause problems if // an iterator is used on the ArrayList {@link mObservers}. // to avoid such problems, just march thru the list in the reverse order. for (int i = mObservers.size() - 1; i >= 0; i--) { mObservers.get(i).onChanged(); } } } |