摘要 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 ArrayListPersonBean mList; private Context mContext; public PersonAdapter(Arr
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：
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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：
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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。
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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：
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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(); }} |
先说一个小bug吧，看一下在new Thread上面有一句注释掉的
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final Message message = new Message(); |
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：
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message = mHandler.obtainMessage(); |
里面主要看一下handler中重写handlerMessage这个方法：
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@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：
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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：
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/** * 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(); } } } |
看一下他的方法说明：当数据被观察到已经改变，调用每一个观察者的onChanged方法去读取数据的最新状态。
mObservers的定义如下：
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protected final ArrayList<T> mObservers = new ArrayList<T>(); |
通过遍历一个ArrayList来通知各个观察者。
前面说到了，我们可以调用registerDataSetObserver注册为观察者，但是是在哪注册的呢？因为如果没有注册，adapter就不应该发生变化。所以，我们看下ListView的SetAdapter这个方法：
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@Override public void setAdapter(ListAdapter adapter) { if (mAdapter != null && mDataSetObserver != null) { mAdapter.unregisterDataSetObserver(mDataSetObserver); } resetList(); mRecycler.clear(); if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) { mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter); } else { mAdapter = adapter; } mOldSelectedPosition = INVALID_POSITION; mOldSelectedRowId = INVALID_ROW_ID; // AbsListView#setAdapter will update choice mode states. super.setAdapter(adapter); if (mAdapter != null) { mAreAllItemsSelectable = mAdapter.areAllItemsEnabled(); mOldItemCount = mItemCount; mItemCount = mAdapter.getCount(); checkFocus(); mDataSetObserver = new AdapterDataSetObserver(); mAdapter.registerDataSetObserver(mDataSetObserver); mRecycler.setViewTypeCount(mAdapter.getViewTypeCount()); int position; if (mStackFromBottom) { position = lookForSelectablePosition(mItemCount - 1, false); } else { position = lookForSelectablePosition(0, true); } setSelectedPositionInt(position); setNextSelectedPositionInt(position); if (mItemCount == 0) { // Nothing selected checkSelectionChanged(); } } else { mAreAllItemsSelectable = true; checkFocus(); // Nothing selected checkSelectionChanged(); } requestLayout(); } |
如果mAdapter和mDataSetObserver都不为空的话，取消mAdapter对mDataSetObserver的注册。
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if (mAdapter != null && mDataSetObserver != null) { mAdapter.unregisterDataSetObserver(mDataSetObserver); } |
然后，把传入的adapter这个参数，赋值给mAdapter：
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if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) { mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter); } else { mAdapter = adapter; } |
赋值成功后：
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if (mAdapter != null) { mDataSetObserver = new AdapterDataSetObserver(); mAdapter.registerDataSetObserver(mDataSetObserver); |
重新为mDataSetObserver赋值，然后把mAdapter注册为mDataSetObserver的观察者。
至此，思路应该清晰了：在listview的setAdapter中把adapter注册为mDataSetObserver的观察者。当数据变化时，就可以调用notifyDataSetChanged方法来提示观察者数据已经变化。
最后就是说一下，里面PersonBean类，就是一个实体类，很简单，不在详述。