Android从源码看ListView的重用机制
不管是android还是iOS，列表视图应该是最复杂的控件了。android中的listview从命名能够看出是个一维数组，而iOS中的tableview则是二维数组。但事实上须要注意的地方是差点儿相同的。都是重用机制。这是考量你对listview是否能掌握的最好的方法。
常见的listview的初始化以及设置适配器的代码例如以下：
ListView listView;
MyAdapter listAdapter;
ArrayList<String> listString;
listView = (ListView)this.findViewById(R.id.listview);
listString = new ArrayList<String>();
for(int i = 0 ; i < 100 ; i++)
{
listString.add(Integer.toString(i));
}
listAdapter = new MyAdapter(this);
listView.setAdapter(listAdapter);
}
class MyAdapter extends BaseAdapter{
Context mContext;
LinearLayout linearLayout = null;
LayoutInflater inflater;
public MyAdapter(Context context) {
// TODO Auto-generated constructor stub
mContext = context;
inflater = LayoutInflater.from(mContext);
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return listString.size();
}
@Override
public Object getItem(int arg0) {
// TODO Auto-generated method stub
return listString.get(arg0);
}
@Override
public long getItemId(int position) {
// TODO Auto-generated method stub
return position;
}
public final class ViewHolder{
public ImageView img;
public TextView title;
public TextView info;
public Button viewBtn;
}
public class MyAdapter extends BaseAdapter{
private LayoutInflater mInflater;
public MyAdapter(Context context){
this.mInflater = LayoutInflater.from(context);
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return mData.size();
}
@Override
public Object getItem(int arg0) {
// TODO Auto-generated method stub
return null;
}
@Override
public long getItemId(int arg0) {
// TODO Auto-generated method stub
return 0;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder = null;
if (convertView == null) {
holder=new ViewHolder();
convertView = mInflater.inflate(R.layout.vlist2, null);
holder.img = (ImageView)convertView.findViewById(R.id.img);
holder.title = (TextView)convertView.findViewById(R.id.title);
holder.info = (TextView)convertView.findViewById(R.id.info);
holder.viewBtn = (Button)convertView.findViewById(R.id.view_btn);
convertView.setTag(holder);
}else {
holder = (ViewHolder)convertView.getTag();
}
holder.img.setBackgroundResource((Integer)mData.get(position).get("img"));
holder.title.setText((String)mData.get(position).get("title"));
holder.info.setText((String)mData.get(position).get("info"));
holder.viewBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
showInfo();
}
});
return convertView;
}
}
当中setAdapter是主要用来设置数据的。我们不防看一下ListView（源代码在此）的setAdapter源代码
@Override
public void setAdapter(ListAdapter adapter) {
if (null != mAdapter) {
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;
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();
}
if (mCheckStates != null) {
mCheckStates.clear();
}
requestLayout();
}
从以上代码能够看出其分为两步，第一步是当前adapter不为空的话。先清空本地adapter数据。然后是设置新的数据到listview。
当中里面有个重要的类
AdapterDataSetObserver
是用来存储数据的，我们看一下里面的代码,有个成员变量
mDataSetObserver
<span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">就是这个对象申明在listview的父类AbsListView（<a target=_blank href="http://grepcode.com/file_/repository.grepcode.com/java/ext/com.google.android/android/2.2_r1.1/android/widget/AbsListView.java/?v=source">源代码在此</a>）中</span>
那么这个AdapterDataSetObserver到底是个什么东西呢，我们还是到AbsListView的父类AdapterView（源代码在此）中一探到底吧。
class AdapterDataSetObserver extends DataSetObserver {
private Parcelable mInstanceState = null;
@Override
public void onChanged() {
mDataChanged = true;
mOldItemCount = mItemCount;
mItemCount = getAdapter().getCount();
// Detect the case where a cursor that was previously invalidated has
// been repopulated with new data.
if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
&& mOldItemCount == 0 && mItemCount > 0) {
AdapterView.this.onRestoreInstanceState(mInstanceState);
mInstanceState = null;
} else {
rememberSyncState();
}
checkFocus();
requestLayout();
}
@Override
public void onInvalidated() {
mDataChanged = true;
if (AdapterView.this.getAdapter().hasStableIds()) {
// Remember the current state for the case where our hosting activity is being
// stopped and later restarted
mInstanceState = AdapterView.this.onSaveInstanceState();
}
// Data is invalid so we should reset our state
mOldItemCount = mItemCount;
mItemCount = 0;
mSelectedPosition = INVALID_POSITION;
mSelectedRowId = INVALID_ROW_ID;
mNextSelectedPosition = INVALID_POSITION;
mNextSelectedRowId = INVALID_ROW_ID;
mNeedSync = false;
checkSelectionChanged();
checkFocus();
requestLayout();
}
public void clearSavedState() {
mInstanceState = null;
}
}从以上代码能够看出，这个类事实上是继承自DataSetObserver(源代码在此)。
使用的是观察者模式，用于listview的数据处理。
这是一个抽象类，不多说了。
未完待续。
。。
