工作原理:
- ListView 針對List中每個item,要求 adapter “給我一個視圖” (getView)。
- 一個新的視圖被傳回并顯示
如果我們有上億個項目要顯示怎麼辦?為每個項目建立一個新視圖?NO!這不可能!
實際上Android為你緩存了視圖。
Android中有個叫做Recycler的構件,下圖是他的工作原理:
- 如果你有10億個項目(item),其中隻有可見的項目存在記憶體中,其他的在Recycler中。
- ListView先請求一個type1視圖(getView)然後請求其他可見的項目。convertView在getView中是空(null)的。
- 當item1滾出螢幕,并且一個新的項目從螢幕低端上來時,ListView再請求一個type1視圖。convertView此時不是空值了,它的值是item1。你隻需設定新的資料然後傳回convertView,不必重新建立一個視圖。
請看下面的示例代碼,這裡在getView中使用了System.out進行輸出:
public class MultipleItemsList extends ListActivity {
private MyCustomAdapter mAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mAdapter = new MyCustomAdapter();
for (int i = 0; i < 50; i++) {
mAdapter.addItem("item " + i);
}
setListAdapter(mAdapter);
}
private class MyCustomAdapter extends BaseAdapter {
private ArrayList mData = new ArrayList();
private LayoutInflater mInflater;
public MyCustomAdapter() {
mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}
public void addItem(final String item) {
mData.add(item);
notifyDataSetChanged();
}
@Override
public int getCount() {
return mData.size();
}
@Override
public String getItem(int position) {
return mData.get(position);
}
@Override
public long getItemId(int position) {
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
System.out.println("getView " + position + " " + convertView);
ViewHolder holder = null;
if (convertView == null) {
convertView = mInflater.inflate(R.layout.item1, null);
holder = new ViewHolder();
holder.textView = (TextView)convertView.findViewById(R.id.text);
convertView.setTag(holder);
} else {
holder = (ViewHolder)convertView.getTag();
}
holder.textView.setText(mData.get(position));
return convertView;
}
}
public static class ViewHolder {
public TextView textView;
}
}
執行程式,然後在Logcat中檢視日志:
getView 被調用 9 次 ,convertView 對于所有的可見項目是空值(如下)
? 02-0513:47:32.559: INFO/System.out(947): getView 0null
02-0513:47:32.570: INFO/System.out(947): getView 1null
02-0513:47:32.589: INFO/System.out(947): getView 2null
02-0513:47:32.599: INFO/System.out(947): getView 3null
02-0513:47:32.619: INFO/System.out(947): getView 4null
02-0513:47:32.629: INFO/System.out(947): getView 5null
02-0513:47:32.708: INFO/System.out(947): getView 6null
02-0513:47:32.719: INFO/System.out(947): getView 7null
02-0513:47:32.729: INFO/System.out(947): getView 8null 然後稍微向下滾動List,直到item10出現:
convertView仍然是空值,因為recycler中沒有視圖(item1的邊緣仍然可見,在頂端)
?
|
再滾動List
convertView不是空值了!item1離開螢幕到Recycler中去了,然後item11被建立
?
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再滾動:
?
02-0514:01:31.069: INFO/System.out(947): getView [email protected]
02-0514:01:31.142: INFO/System.out(947): getView [email protected]
02-0514:01:31.279: INFO/System.out(947): getView [email protected]
02-0514:01:31.350: INFO/System.out(947): getView [email protected]
02-0514:01:31.429: INFO/System.out(947): getView [email protected]
02-0514:01:31.550: INFO/System.out(947): getView [email protected]
02-0514:01:31.669: INFO/System.out(947): getView [email protected]
02-0514:01:31.839: INFO/System.out(947): getView [email protected]
02-0514:03:30.900: INFO/System.out(947): getView [email protected]
02-0514:03:32.069: INFO/System.out(947): getView [email protected]
convertView 如我們所期待的非空了,在item11離開螢幕之後,它的視圖(@437430f8)作為convertView容納item21了
-------------->詳細見 http://www.cnblogs.com/xiaowenji/archive/2010/12/08/1900579.html