CursorAdapter中getView newView bindView异同

Adapter的作用是界面与数据之间的桥梁，通过设置适配器至ListView控件后(如调用ListView的 setAdapter(ListAdapter adapter)                              
)，列表的每一项会显示至页面中。其实，当列表里的每一项显示到页面时，都会调用Adapter的getView方法返回一个View，如：                                 
                @Override                                                                                                                         
                public View getView(int position, View convertView, ViewGroup parent) {                                                           
                        return super.getView(position, convertView, parent);                                                                      
                }                                                                                                     
CursorAdapter中提供了这两个抽象方法：
//Makes a new view to hold the data pointed to by cursor.
public abstract View newView(Context context, Cursor cursor, ViewGroup parent);
//Bind an existing view to the data pointed to by cursor
public abstract void bindView(View view, Context context, Cursor cursor);

newView和bindView细化了getView中的功能实现，均可写在getView中代替。

三者的调用顺序为：
getView——>newView——>bindView

转：http://hi.baidu.com/spare_h/blog/item/a826d9d873f7053410df9b52.html

ListView 中getView的原理

ListView 和 Adapter 的基础

工作原理:

ListView 针对List中每个item，要求 adapter “给我一个视图” (getView)。一个新的视图被返回并显示

如果我们有上亿个项目要显示怎么办？为每个项目创建一个新视图？NO!这不可能！

实际上Android为你缓存了视图。

Android中有个叫做Recycler的构件，下图是他的工作原理：

如果你有10亿个项目(item)，其中只有可见的项目存在内存中，其他的在Recycler中。ListView先请求一个type1视图(getView)然后请求其他可见的项目。convertView在getView中是空 (null)的。当item1滚出屏幕，并且一个新的项目从屏幕低端上来时，ListView再请求一个type1视图。convertView此时不是空值了， 它的值是item1。你只需设定新的数据然后返回convertView，不必重新创建一个视图。

请看下面的示例代码，这里在getView中使用了System.out进行输出

view sourceprint?

01	publicclassMultipleItemsList extendsListActivity {

02

03	privateMyCustomAdapter mAdapter;

04

05

@Override

06	publicvoidonCreate(Bundle savedInstanceState) {

07	super.onCreate(savedInstanceState);

08	mAdapter = newMyCustomAdapter();

09	for(inti = 0; i < 50; i++) {

10	mAdapter.addItem("item "+ i);

11

}

12	setListAdapter(mAdapter);

13

}

14

15	privateclassMyCustomAdapter extendsBaseAdapter {

16

17	privateArrayList mData = newArrayList();

18	privateLayoutInflater mInflater;

19

20	publicMyCustomAdapter() {

21	mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);

22

}

23

24	publicvoidaddItem(finalString item) {

25	mData.add(item);

26	notifyDataSetChanged();

27

}

28

29

@Override

30	publicintgetCount() {

31	returnmData.size();

32

}

33

34

@Override

35	publicString getItem(intposition) {

36	returnmData.get(position);

37

}

38

39

@Override

40	publiclonggetItemId(intposition) {

41	returnposition;

42

}

43

44

@Override

45	publicView getView(intposition, View convertView, ViewGroup parent) {

46	System.out.println("getView "+ position + " "+ convertView);

47	ViewHolder holder = null;

48	if(convertView == null) {

49	convertView = mInflater.inflate(R.layout.item1, null);

50	holder = newViewHolder();

51	holder.textView = (TextView)convertView.findViewById(R.id.text);

52	convertView.setTag(holder);

53

} else{

54	holder = (ViewHolder)convertView.getTag();

55

}

56	holder.textView.setText(mData.get(position));

57	returnconvertView;

58

}

59

60

}

61

62	publicstaticclassViewHolder {

63	publicTextView textView;

64

}

65

}

执行程序，然后在Logcat中查看日志

getView 被调用 9 次 ，convertView 对于所有的可见项目是空值（如下）

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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

然后稍微向下滚动List，直到item10出现：

 

convertView仍然是空值，因为recycler中没有视图（item1的边缘仍然可见，在顶端）

 

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02-0513:48:25.169: INFO/System.out(947): getView 9null

再滚动List

convertView不是空值了！item1离开屏幕到Recycler中去了，然后item11被创建

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02-0513:48:42.879: INFO/System.out(947): getView 10android.widget.LinearLayout@437430f8

再滚动：

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02-0514:01:31.069: INFO/System.out(947): getView 11android.widget.LinearLayout@437447d0

02-0514:01:31.142: INFO/System.out(947): getView 12android.widget.LinearLayout@43744ff8

02-0514:01:31.279: INFO/System.out(947): getView 13android.widget.LinearLayout@43743fa8

02-0514:01:31.350: INFO/System.out(947): getView 14android.widget.LinearLayout@43745820

02-0514:01:31.429: INFO/System.out(947): getView 15android.widget.LinearLayout@43746048

02-0514:01:31.550: INFO/System.out(947): getView 16android.widget.LinearLayout@43746870

02-0514:01:31.669: INFO/System.out(947): getView 17android.widget.LinearLayout@43747098

02-0514:01:31.839: INFO/System.out(947): getView 18android.widget.LinearLayout@437478c0

02-0514:03:30.900: INFO/System.out(947): getView 19android.widget.LinearLayout@43748df0

02-0514:03:32.069: INFO/System.out(947): getView 20android.widget.LinearLayout@437430f8

convertView 如我们所期待的非空了，在item11离开屏幕之后，它的视图(@437430f8)作为convertView容纳item21了

ListView优化
http://blog.csdn.net/yucf1988/article/details/6365297