ListView和适配器的基础：
它是如何工作的：
- ListView 向适配器说“给我的每个条目一个布局”
- 一个新的布局创建出来并显示出来
下一个问题：当我们有10亿个条目的时候怎么办，难道新创建一个新的布局并显示出来吗？答案肯定是“不”。Android会为你把布局缓存起来。
这一部分在Android中称呼为"Recycle - 回收利用"。以下为它的具体实现过程图。
- 当你有一亿个条目的时候， 只有可看见的View保存在内存中+Recycle过的View
- 当ListView第一次向适配器请求一个VIew的时候，convertView为null，因此需要新建一个convertView.
- 当ListView请求一个条目item1的VIew，并且item1已经超出屏幕之外，并进来一个相同类型的条目从底部进入到屏幕里面，这时convertVIew 不为null，而是等于item1。 你只需要获取新的数据装载到该View里面并返回回去。而不必要重新创建一个新的VIew
下面为简单的代码实现：
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; }} 运行程序并观察发生了什么 
getView方法调用了9次。对于可以看见的VIew中，convertView一直为null。
02-05 13:47:32.559: INFO/System.out(947): getView 0 null02-05 13:47:32.570: INFO/System.out(947): getView 1 null02-05 13:47:32.589: INFO/System.out(947): getView 2 null02-05 13:47:32.599: INFO/System.out(947): getView 3 null02-05 13:47:32.619: INFO/System.out(947): getView 4 null02-05 13:47:32.629: INFO/System.out(947): getView 5 null02-05 13:47:32.708: INFO/System.out(947): getView 6 null02-05 13:47:32.719: INFO/System.out(947): getView 7 null02-05 13:47:32.729: INFO/System.out(947): getView 8 null
拖动以下，并查看输出的状态。
02-05 14:01:31.069: INFO/System.out(947): getView 11 android.widget.LinearLayout@437447d002-05 14:01:31.142: INFO/System.out(947): getView 12 android.widget.LinearLayout@43744ff802-05 14:01:31.279: INFO/System.out(947): getView 13 android.widget.LinearLayout@43743fa802-05 14:01:31.350: INFO/System.out(947): getView 14 android.widget.LinearLayout@4374582002-05 14:01:31.429: INFO/System.out(947): getView 15 android.widget.LinearLayout@4374604802-05 14:01:31.550: INFO/System.out(947): getView 16 android.widget.LinearLayout@4374687002-05 14:01:31.669: INFO/System.out(947): getView 17 android.widget.LinearLayout@4374709802-05 14:01:31.839: INFO/System.out(947): getView 18 android.widget.LinearLayout@437478c002-05 14:03:30.900: INFO/System.out(947): getView 19 android.widget.LinearLayout@43748df002-05 14:03:32.069: INFO/System.out(947): getView 20 android.widget.LinearLayout@437430f8就像我们所想到的一样，convertView不为null.当item11超出屏幕之后，并进来item21的时候2个convertView为同一个View。
不同的列表条目的View
我们举一个更加复杂点的例子吧，我们加入一个分隔符到ListView中。
你需要做的是：
- 重写getViewTypeCount() ->它返回不同的View的个数
- getItemViewType(int) -> 根据它的位置返回正确的View类型
- Create correct convertView (depending on view item type) in getView
下面为代码：
public class MultipleItemsList extends ListActivity { private MyCustomAdapter mAdapter; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); mAdapter = new MyCustomAdapter(); for (int i = 1; i < 50; i++) { mAdapter.addItem("item " + i); if (i % 4 == 0) { mAdapter.addSeparatorItem("separator " + i); } } setListAdapter(mAdapter); } private class MyCustomAdapter extends BaseAdapter { private static final int TYPE_ITEM = 0; private static final int TYPE_SEPARATOR = 1; private static final int TYPE_MAX_COUNT = TYPE_SEPARATOR + 1; private ArrayList mData = new ArrayList(); private LayoutInflater mInflater; private TreeSet mSeparatorsSet = new TreeSet(); public MyCustomAdapter() { mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE); } public void addItem(final String item) { mData.add(item); notifyDataSetChanged(); } public void addSeparatorItem(final String item) { mData.add(item); // save separator position mSeparatorsSet.add(mData.size() - 1); notifyDataSetChanged(); } @Override public int getItemViewType(int position) { return mSeparatorsSet.contains(position) ? TYPE_SEPARATOR : TYPE_ITEM; } @Override public int getViewTypeCount() { return TYPE_MAX_COUNT; } @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) { ViewHolder holder = null; int type = getItemViewType(position); System.out.println("getView " + position + " " + convertView + " type = " + type); if (convertView == null) { holder = new ViewHolder(); switch (type) { case TYPE_ITEM: convertView = mInflater.inflate(R.layout.item1, null); holder.textView = (TextView)convertView.findViewById(R.id.text); break; case TYPE_SEPARATOR: convertView = mInflater.inflate(R.layout.item2, null); holder.textView = (TextView)convertView.findViewById(R.id.textSeparator); break; } convertView.setTag(holder); } else { holder = (ViewHolder)convertView.getTag(); } holder.textView.setText(mData.get(position)); return convertView; } } public static class ViewHolder { public TextView textView; } 我们运行并查看以下我们写的代码所输出的东西，我们会发现每隔4个条目它会出现一个分隔符。 
查看输出Log，没有什么特别的对于不同的类型 convertView都为Null
02-05 15:19:03.080: INFO/System.out(1035): getView 0 null type = 002-05 15:19:03.112: INFO/System.out(1035): getView 1 null type = 002-05 15:19:03.130: INFO/System.out(1035): getView 2 null type = 002-05 15:19:03.141: INFO/System.out(1035): getView 3 null type = 002-05 15:19:03.160: INFO/System.out(1035): getView 4 null type = 102-05 15:19:03.170: INFO/System.out(1035): getView 5 null type = 002-05 15:19:03.180: INFO/System.out(1035): getView 6 null type = 002-05 15:19:03.190: INFO/System.out(1035): getView 7 null type = 002-05 15:19:03.210: INFO/System.out(1035): getView 8 null type = 002-05 15:19:03.210: INFO/System.out(1035): getView 9 null type = 1拖动一下，看看Log中有什么变化
02-05 15:19:54.160: INFO/System.out(1035): getView 10 null type = 002-05 15:19:57.440: INFO/System.out(1035): getView 11 android.widget.LinearLayout@43744528 type = 002-05 15:20:01.310: INFO/System.out(1035): getView 12 android.widget.LinearLayout@43744eb0 type = 002-05 15:20:01.880: INFO/System.out(1035): getView 13 android.widget.LinearLayout@437456d8 type = 002-05 15:20:02.869: INFO/System.out(1035): getView 14 null type = 102-05 15:20:06.489: INFO/System.out(1035): getView 15 android.widget.LinearLayout@43745f00 type = 002-05 15:20:07.749: INFO/System.out(1035): getView 16 android.widget.LinearLayout@43747170 type = 002-05 15:20:10.250: INFO/System.out(1035): getView 17 android.widget.LinearLayout@43747998 type = 002-05 15:20:11.661: INFO/System.out(1035): getView 18 android.widget.LinearLayout@437481c0 type = 002-05 15:20:13.180: INFO/System.out(1035): getView 19 android.widget.LinearLayout@437468a0 type = 102-05 15:20:16.900: INFO/System.out(1035): getView 20 android.widget.LinearLayout@437489e8 type = 002-05 15:20:25.690: INFO/System.out(1035): getView 21 android.widget.LinearLayout@4374a8d8 type = 0分隔符的convertView为Null，直到第一个分隔符看见为止。当它超出屏幕时，View还会缓存到Recycler中，以使convertView显示出来。
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