ListView适配器及缓存机制

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 null

02-05 13:47:32.570: INFO/System.out(947): getView 1 null

02-05 13:47:32.589: INFO/System.out(947): getView 2 null

02-05 13:47:32.599: INFO/System.out(947): getView 3 null

02-05 13:47:32.619: INFO/System.out(947): getView 4 null

02-05 13:47:32.629: INFO/System.out(947): getView 5 null

02-05 13:47:32.708: INFO/System.out(947): getView 6 null

02-05 13:47:32.719: INFO/System.out(947): getView 7 null

02-05 13:47:32.729: INFO/System.out(947): getView 8 null

复制代码

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@437447d0

02-05 14:01:31.142: INFO/System.out(947): getView 12 android.widget.LinearLayout@43744ff8

02-05 14:01:31.279: INFO/System.out(947): getView 13 android.widget.LinearLayout@43743fa8

02-05 14:01:31.350: INFO/System.out(947): getView 14 android.widget.LinearLayout@43745820

02-05 14:01:31.429: INFO/System.out(947): getView 15 android.widget.LinearLayout@43746048

02-05 14:01:31.550: INFO/System.out(947): getView 16 android.widget.LinearLayout@43746870

02-05 14:01:31.669: INFO/System.out(947): getView 17 android.widget.LinearLayout@43747098

02-05 14:01:31.839: INFO/System.out(947): getView 18 android.widget.LinearLayout@437478c0

02-05 14:03:30.900: INFO/System.out(947): getView 19 android.widget.LinearLayout@43748df0

02-05 14:03:32.069: INFO/System.out(947): getView 20 android.widget.LinearLayout@437430f8

复制代码

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 = 0

02-05 15:19:03.112: INFO/System.out(1035): getView 1 null type = 0

02-05 15:19:03.130: INFO/System.out(1035): getView 2 null type = 0

02-05 15:19:03.141: INFO/System.out(1035): getView 3 null type = 0

02-05 15:19:03.160: INFO/System.out(1035): getView 4 null type = 1

02-05 15:19:03.170: INFO/System.out(1035): getView 5 null type = 0

02-05 15:19:03.180: INFO/System.out(1035): getView 6 null type = 0

02-05 15:19:03.190: INFO/System.out(1035): getView 7 null type = 0

02-05 15:19:03.210: INFO/System.out(1035): getView 8 null type = 0

02-05 15:19:03.210: INFO/System.out(1035): getView 9 null type = 1

复制代码

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 = 0

02-05 15:19:57.440: INFO/System.out(1035): getView 11 android.widget.LinearLayout@43744528 type = 0

02-05 15:20:01.310: INFO/System.out(1035): getView 12 android.widget.LinearLayout@43744eb0 type = 0

02-05 15:20:01.880: INFO/System.out(1035): getView 13 android.widget.LinearLayout@437456d8 type = 0

02-05 15:20:02.869: INFO/System.out(1035): getView 14 null type = 1

02-05 15:20:06.489: INFO/System.out(1035): getView 15 android.widget.LinearLayout@43745f00 type = 0

02-05 15:20:07.749: INFO/System.out(1035): getView 16 android.widget.LinearLayout@43747170 type = 0

02-05 15:20:10.250: INFO/System.out(1035): getView 17 android.widget.LinearLayout@43747998 type = 0

02-05 15:20:11.661: INFO/System.out(1035): getView 18 android.widget.LinearLayout@437481c0 type = 0

02-05 15:20:13.180: INFO/System.out(1035): getView 19 android.widget.LinearLayout@437468a0 type = 1

02-05 15:20:16.900: INFO/System.out(1035): getView 20 android.widget.LinearLayout@437489e8 type = 0

02-05 15:20:25.690: INFO/System.out(1035): getView 21 android.widget.LinearLayout@4374a8d8 type = 0

复制代码

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。