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
public class MultipleItemsList extends ListActivity {
private MyCustomAdapter mAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mAdapter = new MyCustomAdapter();
for (int i = ; i < ; 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;
}
}
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）
- ::32.559:
INFO/System.out():
getView null
- ::32.570:
INFO/System.out():
getView null
- ::32.589:
INFO/System.out():
getView null
- ::32.599:
INFO/System.out():
getView null
- ::32.619:
INFO/System.out():
getView null
- ::32.629:
INFO/System.out():
getView null
- ::32.708:
INFO/System.out():
getView null
- ::32.719:
INFO/System.out():
getView null
- ::32.729:
INFO/System.out():
getView null
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）
- ::25.169:
INFO/System.out():
getView null
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
- ::42.879:
INFO/System.out():
getView android.widget.LinearLayout@437430f8
再滚动：
- ::31.069:
INFO/System.out():
getView android.widget.LinearLayout@437447d0
- ::31.142:
INFO/System.out():
getView android.widget.LinearLayout@43744ff8
- ::31.279:
INFO/System.out():
getView android.widget.LinearLayout@43743fa8
- ::31.350:
INFO/System.out():
getView android.widget.LinearLayout@
- ::31.429:
INFO/System.out():
getView android.widget.LinearLayout@
- ::31.550:
INFO/System.out():
getView android.widget.LinearLayout@
- ::31.669:
INFO/System.out():
getView android.widget.LinearLayout@
- ::31.839:
INFO/System.out():
getView android.widget.LinearLayout@437478c0
- ::30.900:
INFO/System.out():
getView android.widget.LinearLayout@43748df0
- ::32.069:
INFO/System.out():
getView android.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
不同的项目布局(item layout)
我们再举一个稍微复杂的例子，在上例的list中加入一些分隔线
你需要做这些:
- 重(@Override)写 getViewTypeCount() – 返回你有多少个不同的布局
- 重写 getItemViewType(int) – 由position返回view type id
- 根据view item的类型，在getView中创建正确的convertView
以下是代码：
public class MultipleItemsList extends ListActivity {
private MyCustomAdapter mAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mAdapter = new MyCustomAdapter();
for (int i = ; i < ; i++) {
mAdapter.addItem("item " + i);
if (i % == ) {
mAdapter.addSeparatorItem("separator " + i);
}
}
setListAdapter(mAdapter);
}
private class MyCustomAdapter extends BaseAdapter {
private static final int TYPE_ITEM = ;
private static final int TYPE_SEPARATOR = ;
private static final int TYPE_MAX_COUNT = TYPE_SEPARATOR + ;
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() - );
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;
}
}
运行程序，你会看到每4个item一个分割线
看看日志，无异常，所有的convertView都是空的
- ::03.080:
INFO/System.out():
getView null type
=
- ::03.112:
INFO/System.out():
getView null type
=
- ::03.130:
INFO/System.out():
getView null type
=
- ::03.141:
INFO/System.out():
getView null type
=
- ::03.160:
INFO/System.out():
getView null type
=
- ::03.170:
INFO/System.out():
getView null type
=
- ::03.180:
INFO/System.out():
getView null type
=
- ::03.190:
INFO/System.out():
getView null type
=
- ::03.210:
INFO/System.out():
getView null type
=
- ::03.210:
INFO/System.out():
getView null type
=
滚动list：
?
- ::54.160:
INFO/System.out():
getView null type
=
- ::57.440:
INFO/System.out():
getView android.widget.LinearLayout@ type
=
- ::01.310:
INFO/System.out():
getView android.widget.LinearLayout@43744eb0 type
=
- ::01.880:
INFO/System.out():
getView android.widget.LinearLayout@437456d8 type
=
- ::02.869:
INFO/System.out():
getView null type
=
- ::06.489:
INFO/System.out():
getView android.widget.LinearLayout@43745f00 type
=
- ::07.749:
INFO/System.out():
getView android.widget.LinearLayout@ type
=
- ::10.250:
INFO/System.out():
getView android.widget.LinearLayout@ type
=
- ::11.661:
INFO/System.out():
getView android.widget.LinearLayout@437481c0 type
=
- ::13.180:
INFO/System.out():
getView android.widget.LinearLayout@437468a0 type
=
- ::16.900:
INFO/System.out():
getView android.widget.LinearLayout@437489e8 type
=
- ::25.690:
INFO/System.out():
getView android.widget.LinearLayout@4374a8d8 type
=
convertView对于分割线是空的，直到第一个分割线可见，当其离开屏幕，视图去到Recycler并且convertView开始起作用。
代码下载：这里