ListView中getView的原理+如何在ListView中放置多个item
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
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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; } } |
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)
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 |
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)
02 - 05 13 : 48 : 25.169 :
INFO/System.out( 947 ):
getView 9 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
02 - 05 13 : 48 : 42.879 :
INFO/System.out( 947 ):
getView 10 android.widget.LinearLayout @437430f8 |
再滚动:
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 |
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:
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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; } } |
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
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 |
滚动list:
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 |
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。