[Android] 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

01public class MultipleItemsList extends ListActivity { 02    03    private MyCustomAdapter mAdapter; 04    05    @Override 06    public void onCreate(Bundle savedInstanceState) { 07        super.onCreate(savedInstanceState); 08        mAdapter = new MyCustomAdapter(); 09        for (int i = 0; i < 50; i++) { 10            mAdapter.addItem("item " + i); 11        } 12        setListAdapter(mAdapter); 13    } 14    15    private class MyCustomAdapter extends BaseAdapter { 16    17        private ArrayList mData = new ArrayList(); 18        private LayoutInflater mInflater; 19    20        public MyCustomAdapter() { 21            mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE); 22        } 23    24        public void addItem(final String item) { 25            mData.add(item); 26            notifyDataSetChanged(); 27        } 28    29        @Override 30        public int getCount() { 31            return mData.size(); 32        } 33    34        @Override 35        public String getItem(int position) { 36            return mData.get(position); 37        } 38    39        @Override 40        public long getItemId(int position) { 41            return position; 42        } 43    44        @Override 45        public View getView(int position, 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 = new ViewHolder(); 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            return convertView; 58        } 59    60    } 61    62    public static class ViewHolder { 63        public TextView 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）

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：

 

001public class MultipleItemsList extends ListActivity { 002    003    private MyCustomAdapter mAdapter; 004    005    @Override 006    public void onCreate(Bundle savedInstanceState) { 007        super.onCreate(savedInstanceState); 008        mAdapter = new MyCustomAdapter(); 009        for (int i = 1; i < 50; i++) { 010            mAdapter.addItem("item " + i); 011            if (i % 4 == 0) { 012                mAdapter.addSeparatorItem("separator " + i); 013            } 014        } 015        setListAdapter(mAdapter); 016    } 017    018    private class MyCustomAdapter extends BaseAdapter { 019    020        private static final int TYPE_ITEM = 0; 021        private static final int TYPE_SEPARATOR = 1; 022        private static final int TYPE_MAX_COUNT = TYPE_SEPARATOR + 1; 023    024        private ArrayList mData = new ArrayList(); 025        private LayoutInflater mInflater; 026    027        private TreeSet mSeparatorsSet = new TreeSet(); 028    029        public MyCustomAdapter() { 030            mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE); 031        } 032    033        public void addItem(final String item) { 034            mData.add(item); 035            notifyDataSetChanged(); 036        } 037    038        public void addSeparatorItem(final String item) { 039            mData.add(item); 040            // save separator position 041            mSeparatorsSet.add(mData.size() - 1); 042            notifyDataSetChanged(); 043        } 044    045        @Override 046        public int getItemViewType(int position) { 047            return mSeparatorsSet.contains(position) ? TYPE_SEPARATOR : TYPE_ITEM; 048        } 049    050        @Override 051        public int getViewTypeCount() { 052            return TYPE_MAX_COUNT; 053        } 054    055        @Override 056        public int getCount() { 057            return mData.size(); 058        } 059    060        @Override 061        public String getItem(int position) { 062            return mData.get(position); 063        } 064    065        @Override 066        public long getItemId(int position) { 067            return position; 068        } 069    070        @Override 071        public View getView(int position, View convertView, ViewGroup parent) { 072            ViewHolder holder = null; 073            int type = getItemViewType(position); 074            System.out.println("getView " + position + " " + convertView + " type = " + type); 075            if (convertView == null) { 076                holder = new ViewHolder(); 077                switch (type) { 078                    case TYPE_ITEM: 079                        convertView = mInflater.inflate(R.layout.item1, null); 080                        holder.textView = (TextView)convertView.findViewById(R.id.text); 081                        break; 082                    case TYPE_SEPARATOR: 083                        convertView = mInflater.inflate(R.layout.item2, null); 084                        holder.textView = (TextView)convertView.findViewById(R.id.textSeparator); 085                        break; 086                } 087                convertView.setTag(holder); 088            } else { 089                holder = (ViewHolder)convertView.getTag(); 090            } 091            holder.textView.setText(mData.get(position)); 092            return convertView; 093        } 094    095    } 096    097    public static class ViewHolder { 098        public TextView textView; 099    } 100}

 

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。

 

本文翻译自http://android.amberfog.com/?p=296

代码下载：MultipleItemsList.zip – source code

 

转自：http://www.cnblogs.com/xiaowenji/archive/2010/12/08/1900579.html