listview 分析

ListView 和 Adapter 的基础

工作原理:

1. ListView 针对List中每个item，要求 adapter “给我一个视图” (getView)。
2. 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：

1. 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。
2. 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。
3. 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

01 public 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 10android.widget.LinearLayout@437430f8

 

再滚动：

02-05 14:01:31.069: INFO/System.out(947): getView 11android.widget.LinearLayout@437447d0
02-05 14:01:31.142: INFO/System.out(947): getView 12android.widget.LinearLayout@43744ff8
02-05 14:01:31.279: INFO/System.out(947): getView 13android.widget.LinearLayout@43743fa8
02-05 14:01:31.350: INFO/System.out(947): getView 14android.widget.LinearLayout@43745820
02-05 14:01:31.429: INFO/System.out(947): getView 15android.widget.LinearLayout@43746048
02-05 14:01:31.550: INFO/System.out(947): getView 16android.widget.LinearLayout@43746870
02-05 14:01:31.669: INFO/System.out(947): getView 17android.widget.LinearLayout@43747098
02-05 14:01:31.839: INFO/System.out(947): getView 18android.widget.LinearLayout@437478c0
02-05 14:03:30.900: INFO/System.out(947): getView 19android.widget.LinearLayout@43748df0
02-05 14:03:32.069: INFO/System.out(947): getView 20android.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:

1. 重(@Override)写 getViewTypeCount() – 返回你有多少个不同的布局
2. 重写 getItemViewType(int) – 由position返回view type id
3. 根据view item的类型，在getView中创建正确的convertView

以下是代码：

 

001 public 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 11android.widget.LinearLayout@43744528 type = 0
02-05 15:20:01.310: INFO/System.out(1035): getView 12android.widget.LinearLayout@43744eb0 type = 0
02-05 15:20:01.880: INFO/System.out(1035): getView 13android.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 15android.widget.LinearLayout@43745f00 type = 0
02-05 15:20:07.749: INFO/System.out(1035): getView 16android.widget.LinearLayout@43747170 type = 0
02-05 15:20:10.250: INFO/System.out(1035): getView 17android.widget.LinearLayout@43747998 type = 0
02-05 15:20:11.661: INFO/System.out(1035): getView 18android.widget.LinearLayout@437481c0 type = 0
02-05 15:20:13.180: INFO/System.out(1035): getView 19android.widget.LinearLayout@437468a0 type = 1
02-05 15:20:16.900: INFO/System.out(1035): getView 20android.widget.LinearLayout@437489e8 type = 0
02-05 15:20:25.690: INFO/System.out(1035): getView 21android.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。