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:

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

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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:

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：

 

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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。