ListView getView中放置多个item和getItemViewType的用法

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

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：

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。

代码下载：这里

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原文地址：https://www.cnblogs.com/zhujiabin/p/4331524.html