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 = ; i < ; 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）

- ::32.559:
 INFO/System.out():
 getView  null
- ::32.570:
 INFO/System.out():
 getView  null
- ::32.589:
 INFO/System.out():
 getView  null
- ::32.599:
 INFO/System.out():
 getView  null
- ::32.619:
 INFO/System.out():
 getView  null
- ::32.629:
 INFO/System.out():
 getView  null
- ::32.708:
 INFO/System.out():
 getView  null
- ::32.719:
 INFO/System.out():
 getView  null
- ::32.729:
 INFO/System.out():
 getView  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）

- ::25.169:
 INFO/System.out():
 getView  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

- ::42.879:
 INFO/System.out():
 getView  android.widget.LinearLayout@437430f8

再滚动：

- ::31.069:
 INFO/System.out():
 getView  android.widget.LinearLayout@437447d0
- ::31.142:
 INFO/System.out():
 getView  android.widget.LinearLayout@43744ff8
- ::31.279:
 INFO/System.out():
 getView  android.widget.LinearLayout@43743fa8
- ::31.350:
 INFO/System.out():
 getView  android.widget.LinearLayout@
- ::31.429:
 INFO/System.out():
 getView  android.widget.LinearLayout@
- ::31.550:
 INFO/System.out():
 getView  android.widget.LinearLayout@
- ::31.669:
 INFO/System.out():
 getView  android.widget.LinearLayout@
- ::31.839:
 INFO/System.out():
 getView  android.widget.LinearLayout@437478c0
- ::30.900:
 INFO/System.out():
 getView  android.widget.LinearLayout@43748df0
- ::32.069:
 INFO/System.out():
 getView  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 = ; i < ; i++) {
            mAdapter.addItem("item " + i);
            if (i %  == ) {
                mAdapter.addSeparatorItem("separator " + i);
            }
        }
        setListAdapter(mAdapter);
    }  

    private class MyCustomAdapter extends BaseAdapter {  

        private static final int TYPE_ITEM = ;
        private static final int TYPE_SEPARATOR = ;
        private static final int TYPE_MAX_COUNT = TYPE_SEPARATOR + ;  

        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() - );
            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

- ::03.080:
 INFO/System.out():
 getView  null type
 =
- ::03.112:
 INFO/System.out():
 getView  null type
 =
- ::03.130:
 INFO/System.out():
 getView  null type
 =
- ::03.141:
 INFO/System.out():
 getView  null type
 =
- ::03.160:
 INFO/System.out():
 getView  null type
 =
- ::03.170:
 INFO/System.out():
 getView  null type
 =
- ::03.180:
 INFO/System.out():
 getView  null type
 =
- ::03.190:
 INFO/System.out():
 getView  null type
 =
- ::03.210:
 INFO/System.out():
 getView  null type
 =
- ::03.210:
 INFO/System.out():
 getView  null type
 =
滚动list：
?
- ::54.160:
 INFO/System.out():
 getView  null type
 =
- ::57.440:
 INFO/System.out():
 getView  android.widget.LinearLayout@ type
 =
- ::01.310:
 INFO/System.out():
 getView  android.widget.LinearLayout@43744eb0 type
 =
- ::01.880:
 INFO/System.out():
 getView  android.widget.LinearLayout@437456d8 type
 =
- ::02.869:
 INFO/System.out():
 getView  null type
 =
- ::06.489:
 INFO/System.out():
 getView  android.widget.LinearLayout@43745f00 type
 =
- ::07.749:
 INFO/System.out():
 getView  android.widget.LinearLayout@ type
 =
- ::10.250:
 INFO/System.out():
 getView  android.widget.LinearLayout@ type
 =
- ::11.661:
 INFO/System.out():
 getView  android.widget.LinearLayout@437481c0 type
 =
- ::13.180:
 INFO/System.out():
 getView  android.widget.LinearLayout@437468a0 type
 =
- ::16.900:
 INFO/System.out():
 getView  android.widget.LinearLayout@437489e8 type
 =
- ::25.690:
 INFO/System.out():
 getView  android.widget.LinearLayout@4374a8d8 type
 = 

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