ListView中getView的原理＋如何在ListView中放置多个item

转载自OYK博客园   http://www.cnblogs.com/-OYK/archive/2011/05/31/2064781.html

此文最原始被翻译自：http://android.amberfog.com/?p=296

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

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）

02-0513:47:32.559: INFO/System.out(947): getView0null
02-0513:47:32.570: INFO/System.out(947): getView1null
02-0513:47:32.589: INFO/System.out(947): getView2null
02-0513:47:32.599: INFO/System.out(947): getView3null
02-0513:47:32.619: INFO/System.out(947): getView4null
02-0513:47:32.629: INFO/System.out(947): getView5null
02-0513:47:32.708: INFO/System.out(947): getView6null
02-0513:47:32.719: INFO/System.out(947): getView7null
02-0513:47:32.729: INFO/System.out(947): getView8null

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）

02-0513:48:25.169: INFO/System.out(947): getView9null

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

02-0513:48:42.879: INFO/System.out(947): getView10android.widget.LinearLayout@437430f8

再滚动：

02-0514:01:31.069: INFO/System.out(947): getView11android.widget.LinearLayout@437447d0
02-0514:01:31.142: INFO/System.out(947): getView12android.widget.LinearLayout@43744ff8
02-0514:01:31.279: INFO/System.out(947): getView13android.widget.LinearLayout@43743fa8
02-0514:01:31.350: INFO/System.out(947): getView14android.widget.LinearLayout@43745820
02-0514:01:31.429: INFO/System.out(947): getView15android.widget.LinearLayout@43746048
02-0514:01:31.550: INFO/System.out(947): getView16android.widget.LinearLayout@43746870
02-0514:01:31.669: INFO/System.out(947): getView17android.widget.LinearLayout@43747098
02-0514:01:31.839: INFO/System.out(947): getView18android.widget.LinearLayout@437478c0
02-0514:03:30.900: INFO/System.out(947): getView19android.widget.LinearLayout@43748df0
02-0514:03:32.069: INFO/System.out(947): getView20android.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

02-0515:19:03.080: INFO/System.out(1035): getView0null type = 0
02-0515:19:03.112: INFO/System.out(1035): getView1null type = 0
02-0515:19:03.130: INFO/System.out(1035): getView2null type = 0
02-0515:19:03.141: INFO/System.out(1035): getView3null type = 0
02-0515:19:03.160: INFO/System.out(1035): getView4null type = 1
02-0515:19:03.170: INFO/System.out(1035): getView5null type = 0
02-0515:19:03.180: INFO/System.out(1035): getView6null type = 0
02-0515:19:03.190: INFO/System.out(1035): getView7null type = 0
02-0515:19:03.210: INFO/System.out(1035): getView8null type = 0
02-0515:19:03.210: INFO/System.out(1035): getView9null type = 1

滚动list：

02-0515:19:54.160: INFO/System.out(1035): getView10null type = 0
02-0515:19:57.440: INFO/System.out(1035): getView11android.widget.LinearLayout@43744528type =0
02-0515:20:01.310: INFO/System.out(1035): getView12android.widget.LinearLayout@43744eb0type =0
02-0515:20:01.880: INFO/System.out(1035): getView13android.widget.LinearLayout@437456d8type =0
02-0515:20:02.869: INFO/System.out(1035): getView14null type = 1
02-0515:20:06.489: INFO/System.out(1035): getView15android.widget.LinearLayout@43745f00type =0
02-0515:20:07.749: INFO/System.out(1035): getView16android.widget.LinearLayout@43747170type =0
02-0515:20:10.250: INFO/System.out(1035): getView17android.widget.LinearLayout@43747998type =0
02-0515:20:11.661: INFO/System.out(1035): getView18android.widget.LinearLayout@437481c0type =0
02-0515:20:13.180: INFO/System.out(1035): getView19android.widget.LinearLayout@437468a0type =1
02-0515:20:16.900: INFO/System.out(1035): getView20android.widget.LinearLayout@437489e8type =0
02-0515:20:25.690: INFO/System.out(1035): getView21android.widget.LinearLayout@4374a8d8type =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