ListView布局多个样式

http://www.cnblogs.com/xiaowenji/archive/2010/12/08/1900579.html

工作原理:

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

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。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。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:

重(@Override)写 getViewTypeCount() – 返回你有多少个不同的布局重写 getItemViewType(int) – 由position返回view type id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。