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http://blog.csdn.net/yujun411522/article/details/46226001
本文出自:【yujun411522的博客】

 1 重复使用getView中传递过来的convertView，不要重复创建不必要的view对象（节省内存）* - It uses the ViewHolder pattern to avoid calling findViewById() when it is not necessary

          ViewHolder1 holder1 = null;

          ViewHolder2 holder2 = null ;

          ViewHolder3 holder3 = null ;

           // 返回此view是哪个viewType

           int viewType = getItemViewType(position);

           if (convertView == null) {

               switch (viewType) {

              caseviewType1 :

                   convertView = LayoutInflater.from( mContext).inflate( R.layout. listview_item1 , null );

                    // 通过这个模 式，将view中的子view的引用保存起来，下次直接能获得viewHolder对象即可，避免重复调用findViewById方法。

                    // 现在viewHolder就等价于view中的各个子view了。

                   holder1 = new ViewHolder1();

                   holder1. index = (TextView) convertView.findViewById(R.id. my_music_listview_item_index );

                   holder1. title = (TextView) convertView.findViewById(R.id. my_music_listview_item_title );

                   convertView.setTag(holder1);

                    break ;

              case viewType2:

                   convertView = LayoutInflater.from( mContext).inflate(R.layout. listview_item2 , null );

                   holder2 = new ViewHolder2();

                   holder2. index = (TextView) convertView.findViewById(R.id. my_music_listview_item_index );

                   holder2. title = (TextView) convertView.findViewById(R.id. my_music_listview_item_title );

                   convertView.setTag(holder2);

                    break ;

              case viewType3:

                   convertView = LayoutInflater.from( mContext).inflate(R.layout. listview_item3 , null );

                   holder3 = new ViewHolder3();

                   holder3. index = (TextView) convertView.findViewById(R.id. my_music_listview_item_index );

                   holder3. title = (TextView) convertView.findViewById(R.id. my_music_listview_item_title );

                   convertView.setTag(holder3);

                    break ;

              }

          }

           // 如果是复用的view，直接取出复用view中的viewHolder，这样就可以直接通过viewHolder来访问子view了。

           else {

               switch (viewType) {

              case viewType1:

                   holder1 = (ViewHolder1) convertView.getTag();

                    break ;

              case viewType2 :

                   holder2 = (ViewHolder2) convertView.getTag();

                    break ;

              case viewType3:

                   holder3 = (ViewHolder3) convertView.getTag();

                    break ;

              }

          }

          switch(viewType) {

          caseviewType1 :

              holder1. index.setText( "" + (position));

              holder1. title.setText( mData.get(position));

               break ;

          caseviewType2 :

              holder2. index.setText( "" + (position));

              holder2. title.setText( mData.get(position));

               break ;

          caseviewType3 :

              holder3. index.setText( "" + (position));

              holder3. title.setText( mData.get(position));

               break ;

          }

           return convertView;  

}

* Provides methods for (un)registering arbitrary observers in an ArrayList.
          */
     public abstract class Observable<T> {
    /**
     * The list of observers.  An observer can be in the list at most once and will never be null.
     */
    protected final ArrayList<T> mObservers = new ArrayList<T>();
    /**
     * Adds an observer to the list. The observer cannot be null and it must not already be registered.
     * @param observer the observer to register
     * @throws IllegalArgumentException the observer is null
     * @throws IllegalStateException the observer is already registered
     */
    public void registerObserver(T observer) {
        if (observer == null) {
            throw new IllegalArgumentException("The observer is null.");
        }
        synchronized(mObservers) {
            if (mObservers.contains(observer)) {
                throw new IllegalStateException("Observer " + observer + " is already registered.");
            }
            mObservers.add(observer);
        }
    }

/**
     * Removes a previously registered observer. The observer must not be null and it must already have been registered.
     * @param observer the observer to unregister
     * @throws IllegalArgumentException the observer is null
     * @throws IllegalStateException the observer is not yet registered
     */
    public void unregisterObserver(T observer) {
        if (observer == null) {
            throw new IllegalArgumentException("The observer is null.");
        }
        synchronized(mObservers) {
            int index = mObservers.indexOf(observer);
            if (index == -1) {
                throw new IllegalStateException("Observer " + observer + " was not registered.");
            }
            mObservers.remove(index);
        }
    }   
    /**
     * Remove all registered observer
     */
    public void unregisterAll() {
        synchronized(mObservers) {
            mObservers.clear();
        }       
    }
}

public void setAdapter(ListAdapter adapter) {
        if (mAdapter != null && mDataSetObserver != null) {
            mAdapter.unregisterDataSetObserver(mDataSetObserver);
        }

...
        if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
            mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter);
        } else {
            mAdapter = adapter;
        }          
        super.setAdapter(adapter);

if (mAdapter != null) {
            mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
            mOldItemCount = mItemCount;
            mItemCount = mAdapter.getCount();
            checkFocus();

// 在这里为BaseAdapter注册一个AdapterDataSetObserver 对象。