Android从源码看ListView的重用机制

不管是android还是iOS，列表视图应该是最复杂的控件了。android中的listview从命名能够看出是个一维数组，而iOS中的tableview则是二维数组。但事实上须要注意的地方是差点儿相同的。都是重用机制。这是考量你对listview是否能掌握的最好的方法。

常见的listview的初始化以及设置适配器的代码例如以下：

ListView listView;
MyAdapter listAdapter;
ArrayList<String> listString;
listView = (ListView)this.findViewById(R.id.listview);
listString = new ArrayList<String>();
for(int i = 0 ; i < 100 ; i++)
{
listString.add(Integer.toString(i));
}
listAdapter = new MyAdapter(this);
listView.setAdapter(listAdapter);
}

class MyAdapter extends BaseAdapter{

Context mContext;
LinearLayout linearLayout = null;
LayoutInflater inflater;

public MyAdapter(Context context) {
// TODO Auto-generated constructor stub
mContext = context;
inflater = LayoutInflater.from(mContext);
}

@Override
public int getCount() {
// TODO Auto-generated method stub
return listString.size();
}

@Override
public Object getItem(int arg0) {
// TODO Auto-generated method stub
return listString.get(arg0);
}

@Override
public long getItemId(int position) {
// TODO Auto-generated method stub
return position;
}
public final class ViewHolder{
		public ImageView img;
		public TextView title;
		public TextView info;
		public Button viewBtn;
	}

	public class MyAdapter extends BaseAdapter{

		private LayoutInflater mInflater;

		public MyAdapter(Context context){
			this.mInflater = LayoutInflater.from(context);
		}
		@Override
		public int getCount() {
			// TODO Auto-generated method stub
			return mData.size();
		}

		@Override
		public Object getItem(int arg0) {
			// TODO Auto-generated method stub
			return null;
		}

		@Override
		public long getItemId(int arg0) {
			// TODO Auto-generated method stub
			return 0;
		}

		@Override
		public View getView(int position, View convertView, ViewGroup parent) {

			ViewHolder holder = null;
			if (convertView == null) {

				holder=new ViewHolder();  

				convertView = mInflater.inflate(R.layout.vlist2, null);
				holder.img = (ImageView)convertView.findViewById(R.id.img);
				holder.title = (TextView)convertView.findViewById(R.id.title);
				holder.info = (TextView)convertView.findViewById(R.id.info);
				holder.viewBtn = (Button)convertView.findViewById(R.id.view_btn);
				convertView.setTag(holder);

			}else {

				holder = (ViewHolder)convertView.getTag();
			}

			holder.img.setBackgroundResource((Integer)mData.get(position).get("img"));
			holder.title.setText((String)mData.get(position).get("title"));
			holder.info.setText((String)mData.get(position).get("info"));

			holder.viewBtn.setOnClickListener(new View.OnClickListener() {

				@Override
				public void onClick(View v) {
					showInfo();
				}
			});

			return convertView;
		}

	}

当中setAdapter是主要用来设置数据的。我们不防看一下ListView（源代码在此）的setAdapter源代码

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

        resetList();
        mRecycler.clear();

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

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

            mDataSetObserver = new AdapterDataSetObserver();
            mAdapter.registerDataSetObserver(mDataSetObserver);

            mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());

            int position;
            if (mStackFromBottom) {
                position = lookForSelectablePosition(mItemCount - 1, false);
            } else {
                position = lookForSelectablePosition(0, true);
            }
            setSelectedPositionInt(position);
            setNextSelectedPositionInt(position);

            if (mItemCount == 0) {
                // Nothing selected
                checkSelectionChanged();
            }

        } else {
            mAreAllItemsSelectable = true;
            checkFocus();
            // Nothing selected
            checkSelectionChanged();
        }

        if (mCheckStates != null) {
            mCheckStates.clear();
        }

        requestLayout();
    }

从以上代码能够看出其分为两步，第一步是当前adapter不为空的话。先清空本地adapter数据。然后是设置新的数据到listview。

当中里面有个重要的类

AdapterDataSetObserver

是用来存储数据的，我们看一下里面的代码,有个成员变量

mDataSetObserver

<span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">就是这个对象申明在listview的父类AbsListView（<a target=_blank href="http://grepcode.com/file_/repository.grepcode.com/java/ext/com.google.android/android/2.2_r1.1/android/widget/AbsListView.java/?v=source">源代码在此</a>）中</span>

那么这个AdapterDataSetObserver到底是个什么东西呢，我们还是到AbsListView的父类AdapterView（源代码在此）中一探到底吧。

class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;

        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = getAdapter().getCount();

            // Detect the case where a cursor that was previously invalidated has
            // been repopulated with new data.
            if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
                    && mOldItemCount == 0 && mItemCount > 0) {
                AdapterView.this.onRestoreInstanceState(mInstanceState);
                mInstanceState = null;
            } else {
                rememberSyncState();
            }
            checkFocus();
            requestLayout();
        }

        @Override
        public void onInvalidated() {
            mDataChanged = true;

            if (AdapterView.this.getAdapter().hasStableIds()) {
                // Remember the current state for the case where our hosting activity is being
                // stopped and later restarted
                mInstanceState = AdapterView.this.onSaveInstanceState();
            }

            // Data is invalid so we should reset our state
            mOldItemCount = mItemCount;
            mItemCount = 0;
            mSelectedPosition = INVALID_POSITION;
            mSelectedRowId = INVALID_ROW_ID;
            mNextSelectedPosition = INVALID_POSITION;
            mNextSelectedRowId = INVALID_ROW_ID;
            mNeedSync = false;
            checkSelectionChanged();

            checkFocus();
            requestLayout();
        }

        public void clearSavedState() {
            mInstanceState = null;
        }
    }

从以上代码能够看出，这个类事实上是继承自DataSetObserver(源代码在此)。

使用的是观察者模式，用于listview的数据处理。

这是一个抽象类，不多说了。

未完待续。

。。