Android從源代碼看ListView的重用機制

無論是android還是iOS，列表視圖應該是最復雜的控件了，android中的listview從命名可以看出是個一維數組，而iOS中的tableview則是二維數組，但其實需要注意的地方是差不多的，都是重用機制，這是考量你對listview能否掌握的最好的方法。

常見的listview的初始化以及設置適配器的代碼如下：

ListView listView;
MyAdapter listAdapter;
ArrayList listString;
listView = (ListView)this.findViewById(R.id.listview);
listString = new ArrayList();
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;
		}
		
	}

其中setAdapter是主要用來設置數據的，我們不防看一下ListView（源碼在此）的setAdapter源碼

@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();
    }

從以上代碼可以看出其分為兩步，第一步是當前adapter不為空的話，先清空本地adapter數據，然後是設置新的數據到listview。其中裡面有個重要的類

AdapterDataSetObserver

是用來存儲數據的，我們看一下裡面的代碼,有個成員變量

mDataSetObserver

就是這個對象申明在listview的父類AbsListView（源碼在此）中

那麼這個AdapterDataSetObserver究竟是個什麼東西呢，我們還是到AbsListView的父類AdapterView（源碼在此）中一探究竟吧。

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;
        }
    }

從以上代碼可以看出，這個類其實是繼承自DataSetObserver(源碼在此)。使用的是觀察者模式，用於listview的數據處理。

這是一個抽象類，不多說了。

未完待續。。。