Android ListView使用Holder优化原理
2013-12-12 16:58
429 查看
一 ListView使用Holder优化
在adapter中覆写getView方法,例如:
[java] view
plaincopyprint?
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder viewHolder = null;
if (convertView == null) {
viewHolder = new ViewHolder();
convertView = mInflater.inflate(R.layout.listview_item, null);
viewHolder.itemTextView = (TextView) convertView.findViewById(R.id.item_textview);
viewHolder.itemImageView = (ImageView) convertView.findViewById(R.id.item_imageview);
convertView.setTag(viewHolder);
} else {
viewHolder = (ViewHolder) convertView.getTag();
}
String value = String.valueOf( getItem(position) );
viewHolder.itemTextView.setText( value );
viewHolder.itemImageView.setImageResource(R.drawable.ic_launcher);
return convertView;
}
其中converView会反复使用,目的是为了提高ListView效率。但是也会造成异步加载数据后,绑定到相应view会出现item显示顺序混乱问题。
疑问?
从代码分析converView是否复用? 如果是,又怎么复用的?
如果复用猜测肯定用一个数据结构类似堆栈‘先入后出’保存这些View,需要反复使用的时候在依次取出当前没有使用的View
二 分析源码,解决疑问
1. adapter是通过setAdapter与ListView绑定到一起的,先来看ListView.setAdapter部分源码
[java] view
plaincopyprint?
public void setAdapter(ListAdapter adapter) {
......
if (mHeaderViewInfos .size() > 0|| mFooterViewInfos.size() > 0) {
mAdapter = new HeaderViewListAdapter(mHeaderViewInfos , mFooterViewInfos , adapter);
} else {
mAdapter = adapter;
}
...
}
发现ListView中把传入的adapter赋值给了ListView的全局变量mAdapter。
这里用的是适配器模式,View具体如何创建由传入的Adapter决定,Adapter在getView中创建新的View。
ListView想要创建view肯定是调用了mAdapter.getView方法,但在ListView中全局搜索没找到。那去ListView的父类中找,其父类是AbsListView,一样搜索'mAdapter.getView',在AbsListView.obtainView()方法中查找到,查看下obtainView代码:
2. AbsListView.obtainView部分源码:
[java] view
plaincopyprint?
View obtainView( int position, boolean[] isScrap) {
isScrap[0] = false ;
View scrapView;
scrapView = mRecycler .getScrapView(position);
View child;
if (scrapView != null) {
......
child = mAdapter .getView(position, scrapView, this);
......
}
......
}
发现getView的第二个参数,即Adapter.getView( int position,
View convertView, ViewGroup parent )中的converView的由来,原来是通过 mRecycler .getScrapView(position)
获取的, 那继续跟下去全局变量mRecycler
3. mRecycler定义
[java] view
plaincopyprint?
/**
* The data set used to store unused views that should be reused during the next layout
* to avoid creating new ones
*/
final RecycleBin mRecycler = new RecycleBin();
4. 继续查下RecycleBin源码,这个类是AbsListView的内部类,2中使用的 mRecycler .getScrapView(position) 部分源码如下:
[java] view
plaincopyprint?
class RecycleBin {
...
/**
* Views that were on screen at the start of layout. This array is populated at the start of
* layout, and at the end of layout all view in mActiveViews are moved to mScrapViews.
* Views in mActiveViews represent a contiguous range of Views, with position of the first
* view store in mFirstActivePosition.
*/
private View[] mActiveViews = new View[0];
/**
* Unsorted views that can be used by the adapter as a convert view.
*/
private ArrayList<View>[] mScrapViews;
private int mViewTypeCount;
private ArrayList<View> mCurrentScrap;
...
/**
* @return A view from the ScrapViews collection. These are unordered.
*/
View getScrapView(int position) {
ArrayList<View> scrapViews;
if (mViewTypeCount == 1) {
scrapViews = mCurrentScrap;
int size = scrapViews.size();
if (size > 0) {
return scrapViews.remove(size - 1);
} else {
return null;
}
} else {
int whichScrap = mAdapter.getItemViewType(position);
if (whichScrap >= 0 && whichScrap < mScrapViews.length) {
scrapViews = mScrapViews[whichScrap];
int size = scrapViews.size();
if (size > 0) {
return scrapViews.remove(size - 1);
}
}
}
return null;
}
...
}
5 分析RecycleBin使用的数据结构
一共有两个存储结构分别是ActiveViews 和 ScrapViews
ActiveViews储存当前在界面(手机显示区域)中显示View,移出界面会存入ScrapViews
ScrapViews存储当前已经滑动出当前界面(手机显示区域)显示的View,这些view存储起来相当于回收,当再次请求的时候从此存储中取出反复使用
三 结论
1. ListView通过Adapter.getView的第二个参数,可以达到反复利用已经创建的View以提高效率。
2. 反复使用的view由AbsListView的内部类RecycleBin管理,通过调用adapter.getView的第二个参数供开发者使用
四 扩展
《Android 自己动手写ListView学习其原理 3 ItemClick,ItemLongClick,View复用》 原理与ListView相同不过是个简化版
在adapter中覆写getView方法,例如:
[java] view
plaincopyprint?
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder viewHolder = null;
if (convertView == null) {
viewHolder = new ViewHolder();
convertView = mInflater.inflate(R.layout.listview_item, null);
viewHolder.itemTextView = (TextView) convertView.findViewById(R.id.item_textview);
viewHolder.itemImageView = (ImageView) convertView.findViewById(R.id.item_imageview);
convertView.setTag(viewHolder);
} else {
viewHolder = (ViewHolder) convertView.getTag();
}
String value = String.valueOf( getItem(position) );
viewHolder.itemTextView.setText( value );
viewHolder.itemImageView.setImageResource(R.drawable.ic_launcher);
return convertView;
}
其中converView会反复使用,目的是为了提高ListView效率。但是也会造成异步加载数据后,绑定到相应view会出现item显示顺序混乱问题。
疑问?
从代码分析converView是否复用? 如果是,又怎么复用的?
如果复用猜测肯定用一个数据结构类似堆栈‘先入后出’保存这些View,需要反复使用的时候在依次取出当前没有使用的View
二 分析源码,解决疑问
1. adapter是通过setAdapter与ListView绑定到一起的,先来看ListView.setAdapter部分源码
[java] view
plaincopyprint?
public void setAdapter(ListAdapter adapter) {
......
if (mHeaderViewInfos .size() > 0|| mFooterViewInfos.size() > 0) {
mAdapter = new HeaderViewListAdapter(mHeaderViewInfos , mFooterViewInfos , adapter);
} else {
mAdapter = adapter;
}
...
}
发现ListView中把传入的adapter赋值给了ListView的全局变量mAdapter。
这里用的是适配器模式,View具体如何创建由传入的Adapter决定,Adapter在getView中创建新的View。
ListView想要创建view肯定是调用了mAdapter.getView方法,但在ListView中全局搜索没找到。那去ListView的父类中找,其父类是AbsListView,一样搜索'mAdapter.getView',在AbsListView.obtainView()方法中查找到,查看下obtainView代码:
2. AbsListView.obtainView部分源码:
[java] view
plaincopyprint?
View obtainView( int position, boolean[] isScrap) {
isScrap[0] = false ;
View scrapView;
scrapView = mRecycler .getScrapView(position);
View child;
if (scrapView != null) {
......
child = mAdapter .getView(position, scrapView, this);
......
}
......
}
发现getView的第二个参数,即Adapter.getView( int position,
View convertView, ViewGroup parent )中的converView的由来,原来是通过 mRecycler .getScrapView(position)
获取的, 那继续跟下去全局变量mRecycler
3. mRecycler定义
[java] view
plaincopyprint?
/**
* The data set used to store unused views that should be reused during the next layout
* to avoid creating new ones
*/
final RecycleBin mRecycler = new RecycleBin();
4. 继续查下RecycleBin源码,这个类是AbsListView的内部类,2中使用的 mRecycler .getScrapView(position) 部分源码如下:
[java] view
plaincopyprint?
class RecycleBin {
...
/**
* Views that were on screen at the start of layout. This array is populated at the start of
* layout, and at the end of layout all view in mActiveViews are moved to mScrapViews.
* Views in mActiveViews represent a contiguous range of Views, with position of the first
* view store in mFirstActivePosition.
*/
private View[] mActiveViews = new View[0];
/**
* Unsorted views that can be used by the adapter as a convert view.
*/
private ArrayList<View>[] mScrapViews;
private int mViewTypeCount;
private ArrayList<View> mCurrentScrap;
...
/**
* @return A view from the ScrapViews collection. These are unordered.
*/
View getScrapView(int position) {
ArrayList<View> scrapViews;
if (mViewTypeCount == 1) {
scrapViews = mCurrentScrap;
int size = scrapViews.size();
if (size > 0) {
return scrapViews.remove(size - 1);
} else {
return null;
}
} else {
int whichScrap = mAdapter.getItemViewType(position);
if (whichScrap >= 0 && whichScrap < mScrapViews.length) {
scrapViews = mScrapViews[whichScrap];
int size = scrapViews.size();
if (size > 0) {
return scrapViews.remove(size - 1);
}
}
}
return null;
}
...
}
5 分析RecycleBin使用的数据结构
一共有两个存储结构分别是ActiveViews 和 ScrapViews
ActiveViews储存当前在界面(手机显示区域)中显示View,移出界面会存入ScrapViews
ScrapViews存储当前已经滑动出当前界面(手机显示区域)显示的View,这些view存储起来相当于回收,当再次请求的时候从此存储中取出反复使用
三 结论
1. ListView通过Adapter.getView的第二个参数,可以达到反复利用已经创建的View以提高效率。
2. 反复使用的view由AbsListView的内部类RecycleBin管理,通过调用adapter.getView的第二个参数供开发者使用
四 扩展
《Android 自己动手写ListView学习其原理 3 ItemClick,ItemLongClick,View复用》 原理与ListView相同不过是个简化版
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