一步一步实现listview加载的性能优化

listview加载的核心是其adapter，本文针对listview加载的性能优化就是对adpter的优化，总共分四个层次：

0、最原始的加载

1、利用convertView

2、利用ViewHolder

3、实现局部刷新

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〇、最原始的加载

这里是不经任何优化的adapter，为了看起来方便，把listview的数据直接在构造函数里传给adapter了，代码如下：

 1     private class AdapterOptmL0 extends BaseAdapter {

 2         private LayoutInflater mLayoutInflater;

 3         private ArrayList<Integer> mListData;

 4

 5         public AdapterOptmL0(Context context, ArrayList<Integer> data) {

 6             mLayoutInflater = LayoutInflater.from(context);

 7             mListData = data;

 8         }

 9

10         @Override

11         public int getCount() {

12             return mListData == null ? 0 : mListData.size();

13         }

14

15         @Override

16         public Object getItem(int position) {

17             return mListData == null ? 0 : mListData.get(position);

18         }

19

20         @Override

21         public long getItemId(int position) {

22             return position;

23         }

24

25         @Override

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

27             View viewRoot = mLayoutInflater.inflate(R.layout.listitem, parent, false);

28             if (viewRoot != null) {

29                 TextView txt = (TextView)viewRoot.findViewById(R.id.listitem_txt);

30                 txt.setText(getItem(position) + "");

31             }

32             return viewRoot;

33         }

34     }

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一、利用convertView

上述代码的第27行在Eclipse中已经提示警告：

Unconditional layout inflation from view adapter: Should use View Holder pattern (use recycled view passed into this method as the second parameter) for smoother scrolling

这个意思就是说，被移出可视区域的view是可以回收复用的，它作为getview的第二个参数已经传进来了，所以没必要每次都从xml里inflate。

经过优化后的代码如下：

 1     @Override

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

 3         if (convertView == null) {

 4             convertView = mLayoutInflater.inflate(R.layout.listitem, parent, false);

 5         }

 6         if (convertView != null) {

 7             TextView txt = (TextView)convertView.findViewById(R.id.listitem_txt);

 8             txt.setVisibility(View.VISIBLE);

 9             txt.setText(getItem(position) + "");

10         }

11         return convertView;

12     }

上述代码加了判断，如果传入的convertView不为null，则直接复用，否则才会从xml里inflate。

按照上述代码，如果手机一屏最多同时显示5个listitem，则最多需要从xml里inflate 5 次，比AdapterOptmL0中每个listitem都需要inflate显然效率高多了。

上述的用法虽然提高了效率，但带来了一个陷阱，如果复用convertView，则需要重置该view所有可能被修改过的属性。

举个例子：

如果第一个view中的textview在getview中被设置成INVISIBLE了，而现在第一个view在滚动过程中出可视区域，并假设它作为参数传入第十个view的getview而被复用

那么，在第十个view的getview里面不仅要setText，还要重新setVisibility，因为这个被复用的view当前处于INVISIBLE状态！

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二、利用ViewHolder

从AdapterOptmL0第27行的警告中，我们还可以看到编译器推荐了一种模型叫ViewHolder，这是个什么东西呢，先看代码：

 1     private class AdapterOptmL2 extends BaseAdapter {

 2         private LayoutInflater mLayoutInflater;

 3         private ArrayList<Integer> mListData;

 4

 5         public AdapterOptmL2(Context context, ArrayList<Integer> data) {

 6             mLayoutInflater = LayoutInflater.from(context);

 7             mListData = data;

 8         }

 9

10         private class ViewHolder {

11             public ViewHolder(View viewRoot) {

12                 txt = (TextView)viewRoot.findViewById(R.id.listitem_txt);

13             }

14             public TextView txt;

15         }

16

17         @Override

18         public int getCount() {

19             return mListData == null ? 0 : mListData.size();

20         }

21

22         @Override

23         public Object getItem(int position) {

24             return mListData == null ? 0 : mListData.get(position);

25         }

26

27         @Override

28         public long getItemId(int position) {

29             return position;

30         }

31

32         @Override

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

34             if (convertView == null) {

35                 convertView = mLayoutInflater.inflate(R.layout.listitem, parent, false);

36                 ViewHolder holder = new ViewHolder(convertView);

37                 convertView.setTag(holder);

38             }

39             if (convertView != null && convertView.getTag() instanceof ViewHolder) {

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

41                 holder.txt.setVisibility(View.VISIBLE);

42                 holder.txt.setText(getItem(position) + "");

43             }

44             return convertView;

45         }

46     }

从代码中可以看到，这一步做的优化是用一个类ViewHolder来保存listitem里面所有找到的子控件，这样就不用每次都通过耗时的findViewById操作了。

这一步的优化，在listitem布局越复杂的时候效果越为明显。

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三、实现局部刷新

OK，到目前为止，listview普遍需要的优化已经做的差不多了，那就该考虑实际使用场景中的优化需求了。

实际使用listview过程中，通常会在后台更新listview的数据，然后调用Adatper的notifyDataSetChanged方法来更新listview的UI。

那么问题来了，一般情况下，一次只会更新listview的一条/几条数据，而调用notifyDataSetChanged方法则会把所有可视范围内的listitem都刷新一遍，这是不科学的！

所以，进一步优化的空间在于，局部刷新listview，话不多说见代码：

    private class AdapterOptmL3 extends BaseAdapter {

        private LayoutInflater mLayoutInflater;

        private ListView mListView;

        private ArrayList<Integer> mListData;

        public AdapterOptmL3(Context context, ListView listview, ArrayList<Integer> data) {

            mLayoutInflater = LayoutInflater.from(context);

            mListView = listview;

            mListData = data;

        }

        private class ViewHolder {

            public ViewHolder(View viewRoot) {

                txt = (TextView)viewRoot.findViewById(R.id.listitem_txt);

            }

            public TextView txt;

        }

        @Override

        public int getCount() {

            return mListData == null ? 0 : mListData.size();

        }

        @Override

        public Object getItem(int position) {

            return mListData == null ? 0 : mListData.get(position);

        }

        @Override

        public long getItemId(int position) {

            return position;

        }

        @Override

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

            if (convertView == null) {

                convertView = mLayoutInflater.inflate(R.layout.listitem, parent, false);

                ViewHolder holder = new ViewHolder(convertView);

                convertView.setTag(holder);

            }

            if (convertView != null && convertView.getTag() instanceof ViewHolder) {

                updateView((ViewHolder)convertView.getTag(), (Integer)getItem(position));

            }

            return convertView;

        }

        public void updateView(ViewHolder holder, Integer data) {

            if (holder != null && data != null) {

                holder.txt.setVisibility(View.VISIBLE);

                holder.txt.setText(data + "");

            }

        }

        public void notifyDataSetChanged(int position) {

            final int firstVisiablePosition = mListView.getFirstVisiblePosition();

            final int lastVisiablePosition = mListView.getLastVisiblePosition();

            final int relativePosition = position - firstVisiablePosition;

            if (position >= firstVisiablePosition && position <= lastVisiablePosition) {

                updateView((ViewHolder)mListView.getChildAt(relativePosition).getTag(), (Integer)getItem(position));

            } else {

                //不在可视范围内的listitem不需要手动刷新，等其可见时会通过getView自动刷新

            }

        }

    }

修改后的Adapter新增了一个方法 public void notifyDataSetChanged(int position) 可以根据position只更新指定的listitem。

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局部刷新番外篇

在局部刷新数据的接口中，实际上还可以再干点事情：listview正在滚动的时候不去刷新。

具体的思路是，如果当前正在滚动，则记住一个pending任务，等listview停止滚动的时候再去刷，这样不会造成滚动的时候刷新错乱。代码如下：

private class AdapterOptmL3Plus extends BaseAdapter implements OnScrollListener{
private LayoutInflater mLayoutInflater;
private ListView mListView;
private ArrayList<Integer> mListData;

private int mScrollState = SCROLL_STATE_IDLE;
private List<Runnable> mPendingNotify = new ArrayList<Runnable>();

public AdapterOptmL3Plus(Context context, ListView listview, ArrayList<Integer> data) {
mLayoutInflater = LayoutInflater.from(context);
mListView = listview;
mListData = data;
mListView.setOnScrollListener(this);
}

private class ViewHolder {
public ViewHolder(View viewRoot) {
txt = (TextView)viewRoot.findViewById(R.id.listitem_txt);
}
public TextView txt;
}

@Override
public int getCount() {
return mListData == null ? 0 : mListData.size();
}

@Override
public Object getItem(int position) {
return mListData == null ? 0 : mListData.get(position);
}

@Override
public long getItemId(int position) {
return position;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
if (convertView == null) {
convertView = mLayoutInflater.inflate(R.layout.listitem, parent, false);
ViewHolder holder = new ViewHolder(convertView);
convertView.setTag(holder);
}
if (convertView != null && convertView.getTag() instanceof ViewHolder) {
updateView((ViewHolder)convertView.getTag(), (Integer)getItem(position));
}
return convertView;
}

public void updateView(ViewHolder holder, Integer data) {
if (holder != null && data != null) {
holder.txt.setVisibility(View.VISIBLE);
holder.txt.setText(data + "");
}
}

public void notifyDataSetChanged(final int position) {
final Runnable runnable = new Runnable() {
@Override
public void run() {
final int firstVisiablePosition = mListView.getFirstVisiblePosition();
final int lastVisiablePosition = mListView.getLastVisiblePosition();
final int relativePosition = position - firstVisiablePosition;
if (position >= firstVisiablePosition && position <= lastVisiablePosition) {
if (mScrollState == SCROLL_STATE_IDLE) {
//当前不在滚动，立刻刷新
Log.d("Snser", "notifyDataSetChanged position=" + position + " update now");
updateView((ViewHolder)mListView.getChildAt(relativePosition).getTag(), (Integer)getItem(position));
} else {
synchronized (mPendingNotify) {
//当前正在滚动，等滚动停止再刷新
Log.d("Snser", "notifyDataSetChanged position=" + position + " update pending");
mPendingNotify.add(this);
}
}
} else {
//不在可视范围内的listitem不需要手动刷新，等其可见时会通过getView自动刷新
Log.d("Snser", "notifyDataSetChanged position=" + position + " update skip");
}
}
};
runnable.run();
}

@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
mScrollState = scrollState;
if (mScrollState == SCROLL_STATE_IDLE) {
//滚动已停止，把需要刷新的listitem都刷新一下
synchronized (mPendingNotify) {
final Iterator<Runnable> iter = mPendingNotify.iterator();
while (iter.hasNext()) {
iter.next().run();
iter.remove();
}
}
}
}

@Override
public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
}
}

  1     private class AdapterOptmL3Plus extends BaseAdapter implements OnScrollListener{

  2         private LayoutInflater mLayoutInflater;

  3         private ListView mListView;

  4         private ArrayList<Integer> mListData;

  5

  6         private int mScrollState = SCROLL_STATE_IDLE;

  7         private List<Runnable> mPendingNotify = new ArrayList<Runnable>();

  8

  9         public AdapterOptmL3Plus(Context context, ListView listview, ArrayList<Integer> data) {

 10             mLayoutInflater = LayoutInflater.from(context);

 11             mListView = listview;

 12             mListData = data;

 13             mListView.setOnScrollListener(this);

 14         }

 15

 16         private class ViewHolder {

 17             public ViewHolder(View viewRoot) {

 18                 txt = (TextView)viewRoot.findViewById(R.id.listitem_txt);

 19             }

 20             public TextView txt;

 21         }

 22

 23         @Override

 24         public int getCount() {

 25             return mListData == null ? 0 : mListData.size();

 26         }

 27

 28         @Override

 29         public Object getItem(int position) {

 30             return mListData == null ? 0 : mListData.get(position);

 31         }

 32

 33         @Override

 34         public long getItemId(int position) {

 35             return position;

 36         }

 37

 38         @Override

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

 40             if (convertView == null) {

 41                 convertView = mLayoutInflater.inflate(R.layout.listitem, parent, false);

 42                 ViewHolder holder = new ViewHolder(convertView);

 43                 convertView.setTag(holder);

 44             }

 45             if (convertView != null && convertView.getTag() instanceof ViewHolder) {

 46                 updateView((ViewHolder)convertView.getTag(), (Integer)getItem(position));

 47             }

 48             return convertView;

 49         }

 50

 51         public void updateView(ViewHolder holder, Integer data) {

 52             if (holder != null && data != null) {

 53                 holder.txt.setVisibility(View.VISIBLE);

 54                 holder.txt.setText(data + "");

 55             }

 56         }

 57

 58         public void notifyDataSetChanged(final int position) {

 59             final Runnable runnable = new Runnable() {

 60                 @Override

 61                 public void run() {

 62                     final int firstVisiablePosition = mListView.getFirstVisiblePosition();

 63                     final int lastVisiablePosition = mListView.getLastVisiblePosition();

 64                     final int relativePosition = position - firstVisiablePosition;

 65                     if (position >= firstVisiablePosition && position <= lastVisiablePosition) {

 66                         if (mScrollState == SCROLL_STATE_IDLE) {

 67                             //当前不在滚动，立刻刷新

 68                             Log.d("Snser", "notifyDataSetChanged position=" + position + " update now");

 69                             updateView((ViewHolder)mListView.getChildAt(relativePosition).getTag(), (Integer)getItem(position));

 70                         } else {

 71                             synchronized (mPendingNotify) {

 72                                 //当前正在滚动，等滚动停止再刷新

 73                                 Log.d("Snser", "notifyDataSetChanged position=" + position + " update pending");

 74                                 mPendingNotify.add(this);

 75                             }

 76                         }

 77                     } else {

 78                         //不在可视范围内的listitem不需要手动刷新，等其可见时会通过getView自动刷新

 79                         Log.d("Snser", "notifyDataSetChanged position=" + position + " update skip");

 80                     }

 81                 }

 82             };

 83             runnable.run();

 84         }

 85

 86         @Override

 87         public void onScrollStateChanged(AbsListView view, int scrollState) {

 88             mScrollState = scrollState;

 89             if (mScrollState == SCROLL_STATE_IDLE) {

 90                 //滚动已停止，把需要刷新的listitem都刷新一下

 91                 synchronized (mPendingNotify) {

 92                     final Iterator<Runnable> iter = mPendingNotify.iterator();

 93                     while (iter.hasNext()) {

 94                         iter.next().run();

 95                         iter.remove();

 96                     }

 97                 }

 98             }

 99         }

100

101         @Override

102         public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {

103         }

104     }

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一步一步实现listview加载的性能优化

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