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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