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[操作系统]【Android笔记】listview加载性能优化及有多种listitem布局处理方式


在android开发中Listview是一个很重要的组件,它以列表的形式根据数据的长自适应展示具体内容。

用户可以自由的定义listview每一列的布局,但当listview有大量的数据需要加载的时候,会占据大量内存,影响性能,这时候就需要按需填充并重新使用view来减少对象的创建。

 

ListView加载数据都是在

1 public View getView(int position, View convertView, ViewGroup parent) {2 3   。。。。。。4 5 }

方法中进行的(要自定义listview都需要重写listadapter:如 BaseAdapter,SimpleAdapter,CursorAdapter的等的getvView方法),

优化listview的加载速度就要让 convertView匹配列表类型,并最大程度上的重新使用convertView

 

其中,getview的加载方法一般有以下三种加载方式:

1、最慢的加载方式是每一次都重新定义一个View载入布局,再加载数据

 1 public View getView(int position, View convertView, ViewGroup parent) { 2  3   View item = mInflater.inflate(R.layout.list_item_icon_text, null); 4  5    ((TextView) item.findViewById(R.id.text)).setText(DATA[position]); 6  7   ((ImageView) item.findViewById(R.id.icon)).setImageBitmap((position & 1) == 1 ? mIcon1 : mIcon2); 8  9   return item;10 11 }

 

2、正确的加载方式是当convertView不为空的时候直接重新使用convertView从而减少了很多不必要的View的创建,然后加载数据

 1 public View getView(int position, View convertView, ViewGroup parent) { 2  3   if (convertView == null) { 4  5     convertView = mInflater.inflate(R.layout.item, parent, false); 6  7    } 8  9    ((TextView) convertView.findViewById(R.id.text)).setText(DATA[position]);10 11    ((ImageView) convertView.findViewById(R.id.icon)).setImageBitmap((position & 1) == 1 ? mIcon1 : mIcon2);12 13 14 15   return convertView;16 17 }

3、最快的方式是定义一个ViewHolder,将convetView的tag设置为ViewHolder,不为空时重新使用即可

 1 static class ViewHolder { 2  3   TextView text; 4  5   ImageView icon; 6  7 } 8  9 10 11 public View getView(int position, View convertView, ViewGroup parent) {12 13   ViewHolder holder;14 15   if (convertView == null) {16 17   convertView = mInflater.inflate(R.layout.list_item_icon_text,parent, false);18 19   holder = new ViewHolder();20 21   holder.text = (TextView) convertView.findViewById(R.id.text);22 23   holder.icon = (ImageView) convertView.findViewById(R.id.icon);24 25   convertView.setTag(holder);26 27 } else {28 29   holder = (ViewHolder) convertView.getTag();30 31 }32 33   holder.text.setText(DATA[position]);34 35   holder.icon.setImageBitmap((position & 1) == 1 ? mIcon1 :mIcon2);36 37   return convertView;38 39 }

三种方式加载效率对比如下图所示:

说明:上述三个例子代码摘自google 2010 I/O大会

 

当处理一些耗时的资源加载的时候需要做到以下几点,以使你的加载更快更平滑:

1.   适配器在界面主线程中进行修改

2.   可以在任何地方获取数据但应该在另外一个地方请求数据

3.   在主界面的线程中提交适配器的变化并调用notifyDataSetChanged()方法

 

===============================分割线=========================================

 

那么如果存在多个item样式如何处理呢??

大致思路就是创建多个viewholder,在getViewType的时候设置不同位置的item用不同的viewholder

以下直接上代码:

 1 class MyAdapter extends BaseAdapter{ 2  3   Context mContext; 4   LinearLayout linearLayout = null; 5   LayoutInflater inflater; 6   TextView tex; 7   final int VIEW_TYPE = 3; 8   final int TYPE_1 = 0; 9   final int TYPE_2 = 1; 10   final int TYPE_3 = 2; 11  12   //各个布局的控件资源 13   class viewHolder1{ 14     CheckBox checkBox; 15     TextView textView; 16   } 17   class viewHolder2{ 18     TextView textView; 19   } 20   class viewHolder3{ 21     ImageView imageView; 22     TextView textView; 23   } 24  25   public MyAdapter(Context context) { 26     // TODO Auto-generated constructor stub 27     mContext = context; 28     inflater = LayoutInflater.from(mContext); 29   } 30  31   @Override 32   public int getCount() { 33     // TODO Auto-generated method stub 34     return listString.size(); 35   } 36  37   //每个convert view都会调用此方法,获得当前所需要的view样式 38   @Override 39   public int getItemViewType(int position) { 40     // TODO Auto-generated method stub 41     int p = position%6; 42     if(p == 0) 43     return TYPE_1; 44     else if(p < 3) 45       return TYPE_2; 46     else if(p < 6) 47       return TYPE_3; 48     else 49       return TYPE_1; 50   } 51    52   //返回样式的数量 53   @Override 54   public int getViewTypeCount() { 55     // TODO Auto-generated method stub 56     return 3; 57   } 58  59   @Override 60   public Object getItem(int arg0) { 61     // TODO Auto-generated method stub 62     return listString.get(arg0); 63   } 64  65   @Override 66   public long getItemId(int position) { 67     // TODO Auto-generated method stub 68     return position; 69   } 70  71   @Override 72   public View getView(int position, View convertView, ViewGroup parent) { 73     // TODO Auto-generated method stub 74     viewHolder1 holder1 = null; 75     viewHolder2 holder2 = null; 76     viewHolder3 holder3 = null; 77     int type = getItemViewType(position); 78  79  80   //无convertView,需要new出各个控件 81   if(convertView == null) 82   {  83     Log.e("convertView = ", " NULL"); 84  85   //按当前所需的样式,确定new的布局 86   switch(type) 87   { 88   case TYPE_1: 89   convertView = inflater.inflate(R.layout.listitem1, parent, false); 90   holder1 = new viewHolder1(); 91   holder1.textView = (TextView)convertView.findViewById(R.id.textview1); 92   holder1.checkBox = (CheckBox)convertView.findViewById(R.id.checkbox); 93   Log.e("convertView = ", "NULL TYPE_1"); 94   convertView.setTag(holder1); 95   break; 96   case TYPE_2: 97   convertView = inflater.inflate(R.layout.listitem2, parent, false); 98   holder2 = new viewHolder2(); 99   holder2.textView = (TextView)convertView.findViewById(R.id.textview2);100   Log.e("convertView = ", "NULL TYPE_2");101   convertView.setTag(holder2);102   break;103   case TYPE_3:104   convertView = inflater.inflate(R.layout.listitem3, parent, false);105   holder3 = new viewHolder3();106   holder3.textView = (TextView)convertView.findViewById(R.id.textview3);107   holder3.imageView =   (ImageView)convertView.findViewById(R.id.imageview);108   Log.e("convertView = ", "NULL TYPE_3");109   convertView.setTag(holder3);110   break;111   }112   }113   else114   {115   //有convertView,按样式,取得不用的布局116   switch(type)117   {118   case TYPE_1:119   holder1 = (viewHolder1) convertView.getTag();120   Log.e("convertView !!!!!!= ", "NULL TYPE_1");121   break;122   case TYPE_2:123   holder2 = (viewHolder2) convertView.getTag();124   Log.e("convertView !!!!!!= ", "NULL TYPE_2");125   break;126   case TYPE_3:127   holder3 = (viewHolder3) convertView.getTag();128   Log.e("convertView !!!!!!= ", "NULL TYPE_3");129   break;130   }131   }132 133   //设置资源134   switch(type)135   {136   case TYPE_1:137   holder1.textView.setText(Integer.toString(position));138   holder1.checkBox.setChecked(true);139   break;140   case TYPE_2:141   holder2.textView.setText(Integer.toString(position));142   break;143   case TYPE_3:144   holder3.textView.setText(Integer.toString(position));145   holder3.imageView.setBackgroundResource(R.drawable.icon);146   break;147   }148 149 150   return convertView;151   }152 153 }  


参考原文:listview加载性能优化ViewHolder

参考原文:listView中多个listItem布局时,convertView缓存及使用