CursorAdapter中getView newView bindView异同

Adapter的作用是界面与数据之间的桥梁，通过设置适配器至ListView控件后(如调用ListView的 setAdapter(ListAdapter adapter)                               )，列表的每一项会显示至页面中。其实，当列表里的每一项显示到页面时，都会调用Adapter的getView方法返回一个View，如：                                                  @Override                                                                                                                                          public View getView(int position, View convertView, ViewGroup parent) {                                                                                    return super.getView(position, convertView, parent);                                                                                       }                                                                                                      CursorAdapter中提供了这两个抽象方法： //Makes a new view to hold the data pointed to by cursor. public abstract View newView(Context context, Cursor cursor, ViewGroup parent); //Bind an existing view to the data pointed to by cursor public abstract void bindView(View view, Context context, Cursor cursor); newView和bindView细化了getView中的功能实现，均可写在getView中代替。 三者的调用顺序为： getView——>newView——>bindView 转： http://hi.baidu.com/spare_h/blog/item/a826d9d873f7053410df9b52.html ListView 中getView的原理

ListView 和 Adapter 的基础

工作原理:

ListView 针对List中每个item，要求 adapter “给我一个视图” (getView)。一个新的视图被返回并显示

如果我们有上亿个项目要显示怎么办？为每个项目创建一个新视图？NO!这不可能！

实际上Android为你缓存了视图。

Android中有个叫做Recycler的构件，下图是他的工作原理：

如果你有10亿个项目(item)，其中只有可见的项目存在内存中，其他的在Recycler中。ListView先请求一个type1视图(getView)然后请求其他可见的项目。convertView在getView中是空 (null)的。当item1滚出屏幕，并且一个新的项目从屏幕低端上来时，ListView再请求一个type1视图。convertView此时不是空值了， 它的值是item1。你只需设定新的数据然后返回convertView，不必重新创建一个视图。

请看下面的示例代码，这里在getView中使用了System.out进行输出

view source print ? 01publicclassMultipleItemsList extendsListActivity { 02   03    privateMyCustomAdapter mAdapter; 04   05    @Override 06    publicvoidonCreate(Bundle savedInstanceState) { 07        super.onCreate(savedInstanceState); 08        mAdapter = newMyCustomAdapter(); 09        for(inti = 0; i < 50; i++) { 10            mAdapter.addItem("item "+ i); 11        } 12        setListAdapter(mAdapter); 13    } 14   15    privateclassMyCustomAdapter extendsBaseAdapter { 16   17        privateArrayList mData = newArrayList(); 18        privateLayoutInflater mInflater; 19   20        publicMyCustomAdapter() { 21            mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE); 22        } 23   24        publicvoidaddItem(finalString item) { 25            mData.add(item); 26            notifyDataSetChanged(); 27        } 28   29        @Override 30        publicintgetCount() { 31            returnmData.size(); 32        } 33   34        @Override 35        publicString getItem(intposition) { 36            returnmData.get(position); 37        } 38   39        @Override 40        publiclonggetItemId(intposition) { 41            returnposition; 42        } 43   44        @Override 45        publicView getView(intposition, View convertView, ViewGroup parent) { 46            System.out.println("getView "+ position + " "+ convertView); 47            ViewHolder holder = null; 48            if(convertView == null) { 49                convertView = mInflater.inflate(R.layout.item1, null); 50                holder = newViewHolder(); 51                holder.textView = (TextView)convertView.findViewById(R.id.text); 52                convertView.setTag(holder); 53            } else{ 54                holder = (ViewHolder)convertView.getTag(); 55            } 56            holder.textView.setText(mData.get(position)); 57            returnconvertView; 58        } 59   60    } 61   62    publicstaticclassViewHolder { 63        publicTextView textView; 64    } 65}

 

执行程序，然后在Logcat中查看日志

 

getView 被调用 9 次 ，convertView 对于所有的可见项目是空值（如下）

 

view source print ? 02-0513:47:32.559: INFO/System.out(947): getView 0null 02-0513:47:32.570: INFO/System.out(947): getView 1null 02-0513:47:32.589: INFO/System.out(947): getView 2null 02-0513:47:32.599: INFO/System.out(947): getView 3null 02-0513:47:32.619: INFO/System.out(947): getView 4null 02-0513:47:32.629: INFO/System.out(947): getView 5null 02-0513:47:32.708: INFO/System.out(947): getView 6null 02-0513:47:32.719: INFO/System.out(947): getView 7null 02-0513:47:32.729: INFO/System.out(947): getView 8null

 

然后稍微向下滚动List，直到item10出现：

 

convertView仍然是空值，因为recycler中没有视图（item1的边缘仍然可见，在顶端）

 

view source print ? 02-0513:48:25.169: INFO/System.out(947): getView 9null

 

再滚动List

 

convertView不是空值了！item1离开屏幕到Recycler中去了，然后item11被创建

 

view source print ? 02-0513:48:42.879: INFO/System.out(947): getView 10android.widget.LinearLayout@437430f8

 

再滚动：

view source print ? 02-0514:01:31.069: INFO/System.out(947): getView 11android.widget.LinearLayout@437447d0 02-0514:01:31.142: INFO/System.out(947): getView 12android.widget.LinearLayout@43744ff8 02-0514:01:31.279: INFO/System.out(947): getView 13android.widget.LinearLayout@43743fa8 02-0514:01:31.350: INFO/System.out(947): getView 14android.widget.LinearLayout@43745820 02-0514:01:31.429: INFO/System.out(947): getView 15android.widget.LinearLayout@43746048 02-0514:01:31.550: INFO/System.out(947): getView 16android.widget.LinearLayout@43746870 02-0514:01:31.669: INFO/System.out(947): getView 17android.widget.LinearLayout@43747098 02-0514:01:31.839: INFO/System.out(947): getView 18android.widget.LinearLayout@437478c0 02-0514:03:30.900: INFO/System.out(947): getView 19android.widget.LinearLayout@43748df0 02-0514:03:32.069: INFO/System.out(947): getView 20android.widget.LinearLayout@437430f8

convertView 如我们所期待的非空了，在item11离开屏幕之后，它的视图(@437430f8)作为convertView容纳item21了

ListView优化http://blog.csdn.net/yucf1988/article/details/6365297