registerDataSetObserver:浅析Andorid ListView和Adapte

最近由于遇到将内容分部绑定到ListView里的需求，追踪源码之后对ListView和Adapter有了点肤浅的认识，在此与大家分享。

这里用到了观察者模式，在ListView的setAdapter里注册一个AdapterDataSetObserver观察者。

1. public void setAdapter(ListAdapter adapter) {
2. ........
3. mDataSetObserver = new AdapterDataSetObserver();//AdapterDataSetObserver是ListView的基类AdapterView的内部类
4. mAdapter.registerDataSetObserver(mDataSetObserver);//注册一个观察者
5. .........
6. requestLayout();//更新视图
7. }

ListAdapter是一接口，BaseAdapter继承了它

1. public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {
2.     private final DataSetObservable mDataSetObservable = new DataSetObservable();
3.  
4.     public boolean hasStableIds() {
5.         return false;
6.     }
7.      
8.     public void registerDataSetObserver(DataSetObserver observer) {
9.         mDataSetObservable.registerObserver(observer);
10.     }
11.  
12.     public void unregisterDataSetObserver(DataSetObserver observer) {
13.         mDataSetObservable.unregisterObserver(observer);
14.     }
15.      
16.     /**
17.      * Notifies the attached observers that the underlying data has been changed
18.      * and any View reflecting the data set should refresh itself.
19.      */
20.     public void notifyDataSetChanged() {
21.         mDataSetObservable.notifyChanged();
22.     }
23.  
24.     /**
25.      * Notifies the attached observers that the underlying data is no longer valid
26.      * or available. Once invoked this adapter is no longer valid and should
27.      * not report further data set changes.
28.      */
29.     public void notifyDataSetInvalidated() {
30.         mDataSetObservable.notifyInvalidated();
31.     }
32.  
33.     public boolean areAllItemsEnabled() {
34.         return true;
35.     }
36.  
37.     public boolean isEnabled(int position) {
38.         return true;
39.     }
40.  
41.     public View getDropDownView(int position, View convertView, ViewGroup parent) {
42.         return getView(position, convertView, parent);
43.     }
44.  
45.     public int getItemViewType(int position) {
46.         return 0;
47.     }
48.  
49.     public int getViewTypeCount() {
50.         return 1;
51.     }
52.      
53.     public boolean isEmpty() {
54.         return getCount() == 0;
55.     }
56. }

DataSetObservable定义如下：

1. public class DataSetObservable extends Observable<DataSetObserver> {
2.     public void notifyChanged() {//通知每个观察者数据已变化
3.         synchronized(mObservers) {
4.             for (DataSetObserver observer : mObservers) {
5.                 observer.onChanged();
6.             }
7.         }
8.     }
9.  
10.     public void notifyInvalidated() {
11.         synchronized (mObservers) {
12.             for (DataSetObserver observer : mObservers) {
13.                 observer.onInvalidated();
14.             }
15.         }
16.     }
17. }

ListView里注册的观察者是AdapterDataSetObserver对象，其定义如下：

1. class AdapterDataSetObserver extends DataSetObserver {
2.  
3.         private Parcelable mInstanceState = null;
4.  
5.         @Override
6.         public void onChanged() {
7.             mDataChanged = true;
8.             mOldItemCount = mItemCount;
9.             mItemCount = getAdapter().getCount();//Adapter现有元素个数
10.  
11.             if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
12.                     && mOldItemCount == 0 && mItemCount > 0) {
13.                 AdapterView.this.onRestoreInstanceState(mInstanceState);
14.                 mInstanceState = null;
15.             } else {
16.                 rememberSyncState();
17.             }
18.             checkFocus();
19.             requestLayout();//更新视图
20.         }
21. .................
22. }

AdapterDataSetObserver里的具体实现我没弄清楚，但感觉是在这里更新了ListView显示的数据，ListView里是怎样更新的没弄清楚，希望哪位高手知道的告诉我。

但是如果只在ListView里显示Adapter的部分数据，该怎么弄呢，可继承BaseAdapter，实现BaseAdapter里未实现的如下方法：

getCount()、getItem(int position)、getItemId(int position)、getView(int position, View convertView, ViewGroup parent)。

只需在getCount里返回想要的大小就行了。