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 * Notifies the attached observers that the underlying data is no longer valid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 Once invoked this adapter is no longer valid and should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

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. public void notifyInvalidated() {
10. synchronized (mObservers) {
11. for (DataSetObserver observer : mObservers) {
12. observer.onInvalidated();
13. }
14. }
15. }
16. }

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

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

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。