一直以来Adapter的使用都仅仅是流于表面，仅仅知道要实现几个抽象的方法，把Adapter设置给某种listView，就能够非常好的工作起来。所谓理解仅仅是建立在主观的猜想上面，认为应该是这样，对，ok，就这样，恩， 明确了。可是其实却没有正真的懂它。这能够说Android的设计模式真的做得非常好，无需了解实现就能够非常好的运用，只是总感觉没有深入的追究还是少了点什么。那就看看它的一些细节吧。尤其来看看我一直疑惑的为什么调用notifyDataSetChanged notifyDataSetInvalidated 就能够重绘View。
首先还是从我掌握的有限的线索開始，notifyDataSetChanged 方法是怎么实现的呢？
依据经验发现，差点儿我们用到的Adapter都是继承自BaseAdapter，借助eclipse 的Type Hierarchy View 也能够确认确实是这么回事，ok，那看看它的实现：
/**
* Notifies the attached observers that the underlying data has been changed
* and any View reflecting the data set should refresh itself.
*/
public void notifyDataSetChanged() {
mDataSetObservable.notifyChanged();
}
/**
* Notifies the attached observers that the underlying data is no longer valid
* or available. Once invoked this adapter is no longer valid and should
* not report further data set changes.
*/
public void notifyDataSetInvalidated() {
mDataSetObservable.notifyInvalidated();
}原来是个Observer的实现，这个倒是和我之前的猜想差点儿相同。
public void notifyChanged() {
synchronized(mObservers) {
// since onChanged() is implemented by the app, it could do anything, including
// removing itself from {@link mObservers} - and that could cause problems if
// an iterator is used on the ArrayList {@link mObservers}.
// to avoid such problems, just march thru the list in the reverse order.
for (int i = mObservers.size() - 1; i >= 0; i--) {
mObservers.get(i).onChanged();
}
}
}看来能够挂非常多个Observer，继续看看onChanged实现发现时个abstract的，怎么办，究竟是谁呢？看来要找个详细的实现。看看比較熟悉的ListView
和比較熟悉的ListAdapter，在ListView的setAdapter里发现
mDataSetObserver = new AdapterDataSetObserver();
mAdapter.registerDataSetObserver(mDataSetObserver);AdapterDataSetObserver的实如今AdapterView：
class AdapterDataSetObserver extends DataSetObserver {
private Parcelable mInstanceState = null;
@Override
public void onChanged() {
mDataChanged = true;
mOldItemCount = mItemCount;
mItemCount = getAdapter().getCount();
// Detect the case where a cursor that was previously invalidated has
// been repopulated with new data.
if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
&& mOldItemCount == 0 && mItemCount > 0) {
AdapterView.this.onRestoreInstanceState(mInstanceState);
mInstanceState = null;
} else {
rememberSyncState();
}
checkFocus();
requestLayout();
}
@Override
public void onInvalidated() {
mDataChanged = true;
if (AdapterView.this.getAdapter().hasStableIds()) {
// Remember the current state for the case where our hosting activity is being
// stopped and later restarted
mInstanceState = AdapterView.this.onSaveInstanceState();
}
// Data is invalid so we should reset our state
mOldItemCount = mItemCount;
mItemCount = 0;
mSelectedPosition = INVALID_POSITION;
mSelectedRowId = INVALID_ROW_ID;
mNextSelectedPosition = INVALID_POSITION;
mNextSelectedRowId = INVALID_ROW_ID;
mNeedSync = false;
checkFocus();
requestLayout();
}
public void clearSavedState() {
mInstanceState = null;
}
}ok，原来是这样来更新的，明确了。
总结起来看，全部的Adapter都是BaseAdapter的子类，全部的带Adapter的view都是AdapterView的子类。AbsListView也是AdapterView的子类。
在View和Adapter关联的时候会把view的Oberver注冊到adapter上，所以在adapter数据变化的时候。view就能够通过observer了解到，从而能够及时更新。
看来Adapter 模式的实现用了Oberver模式，隐隐约约还用到桥模式，不知道我的认识是不是正确呢。