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{ mDataSetObservable.notifyChanged(); } /** * Notifies the attached observers that the underlying data is no longer valid * or available. Once invoked this adapter is no longer valid and should * not report further data set changes.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。