我們都知道ListView都必須有設置適配器的這個步驟,即setAdapter(ListAdapter adapter),而在這裏andriod framework使用到了Java觀察者模式。
在你看下面的解析之前,讀者必須要先對觀察者模式有一定了解,因爲下面的解析都是觀察者的應用實例,沒有對觀察者的概念和簡單實例進行講解
mAdapter = adapter;
mDataSetObserver = new AdapterDataSetObserver();
mAdapter.registerDataSetObserver(mDataSetObserver);
這是ListView 的setAdapter裏面的了兩句代碼,看名字就知道跟觀察者有關係。繼續看下面源代碼
class AdapterDataSetObserver extends AdapterView<ListAdapter>.AdapterDataSetObserver {
@Override
public void onChanged() {
super.onChanged();
if (mFastScroller != null) {
mFastScroller.onSectionsChanged();
}
}
}
這是ListView的直接父類AbsListView裏面的一個類,繼續看AdapterView裏面的源代碼
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();
}
}
下面看DataSetObserver源代碼
public abstract class DataSetObserver {
/**
* This method is called when the entire data set has changed,
* most likely through a call to {@link Cursor#requery()} on a {@link Cursor}.
*/
public void onChanged() {
// Do nothing
}
}
到這裏觀察者模式的觀察者開始出現了,DataSetObserver其實就是觀察者,通過setAdapter就可以執行到mAdapter.registerDataSetObserver(mDataSetObserver)。
接着看mAdapter,
mAdapter在ListView的父類AbsListView中定義,
/**
* The adapter containing the data to be displayed by this view
*/
ListAdapter mAdapter;
ListAdapter是一個接口,其直接父類爲Adapter,Adapter裏面部分源代碼如下
/**
* Register an observer that is called when changes happen to the data used by this adapter.
*
* @param observer the object that gets notified when the data set changes.
*/
void registerDataSetObserver(DataSetObserver observer);
到這裏觀察者模式的通知者信息也知道了,ListView每次setAdapter後都會mAdapter.registerDataSetObserver但是registerDataSetObserver這個方法做了啥呢?看下面
我們setAdapter後會把我們實現的BaseAdapter傳進來也就是mAdapter,在BaseAdapter中源代碼如下
private final DataSetObservable mDataSetObservable = new DataSetObservable();
public boolean hasStableIds() {
return false;
}
public void registerDataSetObserver(DataSetObserver observer) {
mDataSetObservable.registerObserver(observer);
}
接着看下面
DataSetObservable 的直接父類如下
public abstract class Observable<T> {
/**
* The list of observers. An observer can be in the list at most
* once and will never be null.
*/
protected final ArrayList<T> mObservers = new ArrayList<T>();
/**
* Adds an observer to the list. The observer cannot be null and it must not already
* be registered.
* @param observer the observer to register
* @throws IllegalArgumentException the observer is null
* @throws IllegalStateException the observer is already registered
*/
public void registerObserver(T observer) {
if (observer == null) {
throw new IllegalArgumentException("The observer is null.");
}
synchronized(mObservers) {
if (mObservers.contains(observer)) {
throw new IllegalStateException("Observer " + observer + " is already registered.");
}
mObservers.add(observer);
}
}
}
DataSetObservable其實就是就通知者,裏面實現方法有增加觀察者和通知觀察者等
既然是觀察者模式那麼在什麼時候通知者開始通知觀察者呢?請看下面
ListView中的AddHeaderView和removeHeaderView的時候會通知一次觀察者,addFooterView和removeFooterView也同樣會通知
public void addHeaderView(View v, Object data, boolean isSelectable) {
if (mAdapter != null && ! (mAdapter instanceof HeaderViewListAdapter)) {
throw new IllegalStateException(
"Cannot add header view to list -- setAdapter has already been called.");
}
FixedViewInfo info = new FixedViewInfo();
info.view = v;
info.data = data;
info.isSelectable = isSelectable;
mHeaderViewInfos.add(info);
// in the case of re-adding a header view, or adding one later on,
// we need to notify the observer
if (mAdapter != null && mDataSetObserver != null) {
mDataSetObserver.onChanged();
}
}
public boolean removeHeaderView(View v) {
if (mHeaderViewInfos.size() > 0) {
boolean result = false;
if (mAdapter != null && ((HeaderViewListAdapter) mAdapter).removeHeader(v)) {
if (mDataSetObserver != null) {
mDataSetObserver.onChanged();
}
result = true;
}
removeFixedViewInfo(v, mHeaderViewInfos);
return result;
}
return false;
}
我們都知道在使用ListView時都會使用到notifyDataSetChanged()這個方法,那個這個方法做什麼事呢?請下面
notifyDataSetChanged()值BaseAdapter裏面的方法
/**
* 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();
}
調用這個方法後通知者mDataSetObservable執行notifyChanged()操作,
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();
}
}
}
顯然這個方法面同樣通知了觀察者。而ListView每次的通知都會刷新一次ListView。
到此觀察者模式在android應用中的實現就在此結束
最常用的Java設計之一 觀察者模式 在android中的實例 ListView Adapter機制
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