从Android ListView 看Observer 观察者设计模式setOnClickListener

这里我不说观察这模式的理论，定义。举例说明我对观察者设计模式的理解。

Android开发中经常用到:

mBtn.setOnClickListener(new View.OnClickListener() {      @Override      public void onClick(View v) {      }});

这段代码中 mBtn 是被观察者， OnClickListener 是观察者。 二者通过setOnClickListener() 方法达成订阅关系。setOnClickListener方法使得mBtn保存了调用onClick()方法的机会。至于mBtn怎么去调用onClick方法就看mBtn怎么去实现了，很明显当mBtn被点击的时候，就会去判断是否注册了onClick方法，如果有就调用。
总结起来就是：
（Button-> 被观察者( Observable)、OnClickListener ->观察者(Observer)、setOnClickListener() ->订阅(subscribe)，onClick() -> 事件(event)）
观察者模式抽象出来就是： Observable.subscribe(Observer)

开始分析ListView:

mListView = (ListView) findViewById(R.id.listview);mAdapter = new MyListAdapter(mContext, mListData);mListView.setAdapter(mAdapter);mAdapter.notifyDataSetChanged();

 从上面四行对ListView的使用方式来看。 

   /************接口层 begin***********/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) {             synchronized(mObservers) {            mObservers.add(observer);        }    }}/** * Receives call backs when a data set has been changed, or made invalid. The typically data sets * that are observed are {@link Cursor}s or {@link android.widget.Adapter}s. * DataSetObserver must be implemented by objects which are added to a DataSetObservable. */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    }}public class DataSetObservable extends Observable<DataSetObserver> {    /**     * Invokes {@link DataSetObserver#onChanged} on each observer.     * Called when the contents of the data set have changed.  The recipient     * will obtain the new contents the next time it queries the data set.     */    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();            }        }    }}   /************接口层 end***********//************中间层 begin***********/public abstract class AdapterView<T extends Adapter> extends ViewGroup {/* 改变View 的展示 */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();//刷新view布局        }    }}public abstract class AbsListView extends AdapterView<ListAdapter>  {/**     * Should be used by subclasses to listen to changes in the dataset     */    AdapterDataSetObserver mDataSetObserver;    /**     * The adapter containing the data to be displayed by this view     */    ListAdapter mAdapter;    class AdapterDataSetObserver extends AdapterView<ListAdapter>.AdapterDataSetObserver {        @Override        public void onChanged() {            super.onChanged();        }    }}/*************中间层 end****************//*************最终实现层begin****************/@RemoteViewpublic class ListView extends AbsListView {    /**     * Sets the data behind this ListView.     *     * The adapter passed to this method may be wrapped by a {@link WrapperListAdapter},     * depending on the ListView features currently in use. For instance, adding     * headers and/or footers will cause the adapter to be wrapped.     *     * @param adapter The ListAdapter which is responsible for maintaining the     *        data backing this list and for producing a view to represent an     *        item in that data set.     *     * @see #getAdapter()      */    @Override    public void setAdapter(ListAdapter adapter) {mAdapter = adapter;    }    mDataSetObserver = new AdapterDataSetObserver();    mAdapter.registerDataSetObserver(mDataSetObserver);}public abstract class BaseAdapter implements ListAdapter {      private final DataSetObservable mDataSetObservable = new DataSetObservable();       public void registerDataSetObserver(DataSetObserver observer) {        mDataSetObservable.registerObserver(observer);    }    /**     * 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();    }}/*************最终实现层end************/