android ViewPage 的观察者模式实现

【首先是观察者的管理，采用数组型集合，线程安全，泛型可拓展】

package com.android.zgx.androidday.database;import java.util.ArrayList;/** * Provides methods for registering or unregistering arbitrary observers in an {@link ArrayList}. * * This abstract class is intended to be subclassed and specialized to maintain * a registry of observers of specific types and dispatch notifications to them. * * @param T The observer type.  观察者的行为 */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);        }    }    /**     * Removes a previously registered observer. The observer must not be null and it     * must already have been registered.     * @param observer the observer to unregister     * @throws IllegalArgumentException the observer is null     * @throws IllegalStateException the observer is not yet registered     */    public void unregisterObserver(T observer) {        if (observer == null) {            throw new IllegalArgumentException("The observer is null.");        }        synchronized(mObservers) {            int index = mObservers.indexOf(observer);            if (index == -1) {                throw new IllegalStateException("Observer " + observer + " was not registered.");            }            mObservers.remove(index);        }    }    /**     * Remove all registered observers.     */    public void unregisterAll() {        synchronized(mObservers) {            mObservers.clear();        }    }}

【接着，定义数据变化的行为】

package com.android.zgx.androidday.database;import android.database.Cursor;/** * 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    }    /**     * This method is called when the entire data becomes invalid,     * most likely through a call to {@link Cursor#deactivate()} or {@link Cursor#close()} on a     * {@link Cursor}.     */    public void onInvalidated() {        // Do nothing    }}

【然后，组织调用，数组遍历调用，线程安全】

package com.android.zgx.androidday.database;/** * A specialization of {@link Observable} for {@link DataSetObserver} * that provides methods for sending notifications to a list of * {@link DataSetObserver} objects. * * 一个个调用 */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();            }        }    }    /**     * Invokes {@link DataSetObserver#onInvalidated} on each observer.     * Called when the data set is no longer valid and cannot be queried again,     * such as when the data set has been closed.     */    public void notifyInvalidated() {        synchronized (mObservers) {            for (int i = mObservers.size() - 1; i >= 0; i--) {                mObservers.get(i).onInvalidated();            }        }    }}

【最后才是ViewPage数据变化，观察者的实现】

   private class PagerObserver extends DataSetObserver {        @Override        public void onChanged() {            dataSetChanged();        }        @Override        public void onInvalidated() {            dataSetChanged();        }    }

【结束语】

这里主要讲解的是设计模式，ViewPage 的具体观察实现，后续.....