观察者模式及在Android源码中的应用

/**
 *  抽象观察者类，为所有具体观察者定义一个接口，在得到通知时更新自己
 */
public interface Observer {
    /**
     *  有更新
     *  
     *  @param message 消息
     */
    public void update(String message);

}

/**
 * 抽象被观察者类
 */
public interface Observable {

    /**
     * 推送消息
     * 
     * @param message 内容
     */
    void push(String message);

    /**
     * 订阅
     * 
     * @param observer 订阅者
     */
    void register(Observer observer);
}

/**
 * 具体的观察者类，也就是订阅者
 */
public class User implements Observer {
    // 订阅者的名字
    private String name;

    public User(String name) {
        this.name = name;
    }

    @Override
    public void update(String message) {
        System.out.println(name + "," + message + "更新了！");

    }
}

/**
 *  具体的被观察者类，也就是订阅的节目
 */
public class Teleplay implements Observable{

    private List<Observer> list = new ArrayList<Observer>();//储存订阅者

    @Override
    public void push(String message) {
        for(Observer observer:list){
            observer.update(message);
        }
    }

    @Override
    public void register(Observer observer) {
        list.add(observer);
    }

}

public class Client {
    public static void main(String[] args) {
        //被观察者，这里就是用户订阅的电视剧
        Teleplay teleplay = new Teleplay();
        //观察者，这里就是订阅用户
        User user1 = new User("小明");
        User user2 = new User("小光");
        User user3 = new User("小兰");
        //订阅
        teleplay.register(user1);
        teleplay.register(user2);
        teleplay.register(user3);
        //推送新消息
        teleplay.push("xxx电视剧");
    }
}

小明,xxx电视剧更新了！
小光,xxx电视剧更新了！
小兰,xxx电视剧更新了！

public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {
    // 数据集观察者
    private final DataSetObservable mDataSetObservable = new DataSetObservable();

    // 代码省略

    public void registerDataSetObserver(DataSetObserver observer) {
        mDataSetObservable.registerObserver(observer);
    }

    public void unregisterDataSetObserver(DataSetObserver observer) {
        mDataSetObservable.unregisterObserver(observer);
    }

    /**
     * Notifies the attached observers that the underlying data has been changed
     * and any View reflecting the data set should refresh itself.
     * 当数据集用变化时通知所有观察者
     */
    public void notifyDataSetChanged() {
        mDataSetObservable.notifyChanged();
    }
}

public class DataSetObservable extends Observable<DataSetObserver> {
    /**
     * Invokes onChanged on each observer. Called when the data set being observed has
     * changed, and which when read contains the new state of the data.
     */
    public void notifyChanged() {
        synchronized(mObservers) {
            // 调用所有观察者的onChanged方式
            for (int i = mObservers.size() - 1; i >= 0; i--) {
                mObservers.get(i).onChanged();
            }
        }
    }

}

    @Override
    public void setAdapter(ListAdapter adapter) {
        // 如果已经有了一个adapter,那么先注销该Adapter对应的观察者
        if (mAdapter != null && mDataSetObserver != null) {
            mAdapter.unregisterDataSetObserver(mDataSetObserver);
        }

        // 代码省略

        if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
            mAdapter = wrapHeaderListAdapterInternal(mHeaderViewInfos, mFooterViewInfos, adapter);
        } else {
            mAdapter = adapter;
        }

        super.setAdapter(adapter);

        if (mAdapter != null) {
            mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
            mOldItemCount = mItemCount;
            // 获取数据的数量
            mItemCount = mAdapter.getCount();
            checkFocus();
            // 注意这里 : 创建一个数据集观察者
            mDataSetObserver = new AdapterDataSetObserver();
            // 将这个观察者注册到Adapter中，实际上是注册到DataSetObservable中
            mAdapter.registerDataSetObserver(mDataSetObserver);

            // 代码省略
        } else {
            // 代码省略
        }

        requestLayout();
    }

 class AdapterDataSetObserver extends AdapterView<ListAdapter>.AdapterDataSetObserver {
        @Override
        public void onChanged() {
            super.onChanged();
            if (mFastScroll != null) {
                mFastScroll.onSectionsChanged();
            }
        }

        @Override
        public void onInvalidated() {
            super.onInvalidated();
            if (mFastScroll != null) {
                mFastScroll.onSectionsChanged();
            }
        }
    }

class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;
        // 调用Adapter的notifyDataSetChanged的时候会调用所有观察者的onChanged方法,核心实现就在这里
        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            // 获取Adapter中数据的数量
            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();
            // 重新布局ListView、GridView等AdapterView组件
            requestLayout();
        }

        // 代码省略

        public void clearSavedState() {
            mInstanceState = null;
        }
    }