Android编程设计模式之观察者模式实例详解

/**
 * 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 {
  @Override
  public void update(String message) {
    System.out.println(name + "," + message + "更新了！");
  }
  // 订阅者的名字
  private String name;
  public User(String name) {
    this.name = name;
  }
}

/**
 * 具体的被观察者类，也就是订阅的节目
 */
public class Teleplay implements Observable{
  private List list = new ArrayList();//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();
  public boolean hasStableIds() {
    return false;
  }
  public void registerDataSetObserver(DataSetObserver observer) {
    mDataSetObservable.registerObserver(observer);
  }
  public void unregisterDataSetObserver(DataSetObserver observer) {
    mDataSetObservable.unregisterObserver(observer);
  }
  /**
   * 当数据集变化时，通知所有观察者
   */
  public void notifyDataSetChanged() {
    mDataSetObservable.notifyChanged();
  }
}

public class DataSetObservable extends Observable {
  /**
   * 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();
      }
    }
  }
}

@Override
public void setAdapter(ListAdapter adapter) {
    if (mAdapter != null && mDataSetObserver != null) {
      mAdapter.unregisterDataSetObserver(mDataSetObserver);
    }
    resetList();
    mRecycler.clear();
    if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
      mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter);
    } else {
      mAdapter = adapter;
    }
    mOldSelectedPosition = INVALID_POSITION;
    mOldSelectedRowId = INVALID_ROW_ID;
    // AbsListView#setAdapter will update choice mode states.
    super.setAdapter(adapter);
    if (mAdapter != null) {
      mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
      mOldItemCount = mItemCount;
      mItemCount = mAdapter.getCount();
      checkFocus();
      mDataSetObserver = new AdapterDataSetObserver();
      mAdapter.registerDataSetObserver(mDataSetObserver);//注册观察者
      ......省略
    }
}

class AdapterDataSetObserver extends AdapterView.AdapterDataSetObserver {
  @Override
  public void onChanged() {
    super.onChanged();
    if (mFastScroller != null) {
      mFastScroller.onSectionsChanged();
    }
  }
  @Override
  public void onInvalidated() {
    super.onInvalidated();
    if (mFastScroller != null) {
      mFastScroller.onSectionsChanged();
    }
  }
}

class AdapterDataSetObserver extends DataSetObserver {
  private Parcelable mInstanceState = null;
  // 上文有说道，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;
  }
}