Android編程設(shè)計模式之觀察者模式實例詳解

/**
 * 抽象觀察者類，為所有具體觀察者定義一個接口，在得到通知時更新自己
 */
public interface Observer {
  /**
   * 有更新
   * 
   * @param message 消息
   */
  public void update(String message);
}

/**
 * 抽象被觀察者類
 */
public interface Observable {
  /**
   * 推送消息
   * 
   * @param message 內(nèi)容
   */
  void push(String message);
  /**
   * 訂閱
   * 
   * @param observer 訂閱者
   */
  void register(Observer observer);
}

/**
 * 具體的觀察者類，也就是訂閱者
 */
public class User implements Observer {
  @Override
  public void update(String message) {
    System.out.println(name + "," + message + "更新了！");
  }
  // 訂閱者的名字
  private String name;
  public User(String name) {
    this.name = name;
  }
}

/**
 * 具體的被觀察者類，也就是訂閱的節(jié)目
 */
public class Teleplay implements Observable{
  private List<Observer> list = new ArrayList<Observer>();//儲存訂閱者
  @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) {
    //被觀察者，這里就是用戶訂閱的電視劇
    Teleplay teleplay = new Teleplay();
    //觀察者，這里就是訂閱用戶
    User user1 = new User("小明");
    User user2 = new User("小光");
    User user3 = new User("小蘭");
    //訂閱
    teleplay.register(user1);
    teleplay.register(user2);
    teleplay.register(user3);
    //推送新消息
    teleplay.push("xxx電視劇");
  }
}

小明,xxx電視劇更新了！
小光,xxx電視劇更新了！
小蘭,xxx電視劇更新了！

public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {
  //數(shù)據(jù)集觀察者
  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);
  }
  /**
   * 當數(shù)據(jù)集變化時，通知所有觀察者
   */
  public void notifyDataSetChanged() {
    mDataSetObservable.notifyChanged();
  }
}

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();
      }
    }
  }
}

@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);//注冊觀察者
      ......省略
    }
}

class AdapterDataSetObserver extends AdapterView<ListAdapter>.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;
  // 上文有說道，調(diào)用Adapter的notifyDataSetChanged的時候會調(diào)用所有觀察者的onChanged方法,核心實現(xiàn)就在這里
  @Override
  public void onChanged() {
    mDataChanged = true;
    mOldItemCount = mItemCount;
    // 獲取Adapter中數(shù)據(jù)的數(shù)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組件
    requestLayout();
  }
  // 代碼省略
  public void clearSavedState() {
    mInstanceState = null;
  }
}