Android 设计模式之观察者模式

Jcheng一霸 · 发表于 2013-8-28 16:27

4/*
* 观察者模式
*                定义对象间的一种一个(Subject)对多(Observer)的依赖关系，当一个对象的状态发送改变时，所以依赖于它的
* 对象都得到通知并被自动更新
*
* 当然，MVC只是Observer模式的一个实例。Observer模式要解决的问题为：
* 建立一个一（Subject）对多（Observer）的依赖关系，并且做到当“一”变化的时候，
* 依赖这个“一”的多也能够同步改变。最常见的一个例子就是：对同一组数据进行统计分析时候，
* 我们希望能够提供多种形式的表示（例如以表格进行统计显示、柱状图统计显示、百分比统计显示等）。
* 这些表示都依赖于同一组数据，我们当然需要当数据改变的时候，所有的统计的显示都能够同时改变。
* Observer模式就是解决了这一个问题。
*
* 适用性：
*                1. 当一个抽象模型有两个方面，其中一个方面依赖于另一方面
*                将这两者封装成独立的对象中以使它们可以各自独立的改变和服用
*
*                2. 当对一个对象的改变需要同时改变其他对象，而不知道具体有多少对象有待改变
*
*                3. 当一个对象必须通知其它对象，而它又不能假定其它对象是谁
*
* 参与者：
*                1. Subject（目标）
*                目标知道它的观察者，可以有任意多个观察者观察同一个目标
*                提供注册和删除观察者对象的接口
*
*                2. Observer（观察者）
*                为那些在目标发生改变时需获得通知的对象定义个更新的接口
*
*                3. ConcreteSubject（具体目标）
*                将有关状态存入各ConcreteObserver对象
*                当它的状态发送改变时，向它的各个观察者发出通知
*
*                4. ConcreteObserver（具体观察者）
*                维护一个指向ConcreteObserver对象的引用
*                存储有关状态，这些状态应与目标的状态保持一致
*                实现Observer的更新接口是自身状态与目标的状态保持一致
*
*
* */

下面看看Android使用到的观察者模式.观察者（DataSetObserver），目标（Observable<T>），具体目标（DataSetObserverable），DataSetObserver抽象2个方法，一个是观察数据改变的方法，一个是观察数据变成无效（或者不可用）时的方法。源码路径：framework/base/core/java/android/database/DataSetObserver.java

package android.database;

/**
* 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
}
}

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，Observable<T>是一个泛型的抽象类，主要功能是注册和撤销observer。源码路径：framework/base/core/java/android/database/Observable.java

package android.database;

import java.util.ArrayList;

/**
* Provides methods for (un)registering arbitrary observers in an ArrayList.
*/
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 observer
   */
public void unregisterAll() {
      synchronized(mObservers) {
         mObservers.clear();
      }
}
}

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，实现的方法同Oberver一样，只不过它是通知ArrayList<Observer>下的每个Oberver去执行各自的action。源码路径：framework/base/core/java/android/database/DataSetObservable.java

package android.database;

/**
* A specialization of Observable for DataSetObserver that provides methods for
* invoking the various callback methods of DataSetObserver.
*/
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) {
         // 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 onInvalidated on each observer. Called when the data set being monitored
   * has changed such that it is no longer valid.
   */
public void notifyInvalidated() {
      synchronized (mObservers) {
         for (int i = mObservers.size() - 1; i >= 0; i--) {
            mObservers.get(i).onInvalidated();
         }
      }
}
}

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,具体观察者的任务是实实在在执行action的类，一般由开发者根据实际情况，自己实现。android也有实现的例子源码路径：framework/base/core/java/android/widget/AbsListView.java

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();
         }
      }
}

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framework/base/core/java/android/widget/AdapterView.java

class AdapterDataSetObserver extends DataSetObserver {

      private Parcelable mInstanceState = null;

      @Override
      public void onChanged() {
         mDataChanged = true;
         mOldItemCount = mItemCount;
         mItemCount = getAdapter().getCount();
         if (DBG) {
            Xlog.d(TAG, "AdapterView onChanged: mOldItemCount = " + mOldItemCount
                     + ",mItemCount = " + mItemCount + ",getAdapter() = " + getAdapter()
                     + ",AdapterView = " + AdapterView.this, new Throwable("onChanged"));
         }

         // 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();
      }

      @Override
      public void onInvalidated() {
         mDataChanged = true;

         if (DBG) {
            Xlog.d(TAG, "AdapterView onInvalidated: mOldItemCount = " + mOldItemCount
                     + ",mItemCount = " + mItemCount + ",getAdapter() = " + getAdapter()
                     + ",AdapterView = " + AdapterView.this, new Throwable("onInvalidated"));
         }

         if (AdapterView.this.getAdapter().hasStableIds()) {
            // Remember the current state for the case where our hosting activity is being
            // stopped and later restarted
            mInstanceState = AdapterView.this.onSaveInstanceState();
         }

         // Data is invalid so we should reset our state
         mOldItemCount = mItemCount;
         mItemCount = 0;
         mSelectedPosition = INVALID_POSITION;
         mSelectedRowId = INVALID_ROW_ID;
         mNextSelectedPosition = INVALID_POSITION;
         mNextSelectedRowId = INVALID_ROW_ID;
         mNeedSync = false;

         checkFocus();
         requestLayout();
      }

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

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型运用是大家熟悉的BaseAdapter，BaseAdapter关联了一个DataSetObservable对象，并实现registerDataSetObserver和unregisterDataSetObserver两个方法实现注册和撤销Observer，方法notifyDataSetChanged间接调用Observer的实现者的onChange()方法，以达到通知数据改变的作用。使用ListView和BaseAdapter组合时，当BaseAdapter的item改变时，我们经常会调用notifyDataSetChanged()，通知Listview刷新。但是，但是，但是，我们从来没有调用BaseAdapter的registerDataSetObserver(DataSetObserver observer)注册Observer，那么Listview如何接收到通知，并执行刷新动作呢？我们来看看ListView做了什么

/**
   * 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) {
      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);

         mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());

         int position;
         if (mStackFromBOTTom) {
            position = lookForSelectablePosition(mItemCount - 1, false);
         } else {
            position = lookForSelectablePosition(0, true);
         }
         setSelectedPositionInt(position);
         setNextSelectedPositionInt(position);

         if (mItemCount == 0) {
            // Nothing selected
            checkSelectionChanged();
         }
      } else {
         mAreAllItemsSelectable = true;
         checkFocus();
         // Nothing selected
         checkSelectionChanged();
      }

      requestLayout();
}

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注意下面3行

mAdapter = adapter;

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mDataSetObserver = new AdapterDataSetObserver();
mAdapter.registerDataSetObserver(mDataSetObserver);

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Android 设计模式 之 观察者模式

Android 设计模式 之 观察者模式

Android 设计模式之观察者模式

Android 设计模式之观察者模式