Android 設計模式之觀察者模式

Jcheng一霸 · 發(fā)表于 2013-8-28 16:27

4/*
* 觀察者模式
*                定義對象間的一種一個(Subject)對多(Observer)的依賴關(guān)系，當一個對象的狀態(tài)發(fā)送改變時，所以依賴于它的
* 對象都得到通知并被自動更新
*
* 當然，MVC只是Observer模式的一個實例。Observer模式要解決的問題為：
* 建立一個一（Subject）對多（Observer）的依賴關(guān)系，并且做到當“一”變化的時候，
* 依賴這個“一”的多也能夠同步改變。最常見的一個例子就是：對同一組數(shù)據(jù)進行統(tǒng)計分析時候，
* 我們希望能夠提供多種形式的表示（例如以表格進行統(tǒng)計顯示、柱狀圖統(tǒng)計顯示、百分比統(tǒng)計顯示等）。
* 這些表示都依賴于同一組數(shù)據(jù)，我們當然需要當數(shù)據(jù)改變的時候，所有的統(tǒng)計的顯示都能夠同時改變。
* Observer模式就是解決了這一個問題。
*
* 適用性：
*                1. 當一個抽象模型有兩個方面，其中一個方面依賴于另一方面
*                將這兩者封裝成獨立的對象中以使它們可以各自獨立的改變和服用
*
*                2. 當對一個對象的改變需要同時改變其他對象，而不知道具體有多少對象有待改變
*
*                3. 當一個對象必須通知其它對象，而它又不能假定其它對象是誰
*
* 參與者：
*                1. Subject（目標）
*                目標知道它的觀察者，可以有任意多個觀察者觀察同一個目標
*                提供注冊和刪除觀察者對象的接口
*
*                2. Observer（觀察者）
*                為那些在目標發(fā)生改變時需獲得通知的對象定義個更新的接口
*
*                3. ConcreteSubject（具體目標）
*                將有關(guān)狀態(tài)存入各ConcreteObserver對象
*                當它的狀態(tài)發(fā)送改變時，向它的各個觀察者發(fā)出通知
*
*                4. ConcreteObserver（具體觀察者）
*                維護一個指向ConcreteObserver對象的引用
*                存儲有關(guān)狀態(tài)，這些狀態(tài)應與目標的狀態(tài)保持一致
*                實現(xiàn)Observer的更新接口是自身狀態(tài)與目標的狀態(tài)保持一致
*
*
* */

下面看看Android使用到的觀察者模式.觀察者（DataSetObserver），目標（Observable<T>），具體目標（DataSetObserverable），DataSetObserver抽象2個方法，一個是觀察數(shù)據(jù)改變的方法，一個是觀察數(shù)據(jù)變成無效（或者不可用）時的方法。源碼路徑：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>是一個泛型的抽象類，主要功能是注冊和撤銷observer。源碼路徑：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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，實現(xiàn)的方法同Oberver一樣，只不過它是通知ArrayList<Observer>下的每個Oberver去執(zhí)行各自的action。源碼路徑：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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,具體觀察者的任務是實實在在執(zhí)行action的類，一般由開發(fā)者根據(jù)實際情況，自己實現(xiàn)。android也有實現(xiàn)的例子源碼路徑：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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型運用是大家熟悉的BaseAdapter，BaseAdapter關(guān)聯(lián)了一個DataSetObservable對象，并實現(xiàn)registerDataSetObserver和unregisterDataSetObserver兩個方法實現(xiàn)注冊和撤銷Observer，方法notifyDataSetChanged間接調(diào)用Observer的實現(xiàn)者的onChange()方法，以達到通知數(shù)據(jù)改變的作用。使用ListView和BaseAdapter組合時，當BaseAdapter的item改變時，我們經(jīng)常會調(diào)用notifyDataSetChanged()，通知Listview刷新。但是，但是，但是，我們從來沒有調(diào)用BaseAdapter的registerDataSetObserver(DataSetObserver observer)注冊Observer，那么Listview如何接收到通知，并執(zhí)行刷新動作呢？我們來看看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 設計模式 之 觀察者模式

Android 設計模式 之 觀察者模式

Android 設計模式之觀察者模式

Android 設計模式之觀察者模式