Android 設計模式 之 觀察者模式

/*
 * 觀察者模式
 *      定義對象間的一種一個(Subject)對多(Observer)的依賴關係，當一個對象的狀態發送改變時，所以依賴於它的
 * 對象都得到通知並被自動更新
 *
 * 當然，MVC只是Observer模式的一個實例。Observer模式要解決的問題爲：
 * 建立一個一（Subject）對多（Observer）的依賴關係，並且做到當“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：對同一組數據進行統計分析時候，
 * 我們希望能夠提供多種形式的表示（例如以表格進行統計顯示、柱狀圖統計顯示、百分比統計顯示等）。
 * 這些表示都依賴於同一組數據，我們當然需要當數據改變的時候，所有的統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，其中一個方面依賴於另一方面
 *      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
 *      提供註冊和刪除觀察者對象的接口
 *
 *      2. Observer（觀察者）
 *      爲那些在目標發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）
 *      將有關狀態存入各ConcreteObserver對象
 *      當它的狀態發送改變時，向它的各個觀察者發出通知
 *
 *      4. ConcreteObserver（具體觀察者）
 *      維護一個指向ConcreteObserver對象的引用
 *      存儲有關狀態，這些狀態應與目標的狀態保持一致
 *      實現Observer的更新接口是自身狀態與目標的狀態保持一致
 *
 *
 * */

有空我將把UML圖補上。

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）

Observer(觀察者)，DataSetObserver抽象2個方法，一個是觀察數據改變的方法，一個是觀察數據變成無效（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
    }
}

Subject(目標)，Observable<T>是一個泛型的抽象類，主要功能是註冊和撤銷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();
        }
    }
}

ConcreateSubject(具體目標)，實現的方法同Oberver一樣，只不過它是通知ArrayList<Observer>下的每個Oberver去執行各自的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();
            }
        }
    }
}

ConcreateObserver(具體觀察者),具體觀察者的任務是實實在在執行action的類，一般由開發者根據實際情況，自己實現。android也有實現的例子

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();
        }
    }
}

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;
    }
}

實例：

型運用是大家熟悉的BaseAdapter，BaseAdapter關聯了一個DataSetObservable對象，並實現registerDataSetObserver和unregisterDataSetObserver兩個方法實現註冊和撤銷Observer，方法notifyDataSetChanged間接調用Observer的實現者的onChange()方法，以達到通知數據改變的作用。使用ListView和BaseAdapter組合時，當BaseAdapter的item改變時，我們經常會調用notifyDataSetChanged()，通知Listview刷新。

但是，但是，但是，我們從來沒有調用BaseAdapter的registerDataSetObserver(DataSetObserver observer)註冊Observer，那麼Listview如何接收到通知，並執行刷新動作呢？

我們來看看ListView做了什麼

/**
 * 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();
}

注意下面3行

mAdapter = adapter;

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

當我們setAdapter(ListAdapter adapter)時，BaseAdapter同時註冊了AdapterDataSetObserver()，至於AdapterDataSetObserver是如何通知Listvew和每個子item刷新（invalidate）的，這裏涉及到的內容已經超出文章的範圍，具體請查看源碼。

其實，Android用到DataSetObserver的地方很多，Cursor,WebView,Adapter,...非常之多。