设计模式15---Android 观察者模式（转载自：“http://blog.csdn.net/fangchongbory/article/details/7774044”）

/*
 * 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
 *
 *
 * */

有空我将把UML图补上。

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）

Observer(观察者)，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
    }
}

Subject(目标)，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();
        }
    }
}

ConcreateSubject(具体目标)，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();
            }
        }
    }
}

ConcreateObserver(具体观察者),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();
        }
    }
}

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;
    }
}

实例：

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();
}

注意下面3行

mAdapter = adapter;

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

当我们setAdapter(ListAdapter adapter)时，BaseAdapter同时注册了AdapterDataSetObserver()，至于AdapterDataSetObserver是如何通知Listvew和每个子item刷新（invalidate）的，这里涉及到的内容已经超出文章的范围，具体请查看源码。

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