Android 设计模式 之 观察者模式

/* * 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。