安卓设计模式之观察者模式

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/*

* 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

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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

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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

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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

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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

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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

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/**

* 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

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mAdapter = adapter;

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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。