Android设计模式之观察者模式

1、 /* * 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： 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、 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

2、变的时候， 够同时改变。 * Observer 模式就是解决了这一个问题。 对同一组数据进行统计分 柱状图统计显示、 百分比 所有的统计的显示都能 * 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

3、） 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(Observabl

4、eT)，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; /* The typically added to a * Receives call backs when a data set has been changed, or madein

5、valid. data sets * that are observed are link Cursors or link android.widget.Adapters. * DataSetObserver must be implemented by objects which are DataSetObservable. */ public abstract class DataSetObserver /* * This method is called when the entire data set has changed, * most likely through a call

6、to link Cursor#requery() on a link Cursor. */ public void onChanged() / Do nothing /* * This method is called when the entire data becomes invalid, or link 撤销 * most likely through a call to link Cursor#deactivate() Cursor#close() on a * link Cursor. */ public void onInvalidated() / Do nothing Subje

7、ct( 目标 ) ， ObservableT 是一个泛型的抽象类，主要功能是注册和 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 ObservableT /* * The list

8、 of observers. An observer can be in the list at most * once and will never be null. */ protected final ArrayListT mObservers = new ArrayListT(); /* * 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

9、 IllegalArgumentException the observer is null * throws IllegalStateException the observer is already registered */ public void registerObserver(T observer) if (observer = null) observer is null.); thrownew IllegalArgumentException(The synchronized(mObservers) if (mObservers.contains(observer) + obs

10、erver + throw new IllegalStateException(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 t

11、he observer is null * throws IllegalStateException the observer is not yet registered */ public void unregisterObserver(T observer) if (observer = null) observer is null.); thrownew IllegalArgumentException(The synchronized(mObservers) int index = mObservers.indexOf(observer); if (index = -1) + obse

12、rver + throw new IllegalStateException(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 ArrayListObserver 下的每个

13、 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 ex

14、tends ObservableDataSetObserver /* * 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 a

15、nything, 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();

16、/* * 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

17、 ）, 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 extends class AdapterDataSetObserver AdapterViewListAdapter.AdapterDataSetObserver Override public void onChanged() super.onChanged(); if (mFastScroller != null) mFastScro

18、ller.onSectionsChanged(); Override public void onInvalidated() super.onInvalidated(); if (mFastScroller != null) mFastScroller.onSectionsChanged(); framework/base/core/java/android/widget/AdapterView.java class AdapterDataSetObse( 参 考 ： http:/)rver DataSetObserver private Parcelable mInstanceState =

19、 null; Override public void onChanged() mDataChanged = true; mOldItemCount = mItemCount; mItemCount = getAdapter().getCount(); if (DBG) Xlog.d(TAG, AdapterView onChanged: mOldItemCount = + mOldItemCount ,mItemCount ,getAdapter() = + getAdapter() + ,AdapterView Throwable(onChanged); + mItemCount + +

20、AdapterView.this, new / 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

21、 = null; else rememberSyncState(); checkFocus(); requestLayout(); extends Override public void onInvalidated() mDataChanged = true; if (DBG) Xlog.d(TAG, + mOldItemCount AdapterView onInvalidated: mOldItemCount = + mItemCount + + AdapterView.this, new ,mItemCount ,getAdapter() = + getAdapter() + ,Ada

22、pterView Throwable(onInvalidated); if (AdapterView.this.getAdapter().hasStableIds() / Rememberthe 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 mOldI

23、temCount = 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; 型运用是大家熟悉

24、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 item 实例： 达到通知数据改变的作用。使用 ListView 和 BaseAdapter 组合时，当 BaseAdapter 的 改变时，我们经常会调用 notifyDataSetChanged() ，通知

25、 Listview 刷新。 的 如何 但 是 ， 但 是 ， 但 是 ， 我 们 从 来 没 有 调 用 BaseAdapter registerDataSetObserver(DataSetObserver observer) 注册 Observer ，那么 Listview 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, *

26、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. *

27、 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 HeaderViewListAdapt

28、er(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(); chec