Android设计模式之观察者模式中的DataSetObservable和DataSetObserver
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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
*
*
* */
public abstract class Observable<T> {
protected final ArrayList<T> mObservers = new ArrayList<T>();
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);
}
}
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);
}
}
public void unregisterAll() {
synchronized(mObservers) {
mObservers.clear();
}
}
}Observable的直接继承者有两个：DataSetObservable和ContentObservable。ContentObservable实现比较复杂，不过功能与DataSetObservable类似，这里只讲解DataSetObservable。
观察者（DataSetObserver），目标（Observable<T>），具体目标（DataSetObserverable）
DataSetObservable在很多的Adapter中都用到，像BaseAdapter。DataSetObservable使用DataSetObserver实例化了Observable。DataSetObserver表示了一个数据集对象的观察者，主要提供了两个方法：
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。
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