Android开源代码解读の地图照片应用Panoramio的实现详解（四）

本文继续讲解Panoramio的实现，主要介绍ImageAdapter.java和ImageList.java 这两个文件，这两个文件实现了如下所示的界面，左图是数据从网络加载过程中，有图是加载完成后的效果：

ImageAdapter继承自BaseAdapter类，实现图片适配器的功能，而ImageList则继承自ListActivity，用于以列表形式显示搜索到的图片信息。介绍之前，先来普及下DataSetObserver和DataSetObservable的知识。

从名字可以依稀猜到，DataSetObserver实现了观察者模式中的观察者角色（Observer）。当数据集发生变化或者变为无效时，DataSetObserver中的方法被回调，典型的数据集有Cursor和Adapter，当某个对象要添加到DataSetObservable中时，这个对象必须从DataSetObserver继承，DataSetObserver是一个抽象类，定义如下：

[java]
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
     }
}
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
     }
}
而DataSetObservable实现了观察者模式中的对象角色（Subject），它是Observable的具体实现，提供了调用DataSetObserver中各种回调函数的方法，定义如下所示：

[java]
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) {
             for (DataSetObserver observer : mObservers) {
                 observer.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      * has changed such that it is no longer valid.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 (DataSetObserver observer : mObservers) {
                 observer.onInvalidated();
             }
         }
     }
}
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) {
             for (DataSetObserver observer : mObservers) {
                 observer.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 (DataSetObserver observer : mObservers) {
                 observer.onInvalidated();
             }
         }
     }
}
注意这里的Observable是定义在android.database包中的，而不是JDK中的Observable，它的定义如下：

[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

补充：移动开发 , Android ,