Android开源代码解读の地图照片应用Panoramio的实现详解（四）

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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：

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：

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     * has changed such that it is no longer valid.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：

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();
        }        
    }
}

对观察者模式的详细描述，可见这篇文章http://blog.csdn.net/ace1985/article/details/5753658。OK，言归正传，还是来看下我们的ImageAdapter.java，从上面的分析和代码里面的注释应该很好理解了：

package com.google.android.panoramio;

import android.content.Context;
import android.database.DataSetObserver;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;

/**
 * 用来给ImageList绑定图片资源的适配器
 */
public class ImageAdapter extends BaseAdapter {

    /**
     * Maintains the state of our data
     */
    private ImageManager mImageManager;

    private Context mContext;
    
    private MyDataSetObserver mObserver;

    /**
     * ImageManager扮演的是Subject的角色，这个类的实例将被添加到ImagManager类
     * 中的观察者列表中，当ImageList中的数据发生变化时（由ImageManager来检测）
     * ImageManager将通告MyDataSetObserver实例发生的变化
     */
    private class MyDataSetObserver extends DataSetObserver {
        @Override
        public void onChanged() {
        	//BaseAdapter维护了一个DataSetObservable对象mDataSetObservable
        	//这个函数用于通告mDataSetObservable的所有观察者数据发生变化
            notifyDataSetChanged(); 
        }

        @Override
        public void onInvalidated() {
        	//BaseAdapter维护了一个DataSetObservable对象mDataSetObservable
        	//这个函数用于通告mDataSetObservable的所有观察者数据变为无效
            notifyDataSetInvalidated();
        }
    }
    
    public ImageAdapter(Context c) {
        mImageManager = ImageManager.getInstance(c);
        mContext = c;
        mObserver = new MyDataSetObserver();
        
        mImageManager.addObserver(mObserver); //将mObserver设置为mImageManager的观察者
    }

    /**
     * 返回显示的图片的数目
     * 
     * @see android.widget.Adapter#getCount()
     */
    public int getCount() {
        return mImageManager.size();
    }

    /**
     * 返回指定索引的图片
     * 
     * @see android.widget.Adapter#getItem(int)
     */
    public Object getItem(int position) {
        return mImageManager.get(position);
    }

    /**
     * 返回指定索引的图片ID
     * 
     * @see android.widget.Adapter#getItemId(int)
     */
    public long getItemId(int position) {
        PanoramioItem s = mImageManager.get(position);
        return s.getId();
    }

    /**
     * 返回指定索引处用于显示图片的view
     * 
     * @param position 索引
     * @param convertView 可以重用的view，可能为null.
     * @param parent 返回的view的父view.
     * @return 用于显示指定索引处图片的view
     */
    public View getView(int position, View convertView, ViewGroup parent) {
        View view;
        if (convertView == null) {
            // 创建新的view
            LayoutInflater inflater = (LayoutInflater) mContext
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            view = inflater.inflate(R.layout.image_item, null);
        } else {
            // 使用已经存在的view
            view = convertView;
        }
        PanoramioItem s = mImageManager.get(position);

        ImageView i = (ImageView) view.findViewById(R.id.image);
        i.setImageBitmap(s.getBitmap()); //将位图设置到view上
        i.setBackgroundResource(R.drawable.picture_frame); //设置ImageView的背景图片
        
        TextView t = (TextView) view.findViewById(R.id.title);
        t.setText(s.getTitle()); //设置图片名称
        
        t = (TextView) view.findViewById(R.id.owner);
        t.setText(s.getOwner()); //设置图片作者
        return view;
    }

}

同理，ImageList.java文件内容如下所示：

package com.google.android.panoramio;

import android.app.ListActivity;
import android.content.Context;
import android.content.Intent;
import android.database.DataSetObserver;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.Window;
import android.widget.ListView;

/**
 * 显示图片列表的Activity
 */
public class ImageList extends ListActivity {
    
    ImageManager mImageManager;
    
    private MyDataSetObserver mObserver = new MyDataSetObserver();

    /**
     * 保存用户在主界面选取搜索区域时所用的缩放级别
     */
    private int mZoom;

    /**
     * 保存用户在主界面选取的搜索区域的中心点纬度
     */
    private int mLatitudeE6;

    /**
     * 保存用户在主界面选取的搜索区域的中心点经度
     */
    private int mLongitudeE6;

    /**
     * 注册为ImageManager实例的观察者，用于当ImageManager结束图片下载时将ImageList界面上的加载进度显示关闭
     */
    private class MyDataSetObserver extends DataSetObserver {
        @Override
        public void onChanged() {
            if (!mImageManager.isLoading()) {
                getWindow().setFeatureInt(Window.FEATURE_INDETERMINATE_PROGRESS,
                        Window.PROGRESS_VISIBILITY_OFF);
            }
        }

        @Override
        public void onInvalidated() {
        }
    }
    
    @Override
    public void onCreate(Bundle savedInstanceState) {
        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);//显示不确定进度
        super.onCreate(savedInstanceState);
        
        mImageManager = ImageManager.getInstance(this);
        
        //获取ListView，并在底部添加版权信息
        ListView listView = getListView();
        LayoutInflater inflater = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View footer = inflater.inflate(R.layout.list_footer, listView, false);
        listView.addFooterView(footer, null, false);
        
        //将自定义的ImageAdapter设置给这个ListActivity
        setListAdapter(new ImageAdapter(this));

        //在AndroidManifest.xml文件中为我们的List设置了Theme.Light，这里将背景移除
        listView.setBackgroundDrawable(null);
        //当ImageManager还在下载资源时，显示进度条为忙，并注册观察者用于下载结束时隐藏进度条
        if (mImageManager.isLoading()) {
            getWindow().setFeatureInt(Window.FEATURE_INDETERMINATE_PROGRESS,
                    Window.PROGRESS_VISIBILITY_ON);
            mImageManager.addObserver(mObserver);
        }
        
        //保存从Panoramio主界面传过来的搜索区域相关信息
        Intent i = getIntent();
        mZoom = i.getIntExtra(ImageManager.ZOOM_EXTRA, Integer.MIN_VALUE);
        mLatitudeE6 = i.getIntExtra(ImageManager.LATITUDE_E6_EXTRA, Integer.MIN_VALUE);
        mLongitudeE6 = i.getIntExtra(ImageManager.LONGITUDE_E6_EXTRA, Integer.MIN_VALUE);
    }

    @Override
    protected void onListItemClick(ListView l, View v, int position, long id) {
        PanoramioItem item = mImageManager.get(position);   
        
        //创建Intent用于传递相关数据给ViewImage，用于显示单张图片信息
        Intent i = new Intent(this, ViewImage.class);
        i.putExtra(ImageManager.PANORAMIO_ITEM_EXTRA, item);
        i.putExtra(ImageManager.ZOOM_EXTRA, mZoom);
        i.putExtra(ImageManager.LATITUDE_E6_EXTRA, mLatitudeE6);
        i.putExtra(ImageManager.LONGITUDE_E6_EXTRA, mLongitudeE6);
        startActivity(i);
    }   
    
}

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