异步加载图片

研究了android从网络上异步加载图像，现总结如下：

（1）由于android UI更新支持单一线程原则，所以从网络上取数据并更新到界面上，为了不阻塞主线程首先可能会想到以下方法。

     在主线程中new 一个Handler对象，加载图像方法如下所示

    private void loadImage(final String url, final int id) {
         handler.post(new Runnable() {
                public void run() {
                    Drawable drawable = null;
                    try {
                        drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
                    } catch (IOException e) {
                    }
                    ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable);
                }
            });
    }

上面这个方法缺点很显然，经测试，如果要加载多个图片，这并不能实现异步加载，而是等到所有的图片都加载完才一起显示，因为它们都运行在一个线程中。

然后，我们可以简单改进下，将Handler+Runnable模式改为Handler+Thread+Message模式不就能实现同时开启多个线程吗？

（2）在主线程中new 一个Handler对象，代码如下：

    final Handler handler2=new Handler(){
          @Override
          public void handleMessage(Message msg) {
             ((ImageView) LazyLoadImageActivity.this.findViewById(msg.arg1)).setImageDrawable((Drawable)msg.obj);
          }
      };

    对应加载图像代码如下：

  //采用handler+Thread模式实现多线程异步加载
     private void loadImage2(final String url, final int id) {
         Thread thread = new Thread(){
             @Override
             public void run() {
               Drawable drawable = null;
                    try {
                        drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
                    } catch (IOException e) {
                    }

                Message message= handler2.obtainMessage() ;
                 message.arg1 = id;
                 message.obj = drawable;
                 handler2.sendMessage(message);
             }
         };
         thread.start();
         thread = null;
    }

这样就简单实现了异步加载了。细想一下，还可以优化的，比如引入线程池、引入缓存等，我们先介绍线程池。

（3）引入ExecutorService接口，于是代码可以优化如下：

       在主线程中加入：private ExecutorService executorService = Executors.newFixedThreadPool(5);  

       对应加载图像方法更改如下：

      // 引入线程池来管理多线程
    private void loadImage3(final String url, final int id) {
        executorService.submit(new Runnable() {
            public void run() {
                try {
                    final Drawable drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
                    handler.post(new Runnable() {

                        public void run() {
                            ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable);
                        }
                    });
                } catch (Exception e) {
                    throw new RuntimeException(e);
                }
            }
        });
    }

（4）为了更方便使用我们可以将异步加载图像方法封装一个类，对外界只暴露一个方法即可，考虑到效率问题我们可以引入内存缓存机制，做法是

建立一个HashMap，其键（key）为加载图像url，其值（value）是图像对象Drawable。先看一下我们封装的类

public class AsyncImageLoader3 {
   //为了加快速度，在内存中开启缓存（主要应用于重复图片较多时，或者同一个图片要多次被访问，比如在ListView时来回滚动）
    public Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();
    private ExecutorService executorService = Executors.newFixedThreadPool(5);    //固定五个线程来执行任务
    private final Handler handler=new Handler();

     /**
     *
     * @param imageUrl     图像url地址
     * @param callback     回调接口
     * @return     返回内存中缓存的图像，第一次加载返回null
     */
    public Drawable loadDrawable(final String imageUrl, final ImageCallback callback) {
        //如果缓存过就从缓存中取出数据
        if (imageCache.containsKey(imageUrl)) {
            SoftReference<Drawable> softReference = imageCache.get(imageUrl);
            if (softReference.get() != null) {
                return softReference.get();
            }
        }
        //缓存中没有图像，则从网络上取出数据，并将取出的数据缓存到内存中
         executorService.submit(new Runnable() {
            public void run() {
                try {
                    final Drawable drawable = Drawable.createFromStream(new URL(imageUrl).openStream(), "image.png");

                    imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));

                    handler.post(new Runnable() {
                        public void run() {
                           callback.imageLoaded(drawable);
                        }
                    });
                } catch (Exception e) {
                    throw new RuntimeException(e);
                }
            }
        });
        return null;
    }
     //从网络上取数据方法
    protected Drawable loadImageFromUrl(String imageUrl) {
        try {
            return Drawable.createFromStream(new URL(imageUrl).openStream(), "image.png");
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
    //对外界开放的回调接口
    public interface ImageCallback {
        //注意 此方法是用来设置目标对象的图像资源
        public void imageLoaded(Drawable imageDrawable);
    }
} 这样封装好后使用起来就方便多了。在主线程中首先要引入AsyncImageLoader3 对象，然后直接调用其loadDrawable方法即可，需要注意的是ImageCallback接口的imageLoaded方法是唯一可以把加载的图 像设置到目标ImageView或其相关的组件上。

在主线程调用代码：

  先实例化对象 private AsyncImageLoader3 asyncImageLoader3 = new AsyncImageLoader3();

  调用异步加载方法：

//引入线程池，并引入内存缓存功能,并对外部调用封装了接口，简化调用过程
    private void loadImage4(final String url, final int id) {
          //如果缓存过就会从缓存中取出图像，ImageCallback接口中方法也不会被执行
         Drawable cacheImage = asyncImageLoader.loadDrawable(url,new AsyncImageLoader.ImageCallback() {
             //请参见实现：如果第一次加载url时下面方法会执行
             public void imageLoaded(Drawable imageDrawable) {
               ((ImageView) findViewById(id)).setImageDrawable(imageDrawable);
             }
         });
        if(cacheImage!=null){
          ((ImageView) findViewById(id)).setImageDrawable(cacheImage);
        }
    }

（5）同理，下面也给出采用Thread+Handler+MessageQueue+内存缓存代码，原则同（4），只是把线程池换成了Thread+Handler+MessageQueue模式而已。代码如下：

public class AsyncImageLoader {
   //为了加快速度，加入了缓存（主要应用于重复图片较多时，或者同一个图片要多次被访问，比如在ListView时来回滚动）
    private Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();

     /**
     *
     * @param imageUrl     图像url地址
     * @param callback     回调接口
     * @return     返回内存中缓存的图像，第一次加载返回null
     */
    public Drawable loadDrawable(final String imageUrl, final ImageCallback callback) {
        //如果缓存过就从缓存中取出数据
        if (imageCache.containsKey(imageUrl)) {
            SoftReference<Drawable> softReference = imageCache.get(imageUrl);
            if (softReference.get() != null) {
                return softReference.get();
            }
        }

        final Handler handler = new Handler() {
            @Override
            public void handleMessage(Message msg) {
                callback.imageLoaded((Drawable) msg.obj);
            }
        };
        new Thread() {
            public void run() {
                Drawable drawable = loadImageFromUrl(imageUrl);
                imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
                handler.sendMessage(handler.obtainMessage(0, drawable));

            }

        }.start();
        /*
        下面注释的这段代码是Handler的一种代替方法
         */
//        new AsyncTask() {
//            @Override
//            protected Drawable doInBackground(Object... objects) {
//                  Drawable drawable = loadImageFromUrl(imageUrl);
//                imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
//                return  drawable;
//            }
//
//            @Override
//            protected void onPostExecute(Object o) {
//                  callback.imageLoaded((Drawable) o);
//            }
//        }.execute();
        return null;
    }

    protected Drawable loadImageFromUrl(String imageUrl) {
        try {
            return Drawable.createFromStream(new URL(imageUrl).openStream(), "src");
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
    //对外界开放的回调接口
    public interface ImageCallback {
        public void imageLoaded(Drawable imageDrawable);
    }
}

至此，异步加载就介绍完了，下面给出的代码为测试用的完整代码：

package com.bshark.supertelphone.activity;

import android.app.Activity;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.ImageView;
import com.bshark.supertelphone.R;
import com.bshark.supertelphone.ui.adapter.util.AsyncImageLoader;
import com.bshark.supertelphone.ui.adapter.util.AsyncImageLoader3;

import java.io.IOException;
import java.net.URL;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class LazyLoadImageActivity extends Activity {
       final Handler handler=new Handler();
      final Handler handler2=new Handler(){
          @Override
          public void handleMessage(Message msg) {
             ((ImageView) LazyLoadImageActivity.this.findViewById(msg.arg1)).setImageDrawable((Drawable)msg.obj);
          }
      };
private ExecutorService executorService = Executors.newFixedThreadPool(5);    //固定五个线程来执行任务
    private AsyncImageLoader asyncImageLoader = new AsyncImageLoader();
    private AsyncImageLoader3 asyncImageLoader3 = new AsyncImageLoader3();

@Override
public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
 
//  loadImage("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);
//  loadImage("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);
//  loadImage("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);
//        loadImage("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
//  loadImage("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);

        loadImage2("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);
  loadImage2("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);
  loadImage2("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);
        loadImage2("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
  loadImage2("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);
//        loadImage3("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);
//  loadImage3("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);
//  loadImage3("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);
//        loadImage3("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
//  loadImage3("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);

//        loadImage4("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);
//  loadImage4("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);
//  loadImage4("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);
//        loadImage4("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
//  loadImage4("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);

//        loadImage5("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);
//        //为了测试缓存而模拟的网络延时
//        SystemClock.sleep(2000);
//  loadImage5("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);
//        SystemClock.sleep(2000);
//  loadImage5("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);
//        SystemClock.sleep(2000);
//        loadImage5("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
//        SystemClock.sleep(2000);
//  loadImage5("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);
//        SystemClock.sleep(2000);
//         loadImage5("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
}

@Override
protected void onDestroy() {
  executorService.shutdown();
  super.onDestroy();
}
    //线程加载图像基本原理
    private void loadImage(final String url, final int id) {
         handler.post(new Runnable() {
                public void run() {
                    Drawable drawable = null;
                    try {
                        drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
                    } catch (IOException e) {
                    }
                    ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable);
                }
            });
    }
     //采用handler+Thread模式实现多线程异步加载
     private void loadImage2(final String url, final int id) {
         Thread thread = new Thread(){
             @Override
             public void run() {
               Drawable drawable = null;
                    try {
                        drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
                    } catch (IOException e) {
                    }

                Message message= handler2.obtainMessage() ;
                 message.arg1 = id;
                 message.obj = drawable;
                 handler2.sendMessage(message);
             }
         };
         thread.start();
         thread = null;
    }
    // 引入线程池来管理多线程
    private void loadImage3(final String url, final int id) {
        executorService.submit(new Runnable() {
            public void run() {
                try {
                    final Drawable drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
                    handler.post(new Runnable() {

                        public void run() {
                            ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable);
                        }
                    });
                } catch (Exception e) {
                    throw new RuntimeException(e);
                }
            }
        });
    }
    //引入线程池，并引入内存缓存功能,并对外部调用封装了接口，简化调用过程
    private void loadImage4(final String url, final int id) {
          //如果缓存过就会从缓存中取出图像，ImageCallback接口中方法也不会被执行
         Drawable cacheImage = asyncImageLoader.loadDrawable(url,new AsyncImageLoader.ImageCallback() {
             //请参见实现：如果第一次加载url时下面方法会执行
             public void imageLoaded(Drawable imageDrawable) {
               ((ImageView) findViewById(id)).setImageDrawable(imageDrawable);
             }
         });
        if(cacheImage!=null){
          ((ImageView) findViewById(id)).setImageDrawable(cacheImage);
        }
    }

    //采用Handler+Thread+封装外部接口
    private void loadImage5(final String url, final int id) {
          //如果缓存过就会从缓存中取出图像，ImageCallback接口中方法也不会被执行
         Drawable cacheImage = asyncImageLoader3.loadDrawable(url,new AsyncImageLoader3.ImageCallback() {
             //请参见实现：如果第一次加载url时下面方法会执行
             public void imageLoaded(Drawable imageDrawable) {
               ((ImageView) findViewById(id)).setImageDrawable(imageDrawable);
             }
         });
        if(cacheImage!=null){
                    ((ImageView) findViewById(id)).setImageDrawable(cacheImage);
        }
    }


}

xml文件大致如下：

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="fill_parent"
              android:orientation="vertical"
              android:layout_height="fill_parent" >
  <ImageView android:id="@+id/image1" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView>
   <ImageView android:id="@+id/image2" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView>
    <ImageView android:id="@+id/image3" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView>
    <ImageView android:id="@+id/image5" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView>
    <ImageView android:id="@+id/image4" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView>
</LinearLayout>