Android异步加载图像小结

研究了android从网络上异步加载图像，现总结如下：

（1）由于androidUI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

    privatevoid 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：

    finalHandler 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 voidloadImage3(final String url, final int id) {

       executorService.submit(new Runnable() {

           public void run() {

               try {

                   final Drawable drawable = Drawable.createFromStream(newURL(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）

    publicMap<String,SoftReference<Drawable>>imageCache = new HashMap<String,SoftReference<Drawable>>();

    privateExecutorService executorService =Executors.newFixedThreadPool(5);   //固定五个线程来执行任务

    privatefinal Handler handler=new Handler();

    

    publicDrawable loadDrawable(final String imageUrl, final ImageCallbackcallback) {

       //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(newURL(imageUrl).openStream(), "image.png");

                   imageCache.put(imageUrl, newSoftReference<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

    protectedDrawable 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

    publicinterface ImageCallback {