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);
        }
    }