最近在对编写完后的FileManager做优化，发觉其中异步加载图片的那块代码还需要在重构一下。

首先我先说明一下，该FileManager中显示文件图标的控件为GridView，并且最大可视区域为20个图标，就是因为要同时显示20个才给我惹了大麻烦。

简单地说是由于测试部在对FileManager的稳定性进行非常暴力的测试发生的问题，他们极其迅速地多次上下来回滑动GridView，创建过多AsyncTask导致了CPU无法负荷而发生ANR。这个问题也是由于之前我对android线程的了解还不够深入所引发的。AsyncTask本质是属于线程池，多次new所消耗的资源远远超过了Thread，这就是为什么AsyncTask比较适合简短、少次的异步操作。下面是官方解释：

并且那是每次触发getView时如果cache中没有图片的bitmap话，就会new AsyncTask来执行获取缩略图的操作，这样的后果就是那段代码变成了垃圾。因此我就从网上寻找可行的优化方案，找到了一个比较靠谱的，这回监听了Scroll状态，new的子线程是少了，可是我仔细想了想，感觉他的代码还是可以进一步优化（七楼就是我提的两个改进方案）。我的终极优化方案就是应用启动时就专门创建一个子线程来根据ScrollListener的状态执行解析缩略图的操作。

终于黄天不服有心人，还真让我在MIUI的FileExplore源码和android的Contacts源码中找到了。

SyncThumbnailExtractor是异步提取缩略图的主类，最主要的就是继承于HandlerThread的ExtractorThread，通过mExtractorHandler = new Handler(getLooper(), this)创建唯一的线程，产成消息队列，这样它会无限循环地处理send进来的Message，执行提取缩略图的操作。好了，不废话，直接代码。

public class SyncThumbnailExtractor implements Callback{        private static final String LOADER_THREAD_NAME = "FileIconLoader";    /**     * Type of message sent by the UI thread to itself to indicate that some     * thumbnails need to be extracted.     */    private static final int MESSAGE_REQUEST_EXTRACTING = 1;    /**     * Type of message sent by the loader thread to indicate that some thumbnails     * have been extracted.     */    private static final int MESSAGE_THUMBNAIL_EXTRACTED = 2;	private boolean mPaused;	private boolean mDecodingRequested = false;		final Handler mMainHandler = new Handler(this);	ExtractorThread mExtractorThread;	private Context mContext ;		private final ConcurrentHashMap
    
      mPendingRequests = new ConcurrentHashMap
     
      ();	private final static ConcurrentHashMap
      
        mImageCache = new ConcurrentHashMap
       
        (); 		private static abstract class ImageHolder {        public static final int NEEDED = 0;        public static final int EXTRACTING = 1;        public static final int EXTRACTED = 2;        int state;        public static ImageHolder create(String mime) {            if(mime == null)                return null;            if(mime.contains(ThumbnailUtils.APK)){                return new DrawableHolder();            }            else if(MediaFile.isImageByMimeType(mime) ||                     MediaFile.isVideoByMimeType(mime)){                return new BitmapHolder();            }            return null;        };        public abstract boolean setImageView(ImageView v);        public abstract boolean isNull();        public abstract void setImage(Object image);    }	    private static class BitmapHolder extends ImageHolder {        SoftReference
        
          bitmapRef;        @Override        public boolean setImageView(ImageView v) {            if (bitmapRef.get() == null)                return false;            v.setImageBitmap(bitmapRef.get());            return true;        }        @Override        public boolean isNull() {            return bitmapRef == null;        }        @Override        public void setImage(Object image) {            bitmapRef = image == null ? null : new SoftReference
         
          ((Bitmap) image); } } private static class DrawableHolder extends ImageHolder { SoftReference
          
            drawableRef; @Override public boolean setImageView(ImageView v) { if (drawableRef.get() == null) return false; v.setImageDrawable(drawableRef.get()); return true; } @Override public boolean isNull() { return drawableRef == null; } @Override public void setImage(Object image) { drawableRef = image == null ? null : new SoftReference
           
            ((Drawable) image); } } private static class FileInfo{ public FileInfo(String path,String mime){ this.path = path; this.mime = mime; } public String path; public String mime; } public SyncThumbnailExtractor(Context context) { mContext = context; } public void clear(){ mPaused = false; mImageCache.clear(); mPendingRequests.clear(); }         //当前Activity调用OnDestory时，将ExtractorThread退出，并清空缓存         public void stop(){ pause(); if (mExtractorThread != null) { mExtractorThread.quit(); mExtractorThread = null; } clear(); } public void resume(){ mPaused = false;     if (!mPendingRequests.isEmpty()) {     requestExtracting();     } } public void pause(){ mPaused = true; }     /** * Load thumbnail into the supplied image view. If the thumbnail is already cached, * it is displayed immediately. Otherwise a request is sent to load the * thumbnail from the database. * * @param id, database id */ public boolean decodeThumbnail(ImageView view, String path,String mime) { boolean extracted = loadCache(view, path, mime); if (extracted) { mPendingRequests.remove(view); } else { mPendingRequests.put(view, new FileInfo(path,mime)); if (!mPaused) { // Send a request to start loading thumbnails requestExtracting(); } } return extracted; }     //set default icon by MimeType for unextracted mefile private void setImageByMimeType(ImageView image,String mime){ if( mime.contains(ThumbnailUtils.APK)){ image.setImageResource(R.drawable.apk); } else if (mime.contains(ThumbnailUtils.VIDEO)) { image.setImageResource(R.drawable.video); } else if (mime.contains(ThumbnailUtils.IMAGE)) { image.setImageResource(R.drawable.image); } } /** * Checks if the thumbnail is present in cache. If so, sets the thumbnail on the * view, otherwise sets the state of the thumbnail to * { BitmapHolder#NEEDED} */ private boolean loadCache(ImageView view, String path, String mime) { ImageHolder holder = mImageCache.get(path); if (holder == null) { holder = ImageHolder.create(mime); if (holder == null) return false; mImageCache.put(path, holder); } else if (holder.state == ImageHolder.EXTRACTED) { if (holder.isNull()) { setImageByMimeType(view, mime); return true; } // failing to set imageview means that the soft reference was // released by the GC, we need to reload the thumbnail. if (holder.setImageView(view)) { return true; } holder.setImage(null); } setImageByMimeType(view, mime); holder.state = ImageHolder.NEEDED; return false; } /** * Sends a message to this thread itself to start loading images. If the * current view contains multiple image views, all of those image views will * get a chance to request their respective thumbnails before any of those * requests are executed. This allows us to load images in bulk. */ private void requestExtracting() { if (!mDecodingRequested) { mDecodingRequested = true; mMainHandler.sendEmptyMessage(MESSAGE_REQUEST_EXTRACTING); } } /** * @Description: handle MESSAGE_REQUEST_EXTRACTING message to create ExtractorThread and start    *                to extract thumbnail in mPendingRequests's file * @param msg * */ @Override public boolean handleMessage(Message msg) { switch(msg.what){ case MESSAGE_REQUEST_EXTRACTING: mDecodingRequested = false; if (mExtractorThread == null) { mExtractorThread = new ExtractorThread(); mExtractorThread.start(); } mExtractorThread.requestLoading(); return true; case MESSAGE_THUMBNAIL_EXTRACTED: if (!mPaused) { processExtractThumbnails(); } return true; } return false; } /** * Goes over pending loading requests and displays extracted thumbnails. If some of * the thumbnails still haven't been extracted, sends another request for image * loading. */ private void processExtractThumbnails() { Iterator
            
              iterator = mPendingRequests.keySet().iterator(); while (iterator.hasNext()) { ImageView view = iterator.next(); FileInfo info = mPendingRequests.get(view); boolean extracted = loadCache(view, info.path, info.mime); if (extracted) { iterator.remove(); } } if (!mPendingRequests.isEmpty()) { requestExtracting(); } } private class ExtractorThread extends HandlerThread implements Callback{ private Handler mExtractorHandler; /** * @Description: * @param name */ public ExtractorThread() { super(LOADER_THREAD_NAME); } /** * Sends a message to this thread to extract requested thumbnails. */ public void requestLoading() { if (mExtractorHandler == null) { mExtractorHandler = new Handler(getLooper(), this); } mExtractorHandler.sendEmptyMessage(0); } /** * @Description: extract thumbnail * @param msg * */ @Override public boolean handleMessage(Message msg) { Iterator
             
               iterator = mPendingRequests.values().iterator(); while (iterator.hasNext()) { FileInfo info = iterator.next(); ImageHolder holder = mImageCache.get(info.path); if (holder != null && holder.state == ImageHolder.NEEDED) { // Assuming atomic behavior holder.state = ImageHolder.EXTRACTING; if(info.mime == null){ holder.setImage(FileUtil.sInvalidBmp); } else { if(info.mime.contains(ThumbnailUtils.APK)){ Drawable icon = ThumbnailUtils.getApkIcon(mContext, info.path); holder.setImage(icon); } else if(MediaFile.isVideoByMimeType(info.mime)){ holder.setImage(ThumbnailUtils.getVideoThumb(info.path)); } else if(MediaFile.isImageByMimeType(info.mime)){ holder.setImage(ThumbnailUtils.getScaleImageThumb(mContext, info.path)); } } holder.state = BitmapHolder.EXTRACTED; mImageCache.put(info.path, holder); } } mMainHandler.sendEmptyMessage(MESSAGE_THUMBNAIL_EXTRACTED); return true; } } }
             
            
           
          
         
        
       
      
     
    

然后在Adapter的构造函数中创建该类：

public FileGridQueneAdapter(Context context, GridView gridView) {    mInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);    mContext = context;    syncThumbExtractor = new SyncThumbnailExtractor(context);    mGridView = gridView;    mGridView.setOnScrollListener(this);}

String mime = FileUtil.getMime(path);if(ThumbnailUtils.isNeedSyncDecodeByMime(mime)){    syncThumbExtractor.decodeThumbnail(icon, path, mime);} else {    icon.setImageBitmap(ThumbnailUtils.getThumbnail(mContext, path));}

在Adapter中添加SyncThumbnailExtractor四个操作，供Activity以及ScrollListener使用：

public void clear(){	    syncThumbExtractor.clear();	}		public void pause(){	    syncThumbExtractor.pause();	}		public void stop(){	    syncThumbExtractor.stop();	}		public void resume(){	    syncThumbExtractor.resume();	}

最后给GridView或者ListView添加ScrollListener：

@Override    public void onScrollStateChanged(AbsListView view, int scrollState) {        if(scrollState == OnScrollListener.SCROLL_STATE_FLING){            pause();        } else {            resume();        }    }    @Override    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount,            int totalItemCount) {            }