Android Bitmap的加载优化与Cache相关介绍

public Bitmap decodeSampledBitmapFromResource(Resources res, int resId, int reqWidth, int reqHeight) {
 // First decode with inJustDecodeBounds=true to check dimensions
 final BitmapFactory.Options options = new BitmapFactory.Options();
 options.inJustDecodeBounds = true;
 BitmapFactory.decodeResource(res, resId, options);

 // Calculate inSampleSize
 options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);

 // Decode bitmap with inSampleSize set
 options.inJustDecodeBounds = false;
 return BitmapFactory.decodeResource(res, resId, options);
}

public int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight) {
 if (reqWidth == 0 || reqHeight == 0) {
  return 1;
 }

 // Raw height and width of image
 final int height = options.outHeight;
 final int width = options.outWidth;
 Log.d(TAG, "origin, w= " + width + " h=" + height);
 int inSampleSize = 1;

 if (height > reqHeight || width > reqWidth) {
  final int halfHeight = height / 2;
  final int halfWidth = width / 2;

  // Calculate the largest inSampleSize value that is a power of 2 and
  // keeps both height and width larger than the requested height and width.
  while ((halfHeight / inSampleSize) >= reqHeight && (halfWidth / inSampleSize) >= reqWidth) {
   inSampleSize *= 2;
  }
 }

 Log.d(TAG, "sampleSize:" + inSampleSize);
 return inSampleSize;
}

mImageView.setImageBitmap(
decodeSampledBitmapFromResource(getResource(),R.id.myimage,100,100));

public static DiskLruCache open(File directory,int appversion,int valueCount,long maxSize);

private static final Disk_CACHE_SIZE = 1024*1024*50;//50M

File diskCaCheDir = getDiskCacheDir(mContext,"bitmap");
if(!diskCacheDir.exists()){
 diskCacheDir.mkdirs();
}
mDiskLruCache = DiskLruCache.open(diskCaCheDir,1,1,Disk_CACHE_SIZE);

 Bitmap bitmap = null;
 String key = hashKeyFormUrl(url);
 DiskLruCache.Snapshot snapShot = mDiskLruCache.get(key);
  if (snapShot != null) {
   FileInputStream fileInputStream = (FileInputStream)snapShot.getInputStream(DISK_CACHE_INDEX);
   // 获取文件描述符
   FileDescriptor fileDescriptor = fileInputStream.getFD();
   // 通过 BitmapFactory.decodeFileDescriptor 来加载一张缩放后的图片
   bitmap = mImageResizer.decodeSampledBitmapFromFileDescriptor(fileDescriptor,
     reqWidth, reqHeight);
   if (bitmap != null) {
    addBitmapToMemoryCache(key, bitmap);
   }
  }

  return bitmap;
 }

@Override
public View getView(int position, View convertView, ViewGroup parent) {
   ViewHolder holder = null;
   if (convertView == null) {
    convertView = mInflater.inflate(R.layout.image_list_item,parent, false);
    holder = new ViewHolder();
    holder.imageView = (ImageView) convertView.findViewById(R.id.image);
    convertView.setTag(holder);
   } else {
    holder = (ViewHolder) convertView.getTag();
   }
   ImageView imageView = holder.imageView;
   final String tag = (String)imageView.getTag();
   final String uri = getItem(position);
   if (!uri.equals(tag)) {
    imageView.setImageDrawable(mDefaultBitmapDrawable);
   }
   if (mIsGridViewIdle && mCanGetBitmapFromNetWork) {
    imageView.setTag(uri);
    // 这句话将图片的复杂加载过程交给 ImageLoader 了
    mImageLoader.bindBitmap(uri, imageView, mImageWidth, mImageWidth);
   }
   return convertView;
  }