lazytable 例子
 
 // Set appIcon and clear temporary data/image
     UIImage *image = [[UIImage alloc] initWithData:self.activeDownload];
     
     if (image.size.width != kAppIconHeight && image.size.height != kAppIconHeight)
     {
         CGSize itemSize = CGSizeMake(kAppIconHeight, kAppIconHeight);
         UIGraphicsBeginImageContext(itemSize);
         CGRect imageRect = CGRectMake(0.0, 0.0, itemSize.width, itemSize.height);
         [image drawInRect:imageRect];
         self.appRecord.appIcon = UIGraphicsGetImageFromCurrentImageContext();
         UIGraphicsEndImageContext();
     }
     else
     {
         self.appRecord.appIcon = image;
     }
     
     self.activeDownload = nil;
     [image release];
    
 
 图片缩放到指定大小尺寸
 
 - (UIImage *)scaleToSize:(UIImage *)img size:(CGSize)size{  
     // 创建一个bitmap的context  
 // 并把它设置成为当前正在使用的context  
     UIGraphicsBeginImageContext(size);  
     // 绘制改变大小的图片  
     [img drawInRect:CGRectMake(0, 0, size.width, size.height)];  
     // 从当前context中创建一个改变大小后的图片  
     UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();  
     // 使当前的context出堆栈  
     UIGraphicsEndImageContext();  
     // 返回新的改变大小后的图片  
     return scaledImage;  
 }