计算沙盒中一个目录的大小

+(NSString*)getCacheSize
{
     
    NSDate* begin = [NSDate date];
 
 
    NSFileManager* fm = [NSFileManager defaultManager];
    __block NSError* error = nil;
     
    __block NSUInteger fileSize = 0;
     
     
    //获取Books的缓存
    dispatch_sync(queue, ^{
        NSArray* subFiles = [fm subpathsAtPath:BOOKHEADERPath([LoginPlugin share].userID)];
        NSLog(@"subpath = %@",subFiles);
        for (NSString* fileName in subFiles) {
            if ([fileName hasSuffix:@"png"]||[fileName hasSuffix:@"jpg"]) {
                NSDictionary* dic = [fm attributesOfItemAtPath:BOOKPATH([LoginPlugin share].userID,fileName) error:&error];
                NSUInteger size = (error ? 0:[dic fileSize]);
                fileSize += size;
            }
        }
    });
     
     
    NSString* cacheString = [NSString stringWithFormat:@"%.1fM",fileSize/(1024.0*1024)];
    NSTimeInterval time = [begin timeIntervalSinceNow];
    NSLog(@"便利文件耗费时间:%lf",time/60.0);
    return cacheString;
}