环境变量、block、修饰符：block对环境变量的引用和修改需要通过修饰符来限定。
http://www.cnblogs.com/fengmin/p/5816580.html
- (NSUInteger)getSize {
__block NSUInteger size = 0;
dispatch_sync(self.ioQueue, ^{
NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtPath:self.diskCachePath];
for (NSString *fileName in fileEnumerator) {
NSString *filePath = [self.diskCachePath stringByAppendingPathComponent:fileName];
NSDictionary *attrs = [[NSFileManager defaultManager] attributesOfItemAtPath:filePath error:nil];
size += [attrs fileSize];
}
});
return size;
}