使用嵌套字典按包含浮点数的字符串值对数组进行排序

What i have is

我拥有的是什么

NSSortDescriptor *sortDescriptorNearest = [NSSortDescriptor sortDescriptorWithKey:@"petrolStation.pointCoordinates.distance" ascending:YES];
_nearestList = (NSMutableArray *)[_gasStations sortedArrayUsingDescriptors:@[sortDescriptorNearest]];
NSSortDescriptor *so