iOS版本比较方法

之前一直把版本号转换为floatValue,但是最近在项目中又出现了float的问题，主要是

NSString表示为@"17.30",转换为floatValue 值为17.299999,然后做了一些放大处理，导致不精确，就是精度少了1

所以都不敢在系统版本比较的过程中把值强制转换为floatValue了。

/*
 *  System Versioning Preprocessor Macros hovertree.com
 */
#define SYSTEM_VERSION_EQUAL_TO(v)([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch]==NSOrderedSame)
#define SYSTEM_VERSION_GREATER_THAN(v)([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch]==NSOrderedDescending)
#define SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(v)([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch]!=NSOrderedAscending)
#define SYSTEM_VERSION_LESS_THAN(v)([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch]==NSOrderedAscending)
#define SYSTEM_VERSION_LESS_THAN_OR_EQUAL_TO(v)([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch]!=NSOrderedDescending)
/*
 *  Usage 何问起
 */if(SYSTEM_VERSION_LESS_THAN(@"4.0")){...}
if(SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(@"3.1.1")){...}

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