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Objective-C中经常需要将枚举转换成对应的字符串类型。平时一般用if或switch语句返回枚举值对应的字符串，高级一点的呢就使用字典做映射，如：
typedef NS_ENUM(NSUInteger, RAPDirection) {
RAPDirectionUp,
RAPDirectionDown,
RAPDirectionLeft,
RAPDirectionRight
};
- (NSString *)stringForRAPDirection:(RAPDirection)type {
static NSDictionary *map = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
map = @{
@(RAPDirectionUp): @"上",
@(RAPDirectionDown): @"下",
@(RAPDirectionLeft): @"左",
@(RAPDirectionRight): @"右",
};
});
return map[@(type)] ?: @"unknown";
}
相比之下, Swfit要简单的多。Swift枚举类型引入了Raw Value的概念，每个枚举case的Raw Value可以是其case name。假设有case up，则up.rawValue = "上"。
那有没有更简单的一步到位的方法，在定义时候就榜单相应的字符串呢？看到一篇文章OC枚举值到字符串转换, 作者用宏替换实现了将枚举值映射为对应的字符串，如枚举值RAPDirectionUp映射成@"RAPDirectionUp". 在此基础上，我增加了枚举值映射成自定义文字的功能，如RAPDirectionUp映射成@"上"。
这套宏的用法分为两步：
// M1
#define RAP_DIRECTION(XX) \
XX(RAPDirectionUp, ) \
XX(RAPDirectionDown, ) \
XX(RAPDirectionLeft, = 50, 左) \
XX(RAPDirectionRight, = 100, 右) \
// M2
DECLARE_ENUM(RAPDirection, RAP_DIRECTION)M1生成多个枚举case。枚举case的定义格式为XX(name, assign)，name即case name；assign即对应整数值，格式必须为= Integer，不填表示使用默认值；第三个参数为要映射的描述，不填则返回name。
M2定义枚举，声明反射函数。反射函数的有三个：
DEFINE_ENUM(RAPDirection, RAP_DIRECTION)尝试使用：
NSString *str2 = DescriptionFromRAPDirection(RAPDirectionUp); NSLog(@"RAPDirectionUp has case desc: %@", str2); NSString *str31 = NameFromRAPDirection(RAPDirectionLeft); NSLog(@"RAPDirectionLeft has case name: %@", str31); NSString *str32 = DescriptionFromRAPDirection(RAPDirectionLeft); NSLog(@"RAPDirectionLeft has case desc: %@", str32); RAPDirection dir = RAPDirectionFromNSString(@"RAPDirectionRight"); NSLog(@"RAPDirectionRight has case value: %zd", dir);
输出结果：
RAPDirectionUp has case desc: RAPDirectionUp RAPDirectionLeft has case name: RAPDirectionLeft RAPDirectionLeft has case desc: 左 RAPDirectionRight has case value: 100
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