在.h文件中定義如下：

    @interface ransactionStateMachine : NSObject
  typedef NS_ENUM(NSUInteger, TransactionState) {
    TransactionOpened,
    TransactionPending,
    TransactionClosed,
  };
//加上extern 在外不直接調(diào)用 不加 需要導(dǎo)入頭文件
extern NSString * NSStringFromTransactionState(TransactionState state);
@end

.m文件中定義如下：

@implementation ransactionStateMachine

NSString * NSStringFromTransactionState(TransactionState state) {
    switch (state) {
        case TransactionOpened:
            return @"Opened";
        case TransactionPending:
            return @"Pending";
        case TransactionClosed:
            return @"Closed";
        default:
            return nil;
    }
}