单身人士的持久性属性

Question

只要我在 ViewControllerA ，这样就可以了：

[User sharedUser].email = @"mail@mail.com";
NSLog(@"%@ %@", [User sharedUser].email, [User sharedUser]); // mail@mail.com <User: 0x8d83220>

但如果我在 ViewControllerB 后切换，则电子邮件为空

NSLog(@"%@ %@", [User sharedUser].email, [User sharedUser]); // null <User: 0x8d83220>

为什么会出现这样的错误？

User.m

@implementation User
+ (instancetype)sharedUser
{
    static User *sharedUser = nil;

    if (!sharedUser) {
        sharedUser = [[self alloc] init];
    }

    return sharedUser;
}
@end

User.h

@interface User : NSObject
@property (nonatomic, weak) NSString *email;
@end