I'm using Swift as my project language,and there is a library written in Objective-C.The issue is that the property apnsNickname's type is String!? in Swift.As the compiler's auto completion tells me.

I want to know how to assign a value to this type.The following assignment has a compiler error.
EaseMob.sharedInstance().chatManager.apnsNickname = "Mike"
Cannot assign to the result of this expression
Can anyone help me out?

The definition of the property is:
@protocol IChatManagerSettingOptions <IChatManagerBase>
@optional
@property (strong, nonatomic) NSString *apnsNickname;
...
@end