解决方法
From Apple Documentation:
nil cannot be used with nonoptional constants and variables. If a
constant or variable in your code needs to work with the absence of a
value under certain conditions,always declare it as an optional value
of the appropriate type
也非常重要的注意到：
Swift’s nil is not the same as nil in Objective-C. In Objective-C,nil
is a pointer to a nonexistent object. In Swift,nil is not a
pointer—it is the absence of a value of a certain type. Optionals of
any type can be set to nil,not just object types
我希望能帮助你！
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