？与！的区别
You specify optional chaining by placing a question mark (?) after the optional value on which you wish to call a property, method or subscript if the optional is non-nil. This is very similar to placing an exclamation mark (!) after an optional value to force the unwrapping of its value. The main difference is that optional chaining fails gracefully when the optional is nil, whereas forced unwrapping triggers a runtime error when the optional is nil.
查询值为optional类型
To reflect the fact that optional chaining can be called on a nil value, the result of an optional chaining call is always an optional value,
Specifically, the result of an optional chaining call is of the same type as the expected return value, but wrapped in an optional.
let roomCount = john.residence!.numberOfRooms// this triggers a runtime error
Accessing Subscripts Through Optional Chaining
You can use optional chaining to try to retrieve and set a value from a subscript on an optional value, and to check whether that subscript call is successful.
Accessing Subscripts of Optional Type
If a subscript returns a value of optional type—such as the key subscript of Swift’s Dictionary type—place a question mark after the subscript’s closing bracket to chain on its optional return value:
var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]]testScores["Dave"]?[0] = 91testScores["Bev"]?[0] += 1testScores["Brian"]?[0] = 72// the "Dave" array is now [91, 82, 84] and the "Bev" array is now [80, 94, 81]