Optional type (Optional): It may be nil or a specific value
1. Declare an optional type
var temp:String?
temp="test"
2. Use optional types of variables
print(temp) //The output result is: Optional("test")
print(temp!) //The output result is: test
Summary: add after the type? Indicates an optional type, add after the variable! Indicates that specific values can be obtained by unpacking optional types
3. Optional chain
Shaped like: person?.book!.price! or person.book!.price!
If book is nil, the second will report an error, and the first will return an optional type to continue execution
4. Hermit unpacking
形如：@IBOutlet weak var tableView: UITableView!
Hermit unpacking is equivalent to automatically adding after the variable every time you use it! Unpack
Summary: add after the variable? Indicates an optional chain, add after the type! Means hermit unpacking