learn-co-curriculum / swift-optional-property

Swift — Optional Property

Objectives

1. Use optionals as properties on a class.
2. Print optional properties to observe results.
3. Write a designated initializer for setting an optional property.
4. Write a convenience initializer that passes nil into an optional property.

Paste the following code snippets in a new Xcode Playground file to see them in action.

Optional Properties

Optional properties can be used as part of a class just like other non-optional Types. Any property on a class must have a value assigned to it when it's declared or during the initialization of the class. An exception to this rule is when the property is an optional.

By default optionals have the absence of a value which is referred to as nil. This comes in handy when certain properties on a class may or may not ever have a value. For example, a Student class could have a property called middleName that's declared as an optional. This assumes the application does not need the middle name for its records. Therefore, when the student record is first created, the user will not be required to provide a middle name (it will be optional).

class Student {

    // Value must be assigned during initialization
    let username: String

    // Optionals
    var firstName: String?
    var lastName: String?
    var email: String?
    var phone: String?

    // Declared with a value
    var gpa: Double = 0.0

    init(username: String) {
        self.username = username
    }

}

// Instance of Student class
let joe = Student(username: "joelovescode")

print("username: \(joe.username)")
print("firstName: \(joe.firstName)")
print("lastName: \(joe.lastName)")
print("email: \(joe.email)")
print("phone: \(joe.phone)")
print("GPA: \(joe.gpa)")

This will print:

username: joelovescode
firstName: nil
lastName: nil
email: nil
phone: nil
GPA: 0.0

If values are assigned to the optional properties, they will remain wrapped and require optional binding to check for a value.

// Values assigned to optional properties
joe.firstName = "Joe"
joe.lastName = "Burgess"
joe.email = "[email protected]"
joe.phone = "212-555-1212"

print("firstName: \(joe.firstName)")
print("lastName: \(joe.lastName)")
print("email: \(joe.email)")
print("phone: \(joe.phone)")

This will print:

firstName: Optional("Joe")
lastName: Optional("Burgess")
email: Optional("[email protected]")
phone: Optional("212-555-1212")

Use optional binding to unwrap and assign values to temporary constants.

// Unwrapped optional properties
if
    let firstName = joe.firstName,
    let lastName = joe.lastName,
    let email = joe.email,
    let phone = joe.phone {

    print("firstName: \(firstName)")
    print("lastName: \(lastName)")
    print("email: \(email)")
    print("phone: \(phone)\n")

}

This will print:

firstName: Joe
lastName: Burgess
email: [email protected]
phone: [phone]

Initializers With Optionals

Even though optional properties are not required for the initialization of a class, they can still be used on initializers. Here's an updated version of the current Student class.

class Student {

    // Value must be assigned during initialization
    let username: String

    // Optionals
    var firstName: String?
    var lastName: String?
    var email: String?
    var phone: String?

    // Declared with a value
    var gpa: Double = 0.0

    // Designated initializer with optionals
    init(username: String, firstName: String?, lastName: String?) {
        self.username = username
        self.firstName = firstName
        self.lastName = lastName
    }

}

The original initializer that only took a username argument can now be used as a convenience initializer (Notice that the convenience initializer must include a call to the designated initializer). If multiple initializers seem confusing, remember that they are there to allow for creating instances of class objects under varying circumstances. In the example below the convenience initializer can be used when you know only a username will be provided by the user.

class Student {
    
    // Value must be assigned during initialization
    let username: String
    
    // Optionals
    var firstName: String?
    var lastName: String?
    var email: String?
    var phone: String?
    
    // Declared with a value
    var gpa: Double = 0.0
    
    // Designated initializer with optionals
    init(username: String, firstName: String?, lastName: String?) {
        self.username = username
        self.firstName = firstName
        self.lastName = lastName
    }
    
    // Convenience initializer
    convenience init(username: String) {
        self.init(username: username, firstName: nil, lastName: nil)
    }
    
}

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