Two-phase initialization

Because of Swift’s requirement that all stored properties have initial values, initializers in subclasses must adhere to Swift’s convention of two-phase initialization.

• Phase one: Initialize all of the stored properties in the class instance, from the

bottom to the top of the class hierarchy. You can’t use properties and methods until phase one is complete.

• Phase two: You can now use properties and methods, as well as initializations that require the use of self.

Without two-phase initialization, methods and operations on the class might interact with properties before they’ve been initialized.

The transition from phase one to phase two happens after you’ve initialized all stored properties in the base class of a class hierarchy. In the scope of a subclass initializer, you can think of this as coming after the call to super.init.

Here’s the StudentAthlete class again, with athletes automatically getting a starter grade:

class StudentAthlete: Student { var sports: [String]

init(firstName: String, lastName: String, sports: [String]) {

// 1

self.sports = sports

// 2

let passGrade = Grade(letter: "P", points: 0.0, credits: 0.0)

// 3

super.init(firstName: firstName, lastName: lastName)

// 4 recordGrade(passGrade)

}

//…

}

The above complete initializer shows two-phase initialization in action:

1. First, you initialize the sports property of StudentAthlete. This is part of the first phase of initialization and has to be done early, before you call the superclass initializer.
2. Although you can create local variables for things like grades, you can’t call

recordGrade(_:) yet because the object is still in the first phase.

1. You call super.init. When this returns, you know that you’ve also initialized every class in the hierarchy, because the same rules are applied at every level.
2. After super.init returns, the initializer is in phase 2, so you call

recordGrade(_:).