Answer.

In Swift, all properties of non-optional types must be initialized by the end of the instance initialization. For this, initializers are used: designated (primary), convenience (auxiliary), and required (mandatory for subclasses).

• Designated initializer — the main one, responsible for initializing all properties of the class and calling the designated initializer of the superclass. Each class has at least one designated initializer.
• Convenience initializers — auxiliary ones that delegate the initialization to the same class (calling other initializers within the class using self.init(...)), simplifying the creation of an object with various sets of parameters.
• Required initializer — requires all subclasses to implement this initializer. Declared with the required modifier.

Example:

class Vehicle {
    let numberOfWheels: Int
    required init(numberOfWheels: Int) {
        self.numberOfWheels = numberOfWheels
    }
}

class Car: Vehicle {
    let brand: String
    
    // Designated initializer
    required init(numberOfWheels: Int) {
        self.brand = "Unknown"
        super.init(numberOfWheels: numberOfWheels)
    }
    // Convenience
    convenience init() {
        self.init(numberOfWheels: 4)
        self.brand = "Ford"
    }
}

Key features:

• All properties must be initialized before accessing super.init.
• Convenience initializers must always call a designated or another convenience initializer using self.
• Required initializers must be implemented in all subclasses.

Trick question.

In what order should subclass and superclass properties be initialized when using a designated initializer?

Answer: All properties of the subclass must be initialized before calling the designated initializer of the superclass (super.init), otherwise, it will cause a compilation error. After super.init, no new stored properties of the subclass can be initialized, only used.

class Parent {
    let parentProp: Int
    init(prop: Int) { parentProp = prop }
}
class Child: Parent {
    let childProp: String
    init(prop: Int, childProp: String) {
        self.childProp = childProp // First your properties
        super.init(prop: prop)     // Then parent
    }
}

Examples of real errors due to ignorance of the nuances of the topic.

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Story

A developer in a convenience initializer tried to directly assign a value to a required property without calling designated. As a result, the property was initialized twice with different values, leading to unexpected behavior during testing.

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Story

When inheriting a deep class hierarchy, a required init was forgotten to be declared, which led to the inability to correctly initialize the derived type from the framework via reflection, and a crash during JSON model deserialization.

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Story

In an application, during the extension of the Vehicle class, the designated initializer of the superclass was not called, leading to incorrect initialization of mandatory properties and a runtime crash in production after the data schema update.