TTS Swift Notes: Initialization

Question 1

What is initialization?

Answer 1

The process of preparing an instance of a class, structure, or enumeration for use.

Question 2

What two steps does initialization involve?

Answer 2

Setting an initial value for each stored property on that instance and performing any other setup or initialization that is required before the new instance is ready to use.

Question 3

How do you implement the initialization process?

Answer 3

By defining initializers which are like special methods that can be called to create a new instance of a particular type.

Question 4

What keyword is used for initializers?

Answer 4

init

Question 5

In its simplest form, an initializer is like a what?

Answer 5

An instance method with no parameters.

Question 6

What are the two kinds of initializers that Swift defines for class types?

Answer 6

Designated and convenience initializers.

Question 7

What two things does a designated initializer do?

Answer 7

They are the primary initializers for a class and they fully initialize all properties introduced by that class and calls an appropriate superclass initializer to continue the initialization process up the superclass chain.

Question 8

What does a convenience initializer do?

Answer 8

They are the secondary, supporting initializers for a class. You can define a convenience initializer to call a designated initializer from the same class as the convenience initializer with some of the designated initializer’s parameters set to default values. You can also define a convenience initializer to create an instance of that class for a specific use case or input value type.

Question 9

When should you create a convenience initializer?

Answer 9

Whenever a shortcut to a common initialization pattern will save time or make initialization of the class clearer in intent.

Question 10

How are convenience initializers written?

Answer 10

convenience init(parameters) {

statements

}

Question 11

What are the three rules of initializer delegation?

Answer 11

1) A designated initializer must call a designated initializer from its immediate superclass;
2) A convenience initializer must call another initializer from the same class; and
3) A convenience initializer must ultimately call a designated initializer.

Question 12

Designated initializers must always delegate where?

Answer 12

Up.

Question 13

Convenience initializers must always delegate where?

Answer 13

Across.

Question 14

Class initialization in Swift is a how many phase process?

Answer 14

Two-phase.

Question 15

What is the first phase of class initialization?

Answer 15

Each stored property is assigned an initial value by the class that introduced it.

Question 16

What is the second phase of class initialization?

Answer 16

Each class is given the opportunity to customize its stored properties further before the new instance is considered ready for use.

Question 17

What two things does two-phase class initialization do?

Answer 17

1) Prevents property values from being accessed before they are initialized; and
2) Prevents property values from being set to a different value by another initializer unexpectedly.

Question 18

How many safety checks does Swift’s compiler perform to make sure that two-phase class initialization is completed without error?

Answer 18

Four.

Question 19

What are the four class initialization safety-checks?

Answer 19

1) A designated initializer must ensure that all of the properties introduced by its class are initialized before it delegates up to a superclass initializer;
2) A designated initializer must delegate up to a superclass initializer before assigning a value to an inherited property. If it doesn’t, the new value the designated initializer assigns will be overwritten by the superclass as part of its own initialization.;
3) A convenience initializer must delegate to another initializer before assigning a value to any property (including properties defined by the same class). If it doesn’t, the new value the convenience initializer assigns will be overwritten by its own class’ designated initializer; and
4) An initializer cannot call any instance methods, read the values of any instance properties, or refer to self as a value until after the first phase of initialization is complete.

Question 20

If you write a subclass initializer that matches a superclass designated initializer, what modifier do you have to write before the subclass’ initializer definition?

Answer 20

override

Question 21

If your subclass doesn’t define any designated initializers, does it automatically inherit all of its superclass designated initializers?

Answer 21

Yes.

Question 22

If your subclass provides an implementation of all of its superclass designated initializers - either by inheriting them or by providing a custom implementation as part of its definition - then it automatically inherits what?

Answer 22

All of the superclass convenience initializers.

Question 23

For what three reasons might class initialization fail?

Answer 23

1) Invalid initialization parameter values;
2) the absence of a required external resource; or
3) some other condition

Question 24

How do you write a failable initializer?

Answer 24

(init?)

Question 25

What does a failable initializer create?

Answer 25

An optional value of the type that it initializes.

Question 26

What kind of a return do you write within a failable initializer?

Answer 26

return nil (to indicate a point at which initialization can be triggered)

Question 27

Enumerations with raw values automatically receive what?

Answer 27

A failable initializer ( init?(rawValue:)

Question 28

When can a class failable initializer trigger an initialization failure?

Answer 28

Only after all stored properties introduced by that class have been set to an initial value and any initializer delegation has taken place.

Question 29

How do you write a bailable initializer that creates an implicitly unwrapped optional instance of the appropriate type?

Answer 29

init!

Question 30

What keyword modifier is written before the definition of a class initializer to indicate that every subclass of the class must implement that initializer?

Answer 30

required

class SomeClass {
required init() {

// initializer implementation goes here
}
}