Required initialisers and designated initialisers are not really related, though the associated keywords required and convenience are both used to specify restrictions on subclasses.
Required Initialisers
A required initialiser makes a guarantee that you can initialise a type, or any of its sub-types, with that initialiser. If you have an initialiser in a protocol and you conform something to that protocol, you have to use required (if it's a class) because that protocol guarantees that the initialiser is present on that class, and any of its subclasses. When you use required on an initialiser of a class, that signals that all of its subclasses can also be initialised using that method. This means you also need to add that initialiser to any of its subclasses.
protocol TestProtocol {
init()
}
class TestClass: TestProtocol {
required init() {
}
}
Here, the required keyword must be present because any subclasses of TestClass must also provide init() (because they also conform to TestProtocol).
Having a required initialiser allows you to initialise a class without knowing what it is at compile time, which is useful for a variety of reasons:
let classType: TestProtocol.Type = TestClass.self
let object = classType.init()
If your class conformed to multiple protocols, each with a different initialiser for example, each of those initialisers must also be required:
protocol OtherProtocol {
init(thing: Int)
}
class OtherClass: TestClass, OtherProtocol {
let thing: Int
required init() { // Required from superclass/its protocol
self.thing = 0
}
required init(thing: Int) { // Required from new protocol
self.thing = thing
}
}
Note that adding super.init() isn't required in this special case, because Swift will automatically include the call if it takes no parameters.
In all the above examples, the initialisers are designated because they do not include the convenience keyword.
Even if you didn't have any protocols, you can still make use of required by initialising a type of a class which isn't known at compile time:
class BaseClass {
let value: Int
required init(value: Int) {
self.value = value
}
}
class SubClass: BaseClass {
required init(value: Int) { // Required from superclass
super.init(value: value) // Must call desginated initialiser of superclass
}
}
let someBaseClassType: BaseClass.Type = SubClass.self
let someBaseClassInstance = someBaseClassType.init(value: 1)
Designated Initialisers
A designated initialiser is one which isn't a convenience initialiser (i.e, marked with convenience). A designated initialiser must make sure that all properties of the class have a value before the initialiser finishes (or a super initialiser is called). Convenience initialisers only don't have this requirement because they must themselves call a designated initialiser.
class OtherSubClass: BaseClass {
convenience required init(value: Int) {
self.init() // Must call designated initialiser of this class
}
init() {
super.init(value: 0) // Must call designated initialiser of superclass
}
}
(This is fairly contrived example.)
In my experience, convenience initialisers are rarely useful and I tend to find the problems they solve can be solved using optional arguments on designated initialisers instead. One also needs to consider the fact that initialisers can't call convenience initialisers on their superclass, so make sure you don't have any convenience initialisers which provide functionality that your designated initialisers don't if you intend your class to be subclassed!
Structs and enums don't use the required or convenience keywords because these keywords are both used to indicate initialisation rules for subclasses, which only classes support: The required keyword indicates that subclasses must provide that initialiser, and the convenience keyword indicates that subclasses cannot call that initialiser. Despite not having the keywords, they must still provide initialisers defined in any protocols they conform to, and you can write 'convenient' initialisers which call self.init, just without the convenience keyword.
To respond to your statements:
- Required initialisers don't have to be designated.
- Designated initialisers don't have to be required.
- Classes can have multiple required and designated initialisers.
