Dependency Injection does sort out this case, you're right.
One case it doesn't fix is where your initialised value depends on something
within your superclass to be created first to derive your initialising value.
class MyBaseClass {
let myStateManager: StateManager
init() {
// sets up stateManager
}
}
class MySubclass: MyBaseClass {
var myStateDerivedProperty: Int!
override init() {
super.init()
myStateDerivedProperty = // derive state from myStateManager
}
}
In this case, the writer cannot initialise their state derived properties until
the superclass has completed its initialization because it is waiting on the
stateManager to derive its initial state.
This is a somewhat contrived example but I've actually found myself with
similar patterns where I use other frameworks, where the default is either not
defined in documentation or the object I require as part of initialising the
property does not exist until after the initialisation.
Rod
> On 5 Feb 2017, at 4:04 am, Jean-Daniel <mail...@xenonium.com> wrote:
>
>
>> Le 4 févr. 2017 à 16:52, Rod Brown via swift-evolution
>> <swift-evolution@swift.org> a écrit :
>>
>> Hi Joe,
>>
>> I think this comes back to the idea that a lot of people in the wider Swift
>> community hold that Implicitly Unwrapped Optionals are “bad” and
>> “discouraged", and therefore shouldn’t be used. There seems to have been
>> much pushback on Implicitly Unwrapped Optionals in the Swift 3 timeframe, to
>> try and remove them as much as possible.
>>
>> I definitely see this as a valuable use for them. While an integer is an
>> extremely lightweight example of this, when it could include creating entire
>> object graphs multiple times due to initialiser behaviour, or because you
>> won’t know the correct state of a variable until *after* initialisation has
>> occurred on the superclass, this is a valuable example where IUOs are really
>> the only alternative for performance or correctness reasons.
>
> IUO are useful to workaround poorly designed class. As already said, you
> example could be rewrite like follow to avoid any useless computation
>
> class A {
> let x: Int
> init(_ x: Int = 3) {
> self.x = x
> }
> }
>
> class B : A {
> override init() {
> …
> super.init(1)
> }
> }
>
> No useless initialization of x, and no need to use IUO. Is it a suffisent
> reason to keep them in the language ? Anyway, as long as we need them to
> usefully use IBOutlet, I’m pretty sure they are not going anywhere.
>
>
>>
>> Perhaps this is an area where the Swift Core Team could provide guidance to
>> the community? Do the Core Team see IUOs as “bad” outright, and destined to
>> go away when possible, or are they a tool with specific uses that look to be
>> supported into the future?
>>
>> - Rod
>>
>>
>>
>>>> On 4 Feb 2017, at 5:40 am, Joe Groff via swift-evolution
>>>> <swift-evolution@swift.org> wrote:
>>>>
>>>>
>>>> On Jan 31, 2017, at 3:52 AM, Victor Petrescu via swift-evolution
>>>> <swift-evolution@swift.org> wrote:
>>>>
>>>> 4. Joe Groff says there is already a backdoor of sorts ("There already is
>>>> a backdoor of sorts. This is one of the intended use cases for
>>>> implicitly-unwrapped optionals. If you don't want to be hassled by DI,
>>>> declare a property as T! type, and it will be implicitly initialized to
>>>> nil, and trap if you try to use it as an unwrapped T without initializing
>>>> it first."): I'm assuming by T you mean generics. If that is true that may
>>>> already solve the problem but... generics are a new concept for me (first
>>>> time I really encountered and used them is now, in swift) but to my
>>>> understanding their role is to deal with cases you don't know the type.
>>>> Can you please show how to use this to work around the posted issue?
>>>>
>>>> Sidenote: There may be another workaround using optionals (Joe Groff
>>>> answer made it pop in my mind) but... I know the type and value for the
>>>> variable, it is not optional or nil. Unwrapping each time someone needs it
>>>> does not look like the best solution to me.
>>>
>>> You don't need to understand generics to use implicitly-unwrapped
>>> optionals. By `T!` I was referring to the syntax used to represent them;
>>> you would write Int! or String! or whatever in your code. For your example,
>>> this would let you avoid having to invoke super.init() before resetting `x`:
>>>
>>> class A {
>>> var x:Int! // Int! is nil by default
>>> }
>>>
>>> class B : A {
>>> override init() {
>>> x = 2 // So we can set it here w/o super.init first
>>> }
>>> }
>>>
>>> print(B().x + 1) // and we don't need to explicitly unwrap it to use it,
>>> unlike `Int?`
>>>
>>> You're giving up the static guarantee that `x` has a value, so you'll get a
>>> runtime error if you try to use it before it's initialized, but that's the
>>> same situation you have in Java, where dereferencing an uninitialized
>>> object reference gives an NPE. Whether you want the hard guarantee that `x`
>>> is never optional from the compiler, or the convenience of leaving that up
>>> to runtime checks, is a call you have to make; Swift defaults to the strong
>>> guarantee, but implicitly-unwrapped optional types like Int! are intended
>>> to give you an out if the static model is too strict or inefficient.
>>>
>>> -Joe
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution@swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
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