Swift generic collection of Element cannot convert to collection of Any - swift

I'm facing a problem I don't understand.
In my project, I want to make a collection of elements with some customised methods (like an update from a server). But when I try to group all these collections in an array, I get an error: "Cannot convert value of type MyCollection<someElement> to expected argument type MyCollection<Any>"
What I don't understand is that the same code with Array is working... Array isn't a collection?
// My collection which would contain an update method
class MyCollection<Element> {
var object:Element? = nil
}
let x = MyCollection<Int>()
var list = [MyCollection<Any>]()
list.append(x) //Cannot convert value of type 'MyCollection<In>' to expected argument type 'MyCollection<Any>'
let a = Array<Int>()
var lista = [Array<Any>]()
lista.append(a) //Doesn't get error at all...
I know I can do this with an array of the specific type but by grouping all of MyCollection in an array, I wish to use a code like :
func update() {
for e in list { // array of MyCollection<Any>
e.update()
}
}
Thank you in advance for your help ;)

Being able to convert from SomeType<Subtype> to SomeType<Supertype> is called covariance. In Swift, Array<T> is covariant on T by "compiler magic", and you can't do the same for your own types.
The type checker hardcodes conversions from Array to Array if there is a conversion from T to U. Similar rules exist for Optional and Dictionary. There's no mechanism for doing this with your own types.
Your own generic types are always invariant, meaning that there is never a conversion between SomeType<T> to SomeType<U>, as long as T and U are different types.
Let's imagine what would happen if the conversion on MyCollection were allowed. You could do:
let myCollectionInt = MyCollection<Int>()
let myCollectionAny: MyCollection<Any> = myCollectionInt // suppose you can do this
myCollectionAny.object = "string" // myCollectionAny.object is of type Any?, so this should be ok
We've set myCollectionAny.object to "string", but MyCollection is a reference type, so myCollectionInt.object should also be "string". But myCollectionInt.object is an Int?!
Of course this type-unsafety is also a problem with arrays, but the language designers have decided that casting arrays is a common enough thing to do, that disallowing it would do more hard than good.

Related

Swift 5 storing and passing KeyPaths

Let's say I have the following class:
class User: NSObject {
var name = "Fred"
var age = 24
var email = "fred#freddy.com"
var married = false
}
I want to be able to write a generic function that takes in a list of KeyPaths for a known class type, read the values and print to screen. The problem is, the I can't get the following code to compile as the type of the KeyPath's Value is not known, and will be different for each time. What do I have to do to make this work generically?
Consider the following:
struct KeyPathProperties<T> {
var name: String
var relatedKeyPaths: [KeyPath<T, Any>]
}
extension KeyPath where Root == User {
var properties: KeyPathProperties<Root> {
switch self {
case \Root.name:
return KeyPathProperties(name: "name", relatedKeyPaths: [\Root.age, \Root.email])
default:
fatalError("Unknown key path")
}
}
}
This line fails to compile:
return KeyPathProperties(name: "name", relatedKeyPaths: [\Root.age, \Root.email])
with this error:
Cannot convert value of type 'KeyPath<User, Int>' to expected element type 'KeyPath<User, Any>'
This is what I wish to be able to do, for instance:
let myUser = User()
var keyPathProps = KeyPathProperties(name: "name", relatedKeyPaths: [\User.age, \User.email])
for keyPath in props.relatedKeyPaths {
print("Value: \(myUser[keyPath: keyPath])")
}
The above won't compile of course. Essentially I want to store keyPaths in an array at runtime, so I can generically at some point in time get values out of the User. I need to know if I can re-write the above in some way where the compiler can safely and correctly determine the type of the keyPath's value at runtime.
This is a conceptual use case for a much more complex architectural issue I'm trying to solve with hopefully less code.
MORE INFORMATION:
At runtime I wish to keep track of the properties that get modified - these properties are held in a modifiedProps array in each object / instance. At some point at runtime, I wish to be able to enumerate over this array of KeyPaths and print their values like so:
for modifiedKeyPath in self.modifiedProps {
print ("\(self[keyPath: modifiedKeyPath])"
}
In short - I need to be able to capture the generic type of the KeyPath within KeyPathProperties. How do I achieve this?
SIDE NOTE: I can already easily achieve this by using Swift 3 style string based KeyPaths (by adding #objc to the class properties). I can store an array of keyPaths as strings and later do:
let someKeyPath = #keyPath(User.email)
...
myUser.value(forKeyPath: someKeyPath)
I just cannot do this with Swift 4 KeyPaths generically.
The error tells you what your misconception is:
Cannot convert value of type 'KeyPath<User, Int>'
to expected element type 'KeyPath<User, Any>'
You seem to think that you can use a KeyPath<User, Int> where a KeyPath<User, Any> is expected, ostensibly on the grounds that an Int is an Any. But that's not true. These are generic types, and generic types are not covariant — that is, there is no substitution principle for generics based on their parameterized types. The two types are effectively unrelated.
If you need an array of key paths regardless of their parameterized types, you would need an array of PartialKeyPath or AnyKeyPath. It seems that in your use case the root object is the same throughout, so presumably you want PartialKeyPath.

Realm Swift - Convert an array of results into an array of Ints

I want to convert the results from my Realm data into Int.
Here is an example of how I want to use this.
let results = realm.objects(Data.self)
print(results)
However the result is type Results<Data> and cannot be converted into a Int but the results is an Int.
Just to be clear I want an array of Int from my results
You can simply use Array(realm.objects(RealmType.self)), which will convert the Results<RealmType> instance into an Array<RealmType>.
However, there are other serious flaws with your code. First of all, neither of the last two lines will compile, since firstly realm.objects() accepts a generic input argument of type Object.Type and Data doesn't inherit from Object. You can't directly store Data objects in Realm, you can only store Data as a property of a Realm Object subclass.
Secondly, myArray[results] is simply wrong, since results is supposed to be of type Results<RealmType>, which is a collection, so using it to index an Array cannot work (especially whose Element type is different).
It appears that based on the number of results from the database, you want to select an object from the array.
You can get the number of items in an array with .count. Be certain that an object exists at the specified index in the array, or your application will crash!
let numberOfResults = results.count
if myArray.count > numberOfResults {
let object = myArray[numberOfResults]
print(object)
}

Type "Any" has no subscript members despite casting as AnyObject on Swift 3?

Recently converted code from earlier version of swift to swift 3. Got a lot of errors signifying that type "any" has no subscript members and I thought this could be fixed by casting as AnyObject, but the error persists (and therefore the code I post here does not have this cast in it). Here is the relevant code:
func textfieldTextWasChanged(_ newText: String, parentCell: CustomCell) {
let parentCellIndexPath = tblExpandable.indexPath(for: parentCell)
var address = ""
address = "\(newText)"
// TODO: add a pin to the map from input address
cellDescriptors[0][11].setValue(address, forKey: "primaryTitle")
location = cellDescriptors[0][11]["primaryTitle"]! as! String
tblExpandable.reloadData()
}
Note that cellDescriptors is defined earlier in the code as an NSMutableArray. The error shows up right after cellDescriptors[0] in both lines that it is in. Not sure how to fix this.
It's because you're using more than one subscript operator, because presumably this is something like an array of arrays. But NSMutableArray's subscript operator returns Any. As a result, cellDescriptors[0] is Any. You try to use [11] on the result, but Any doesn't accept subscripts because it's Any, not a collection type.
Casting to AnyObject doesn't help because AnyObject is also not a collection type.
What you should do is cast cellDescriptors[0] to something that accepts subscripts. The right choice depends on what kind of data you're storing in cellDescriptors, but it's presumably some kind of collection, probably an array type.
Another approach would be to change cellDescriptors to be a Swift type instead of NSMutableArray. You could specifically declare the types for each part of your data structure, and then type casting wouldn't be needed.

How do I store a value of type Class<ClassImplementingProtocol> in a Dictionary of type [String:Class<Protocol>] in Swift?

I want to store a more specialized type in a Dictionary of type [String:SomeClass]. Here is some sample code illustrating my problem (also available to play with at https://swiftlang.ng.bluemix.net/#/repl/579756cf9966ba6275fc794a):
class Thing<T> {}
protocol Flavor {}
class Vanilla: Flavor {}
var dict = [String:Thing<Flavor>]()
dict["foo"] = Thing<Vanilla>()
It produces the error ERROR at line 9, col 28: cannot assign value of type 'Thing<Vanilla>' to type 'Thing<Any>?'.
I've tried casting Thing<Vanilla>() as Thing<Flavor> but that produces the error cannot convert value of type 'Thing<Vanilla>' to type 'Thing<Flavor>' in coercion.
I've also tried to define the Dictionary as type [String:Thing<Any>] but that doesn't change anything either.
How do I create a collection of different Things without resorting to plain [String:AnyObject]?
I should also mention that the class Thing is not defined by me (in fact it's about BoltsSwift Tasks), so the solution to create a base class of Thing without a type parameter doesn't work.
A Thing<Vanilla> is not a Thing<Flavor>. Thing is not covariant. There is no way in Swift to express that Thing is covariant. There are good reasons for this. If what you were asking for were allowed without careful rules around it, I would be allowed to write the following code:
func addElement(array: inout [Any], object: Any) {
array.append(object)
}
var intArray: [Int] = [1]
addElement(array: &intArray, object: "Stuff")
Int is a subtype of Any, so if [Int] were a subtype of [Any], I could use this function to append strings to an int array. That breaks the type system. Don't do that.
Depending on your exact situation, there are two solutions. If it is a value type, then repackage it:
let thing = Thing<Vanilla>(value: Vanilla())
dict["foo"] = Thing(value: thing.value)
If it is a reference type, box it with a type eraser. For example:
// struct unless you have to make this a class to fit into the system,
// but then it may be a bit more complicated
struct AnyThing {
let _value: () -> Flavor
var value: Flavor { return _value() }
init<T: Flavor>(thing: Thing<T>) {
_value = { return thing.value }
}
}
var dict = [String:AnyThing]()
dict["foo"] = AnyThing(thing: Thing<Vanilla>(value: Vanilla()))
The specifics of the type eraser may be different depending on your underlying type.
BTW: The diagnostics around this have gotten pretty good. If you try to call my addElement above in Xcode 9, you get this:
Cannot pass immutable value as inout argument: implicit conversion from '[Int]' to '[Any]' requires a temporary
What this is telling you is that Swift is willing to pass [Int] where you ask for [Any] as a special-case for Arrays (though this special treatment isn't extended to other generic types). But it will only allow it by making a temporary (immutable) copy of the array. (This is another example where it can be hard to reason about Swift performance. In situations that look like "casting" in other languages, Swift might make a copy. Or it might not. It's hard to be certain.)
One way to solve this is adding an initialiser to Thing and creating a Thing<Flavor> that will hold a Vanilla object.
It will look something like:
class Thing<T> {
init(thing : T) {
}
}
protocol Flavor {}
class Vanilla: Flavor {}
var dict = [String:Thing<Flavor>]()
dict["foo"] = Thing<Flavor>(thing: Vanilla())

Cannot convert value of type 'Int' to expected argument type '_?'

Note: I'm a rookie in Swift
I'm using Former.
I'm fetching data from a realm model.
let industries = realm.objects(Industry)
Then I try to define a list of InlinePickerItem from it:
$0.pickerItems = industries.map({ industry in
return InlinePickerItem(title: industry.name, value: industry.id)
})
But XCode keeps saying: Cannot convert value of type 'Int' to expected argument type '_?', pointing to industry.id.
Am I missing something? I don't know if the issue comes from Former or from something that I don't understand in Swift. For example, what kind of type is _??
UPDATE:
After #dfri comment, attempt was unsuccessful. From my small understanding of Swift, I get that Swift gets lost. So I extracted the initialisation of the list of InlinePickerItem from the closure.
let industries = realm.objects(Industry)
let inlinePickerItems = industries.map({ industry in
return InlinePickerItem(title: industry.name, displayTitle: nil, value: industry.id)
})
let catRow = InlinePickerRowFormer<ProfileLabelCell, String>(instantiateType: .Nib(nibName: "ProfileLabelCell")) {
$0.titleLabel.text = "CATEGORY".localized
}.configure {
$0.pickerItems = inlinePickerItems
}
The error is disappeared when calling InlinePickerItem(title: industry.name, displayTitle: nil, value: industry.id) but I get something new when assigning it to $0.pickerItems which now is:
Cannot assign value of type '[InlinePickerItem<Int>]' to type '[InlinePickerItem<String>]'
Hope this will provide you with some helpful hints.
Type mismatch when assigning array to different type array
After the re-factoring of your code (after "update") its now apparent what is the source of error.
Immutable catRow is of type InlinePickerRowFormer<ProfileLabelCell, String>. From the source of [InlinePickerRowFormer] we see the that the class and its property pickerItems is declared as follows
public class InlinePickerRowFormer<T: UITableViewCell, S where T: InlinePickerFormableRow>
: ... {
// ...
public var pickerItems: [InlinePickerItem<S>] = []
// ...
}
The key here is that for an instance InlinePickerRowFormer<T,S> its property pickerItems will be an array with elements of type InlinePickerItem<S>. In your example above S is String
let catRow = InlinePickerRowFormer<ProfileLabelCell, String>
/* |
S = String */
Hence pickerItems is an array of InlinePickerItem<String> instances.
You try, however, to append the immutable inlinePickerItems to pickerItems, which means you're trying to assign an array of InlinePickerItem<Int> instances to an array with elements of type InlinePickerItem<String>; naturally leading to a type mismatch.
You can solve this type mismatch by:
Setting your catRow immutable to be of type InlinePickerRowFormer<ProfileLabelCell, Int>.