In the documentation for #dynamicMemberLookup it says,
Apply this attribute to a class, structure, enumeration, or protocol to enable members to be looked up by name at runtime.
If I'm not mistaken, instance methods are considered members of a struct / class. However, when I try to call a function dynamically I get an error saying:
Dynamic key path member lookup cannot refer to instance method foo()
To reproduce the problem:
struct Person {
var name: String
var age: Int
func greet() {
print("hello, my name is \(name)")
}
}
#dynamicMemberLookup
struct Wrapper {
var value: Person
subscript<T>(dynamicMember keypath: KeyPath<Person, T>) -> T {
value[keyPath: keypath]
}
}
let person = Person(name: "John Doe", age: 21)
let wrapper = Wrapper(value: person)
wrapper.greet() // << Error: Dynamic key path member lookup cannot refer to instance method `greet()`
// Or
let function = wrapper.greet // << Error: Dynamic key path member lookup cannot refer to instance method `greet()`
function()
How can I dynamically call greet() using #dynamicMemberLookup? Is there any way to achieve what I'm trying to do?
Thanks in advance!
No, dynamicMemberLookup does not work for methods. As the signature of the subscript suggests, it only works for things that can be represented as a KeyPath. Method calls cannot be part of a key path. :(
Key-Path Expression
A key-path expression refers to a property or subscript of a type.
The path consists of property names, subscripts, optional-chaining
expressions, and forced unwrapping expressions. Each of these key-path
components can be repeated as many times as needed, in any order.
At compile time, a key-path expression is replaced by an instance of
the KeyPath class.
I suspect the reason why it is called "dynamic member lookup" is because it also works with subscripts. The alternative of dynamicPropertyOrSubscriptLookup is rather a mouthful isn't it?
One rather hacky fix would be to change greet into a computed property:
var greet: () -> Void { {
print("hello, my name is \(name)")
} }
If greet has had parameters, you could also change it into a subscript, but I think that is an even uglier solution.
Is there a way to print the runtime type of a variable in swift? For example:
var now = NSDate()
var soon = now.dateByAddingTimeInterval(5.0)
println("\(now.dynamicType)")
// Prints "(Metatype)"
println("\(now.dynamicType.description()")
// Prints "__NSDate" since objective-c Class objects have a "description" selector
println("\(soon.dynamicType.description()")
// Compile-time error since ImplicitlyUnwrappedOptional<NSDate> has no "description" method
In the example above, I'm looking for a way to show that the variable "soon" is of type ImplicitlyUnwrappedOptional<NSDate>, or at least NSDate!.
Update September 2016
Swift 3.0: Use type(of:), e.g. type(of: someThing) (since the dynamicType keyword has been removed)
Update October 2015:
I updated the examples below to the new Swift 2.0 syntax (e.g. println was replaced with print, toString() is now String()).
From the Xcode 6.3 release notes:
#nschum points out in the comments that the Xcode 6.3 release notes show another way:
Type values now print as the full demangled type name when used with
println or string interpolation.
import Foundation
class PureSwiftClass { }
var myvar0 = NSString() // Objective-C class
var myvar1 = PureSwiftClass()
var myvar2 = 42
var myvar3 = "Hans"
print( "String(myvar0.dynamicType) -> \(myvar0.dynamicType)")
print( "String(myvar1.dynamicType) -> \(myvar1.dynamicType)")
print( "String(myvar2.dynamicType) -> \(myvar2.dynamicType)")
print( "String(myvar3.dynamicType) -> \(myvar3.dynamicType)")
print( "String(Int.self) -> \(Int.self)")
print( "String((Int?).self -> \((Int?).self)")
print( "String(NSString.self) -> \(NSString.self)")
print( "String(Array<String>.self) -> \(Array<String>.self)")
Which outputs:
String(myvar0.dynamicType) -> __NSCFConstantString
String(myvar1.dynamicType) -> PureSwiftClass
String(myvar2.dynamicType) -> Int
String(myvar3.dynamicType) -> String
String(Int.self) -> Int
String((Int?).self -> Optional<Int>
String(NSString.self) -> NSString
String(Array<String>.self) -> Array<String>
Update for Xcode 6.3:
You can use the _stdlib_getDemangledTypeName():
print( "TypeName0 = \(_stdlib_getDemangledTypeName(myvar0))")
print( "TypeName1 = \(_stdlib_getDemangledTypeName(myvar1))")
print( "TypeName2 = \(_stdlib_getDemangledTypeName(myvar2))")
print( "TypeName3 = \(_stdlib_getDemangledTypeName(myvar3))")
and get this as output:
TypeName0 = NSString
TypeName1 = __lldb_expr_26.PureSwiftClass
TypeName2 = Swift.Int
TypeName3 = Swift.String
Original answer:
Prior to Xcode 6.3 _stdlib_getTypeName got the mangled type name of a variable. Ewan Swick's blog entry helps to decipher these strings:
e.g. _TtSi stands for Swift's internal Int type.
Mike Ash has a great blog entry covering the same topic.
Edit: A new toString function has been introduced in Swift 1.2 (Xcode 6.3).
You can now print the demangled type of any type using .self and any instance using .dynamicType:
struct Box<T> {}
toString("foo".dynamicType) // Swift.String
toString([1, 23, 456].dynamicType) // Swift.Array<Swift.Int>
toString((7 as NSNumber).dynamicType) // __NSCFNumber
toString((Bool?).self) // Swift.Optional<Swift.Bool>
toString(Box<SinkOf<Character>>.self) // __lldb_expr_1.Box<Swift.SinkOf<Swift.Character>>
toString(NSStream.self) // NSStream
Try calling YourClass.self and yourObject.dynamicType.
Reference: https://devforums.apple.com/thread/227425.
Swift 3.0
let string = "Hello"
let stringArray = ["one", "two"]
let dictionary = ["key": 2]
print(type(of: string)) // "String"
// Get type name as a string
String(describing: type(of: string)) // "String"
String(describing: type(of: stringArray)) // "Array<String>"
String(describing: type(of: dictionary)) // "Dictionary<String, Int>"
// Get full type as a string
String(reflecting: type(of: string)) // "Swift.String"
String(reflecting: type(of: stringArray)) // "Swift.Array<Swift.String>"
String(reflecting: type(of: dictionary)) // "Swift.Dictionary<Swift.String, Swift.Int>"
Is this what you're looking for?
println("\(object_getClassName(now))");
It prints "__NSDate"
UPDATE: Please note this no longer seems to work as of Beta05
My current Xcode is Version 6.0 (6A280e).
import Foundation
class Person { var name: String; init(name: String) { self.name = name }}
class Patient: Person {}
class Doctor: Person {}
var variables:[Any] = [
5,
7.5,
true,
"maple",
Person(name:"Sarah"),
Patient(name:"Pat"),
Doctor(name:"Sandy")
]
for variable in variables {
let typeLongName = _stdlib_getDemangledTypeName(variable)
let tokens = split(typeLongName, { $0 == "." })
if let typeName = tokens.last {
println("Variable \(variable) is of Type \(typeName).")
}
}
Output:
Variable 5 is of Type Int.
Variable 7.5 is of Type Double.
Variable true is of Type Bool.
Variable maple is of Type String.
Variable Swift001.Person is of Type Person.
Variable Swift001.Patient is of Type Patient.
Variable Swift001.Doctor is of Type Doctor.
As of Xcode 6.3 with Swift 1.2, you can simply convert type values into the full demangled String.
toString(Int) // "Swift.Int"
toString(Int.Type) // "Swift.Int.Type"
toString((10).dynamicType) // "Swift.Int"
println(Bool.self) // "Swift.Bool"
println([UTF8].self) // "Swift.Array<Swift.UTF8>"
println((Int, String).self) // "(Swift.Int, Swift.String)"
println((String?()).dynamicType)// "Swift.Optional<Swift.String>"
println(NSDate) // "NSDate"
println(NSDate.Type) // "NSDate.Type"
println(WKWebView) // "WKWebView"
toString(MyClass) // "[Module Name].MyClass"
toString(MyClass().dynamicType) // "[Module Name].MyClass"
You can still access the class, through className (which returns a String).
There are actually several ways to get the class, for example classForArchiver, classForCoder, classForKeyedArchiver (all return AnyClass!).
You can't get the type of a primitive (a primitive is not a class).
Example:
var ivar = [:]
ivar.className // __NSDictionaryI
var i = 1
i.className // error: 'Int' does not have a member named 'className'
If you want to get the type of a primitive, you have to use bridgeToObjectiveC(). Example:
var i = 1
i.bridgeToObjectiveC().className // __NSCFNumber
You can use reflect to get information about object.
For example name of object class:
var classname = reflect(now).summary
Xcode 8 Swift 3.0 use type(of:)
let className = "\(type(of: instance))"
I had luck with:
let className = NSStringFromClass(obj.dynamicType)
SWIFT 5
With the latest release of Swift 3 we can get pretty descriptions of type names through the String initializer. Like, for example print(String(describing: type(of: object))). Where object can be an instance variable like array, a dictionary, an Int, a NSDate, an instance of a custom class, etc.
Here is my complete answer: Get class name of object as string in Swift
That question is looking for a way to getting the class name of an object as string but, also i proposed another way to getting the class name of a variable that isn't subclass of NSObject. Here it is:
class Utility{
class func classNameAsString(obj: Any) -> String {
//prints more readable results for dictionaries, arrays, Int, etc
return String(describing: type(of: obj))
}
}
I made a static function which takes as parameter an object of type Any and returns its class name as String :) .
I tested this function with some variables like:
let diccionary: [String: CGFloat] = [:]
let array: [Int] = []
let numInt = 9
let numFloat: CGFloat = 3.0
let numDouble: Double = 1.0
let classOne = ClassOne()
let classTwo: ClassTwo? = ClassTwo()
let now = NSDate()
let lbl = UILabel()
and the output was:
diccionary is of type Dictionary
array is of type Array
numInt is of type Int
numFloat is of type CGFloat
numDouble is of type Double
classOne is of type: ClassOne
classTwo is of type: ClassTwo
now is of type: Date
lbl is of type: UILabel
In Xcode 8, Swift 3.0
Steps:
1. Get the Type:
Option 1:
let type : Type = MyClass.self //Determines Type from Class
Option 2:
let type : Type = type(of:self) //Determines Type from self
2. Convert Type to String:
let string : String = "\(type)" //String
In Swift 3.0, you can use type(of:), as dynamicType keyword has been removed.
To get a type of object or class of object in Swift, you must need to use a type(of: yourObject)
type(of: yourObject)
When using Cocoa (not CocoaTouch), you can use the className property for objects that are subclasses of NSObject.
println(now.className)
This property is not available for normal Swift objects, which aren't subclasses of NSObject (and in fact, there is no root id or object type in Swift).
class Person {
var name: String?
}
var p = Person()
println(person.className) // <- Compiler error
In CocoaTouch, at this time there is not a way to get a string description of the type of a given variable. Similar functionality also does not exist for primitive types in either Cocoa or CocoaTouch.
The Swift REPL is able to print out a summary of values including its type, so it is possible this manner of introspection will be possible via an API in the future.
EDIT: dump(object) seems to do the trick.
The top answer doesn't have a working example of the new way of doing this using type(of:. So to help rookies like me, here is a working example, taken mostly from Apple's docs here - https://developer.apple.com/documentation/swift/2885064-type
doubleNum = 30.1
func printInfo(_ value: Any) {
let varType = type(of: value)
print("'\(value)' of type '\(varType)'")
}
printInfo(doubleNum)
//'30.1' of type 'Double'
I've tried some of the other answers here but milage seems to very on what the underling object is.
However I did found a way you can get the Object-C class name for an object by doing the following:
now?.superclass as AnyObject! //replace now with the object you are trying to get the class name for
Here is and example of how you would use it:
let now = NSDate()
println("what is this = \(now?.superclass as AnyObject!)")
In this case it will print NSDate in the console.
I found this solution which hopefully might work for someone else.
I created a class method to access the value. Please bear in mind this will work for NSObject subclass only. But at least is a clean and tidy solution.
class var className: String!{
let classString : String = NSStringFromClass(self.classForCoder())
return classString.componentsSeparatedByString(".").last;
}
In the latest XCode 6.3 with Swift 1.2, this is the only way I found:
if view.classForCoder.description() == "UISegment" {
...
}
Many of the answers here do not work with the latest Swift (Xcode 7.1.1 at time of writing).
The current way of getting the information is to create a Mirror and interrogate that. For the classname it is as simple as:
let mirror = Mirror(reflecting: instanceToInspect)
let classname:String = mirror.description
Additional information about the object can also be retrieved from the Mirror. See http://swiftdoc.org/v2.1/type/Mirror/ for details.
Swift version 4:
print("\(type(of: self)) ,\(#function)")
// within a function of a class
Thanks #Joshua Dance
In lldb as of beta 5, you can see the class of an object with the command:
fr v -d r shipDate
which outputs something like:
(DBSalesOrderShipDate_DBSalesOrderShipDate_ *) shipDate = 0x7f859940
The command expanded out means something like:
Frame Variable (print a frame variable) -d run_target (expand dynamic types)
Something useful to know is that using "Frame Variable" to output variable values guarantees no code is executed.
I've found a solution for self-developed classes (or such you have access to).
Place the following computed property within your objects class definition:
var className: String? {
return __FILE__.lastPathComponent.stringByDeletingPathExtension
}
Now you can simply call the class name on your object like so:
myObject.className
Please note that this will only work if your class definition is made within a file that is named exactly like the class you want the name of.
As this is commonly the case the above answer should do it for most cases. But in some special cases you might need to figure out a different solution.
If you need the class name within the class (file) itself you can simply use this line:
let className = __FILE__.lastPathComponent.stringByDeletingPathExtension
Maybe this method helps some people out there.
Based on the answers and comments given by Klass and Kevin Ballard above, I would go with:
println(_stdlib_getDemangledTypeName(now).componentsSeparatedByString(".").last!)
println(_stdlib_getDemangledTypeName(soon).componentsSeparatedByString(".").last!)
println(_stdlib_getDemangledTypeName(soon?).componentsSeparatedByString(".").last!)
println(_stdlib_getDemangledTypeName(soon!).componentsSeparatedByString(".").last!)
println(_stdlib_getDemangledTypeName(myvar0).componentsSeparatedByString(".").last!)
println(_stdlib_getDemangledTypeName(myvar1).componentsSeparatedByString(".").last!)
println(_stdlib_getDemangledTypeName(myvar2).componentsSeparatedByString(".").last!)
println(_stdlib_getDemangledTypeName(myvar3).componentsSeparatedByString(".").last!)
which will print out:
"NSDate"
"ImplicitlyUnwrappedOptional"
"Optional"
"NSDate"
"NSString"
"PureSwiftClass"
"Int"
"Double"
let i: Int = 20
func getTypeName(v: Any) -> String {
let fullName = _stdlib_demangleName(_stdlib_getTypeName(i))
if let range = fullName.rangeOfString(".") {
return fullName.substringFromIndex(range.endIndex)
}
return fullName
}
println("Var type is \(getTypeName(i)) = \(i)")
Swift 4:
// "TypeName"
func stringType(of some: Any) -> String {
let string = (some is Any.Type) ? String(describing: some) : String(describing: type(of: some))
return string
}
// "ModuleName.TypeName"
func fullStringType(of some: Any) -> String {
let string = (some is Any.Type) ? String(reflecting: some) : String(reflecting: type(of: some))
return string
}
Usage:
print(stringType(of: SomeClass())) // "SomeClass"
print(stringType(of: SomeClass.self)) // "SomeClass"
print(stringType(of: String())) // "String"
print(fullStringType(of: String())) // "Swift.String"
There appears to be no generic way to print the type name of an arbitrary value's type. As others have noted, for class instances you can print value.className but for primitive values it appears that at runtime, the type information is gone.
For instance, it looks as if there's not a way to type: 1.something() and get out Int for any value of something. (You can, as another answer suggested, use i.bridgeToObjectiveC().className to give you a hint, but __NSCFNumber is not actually the type of i -- just what it will be converted to when it crosses the boundary of an Objective-C function call.)
I would be happy to be proven wrong, but it looks like the type checking is all done at compile time, and like C++ (with RTTI disabled) much of the type information is gone at runtime.
This is how you get a type string of your object or Type which is consistent and takes into account to which module the object definition belongs to or nested in. Works in Swift 4.x.
#inline(__always) func typeString(for _type: Any.Type) -> String {
return String(reflecting: type(of: _type))
}
#inline(__always) func typeString(for object: Any) -> String {
return String(reflecting: type(of: type(of: object)))
}
struct Lol {
struct Kek {}
}
// if you run this in playground the results will be something like
typeString(for: Lol.self) // __lldb_expr_74.Lol.Type
typeString(for: Lol()) // __lldb_expr_74.Lol.Type
typeString(for: Lol.Kek.self)// __lldb_expr_74.Lol.Kek.Type
typeString(for: Lol.Kek()) // __lldb_expr_74.Lol.Kek.Type
Not exactly what you are after, but you can also check the type of the variable against Swift types like so:
let object: AnyObject = 1
if object is Int {
}
else if object is String {
}
For example.
Xcode 7.3.1, Swift 2.2:
String(instanceToPrint.self).componentsSeparatedByString(".").last
I'm using an enum variable called BuildingType in a class initializer (of a class called Building).
This enum is defined outside the class because I want to use it also in other places.
The autocomplete for this enum is not working properly when initializing the variable typeOfBuilding.
Example code:
enum BuildingType {
case Flat, House, Villa
}
class Building {
var type : BuildingType = BuildingType.House
var floors : Int = 1
init(typeOfBuilding : BuildingType, numFloors : Int) {
self.type = typeOfBuilding
self.floors = numFloors
}
}
var myBuilding : Building = Building(typeOfBuilding: BuildingType.Flat , numFloors: 3)
So if I type "... typeOfBuilding: BuildingType." (when initializing myBuilding) 'floors' and 'type' are shown, and not the enum values.
I must be doing something wrong here but what?
This is a pretty weird bug
It occurs when you attempt to pass an enum into an argument of an initialiser, autocomplete will fail and instead of suggesting the enum cases after typing Enum., it will list the instance members of the class you’re calling the initialiser on. If you try and use the single dot syntax (.Case), autocomplete will also fail, but instead of displaying the list of instance members, it simply won't display anything.
I initially thought it may have had something to do with the naming of your enum and class (BuildingType & Building), but this is not the case.
This bug only appears to be present in initialisers with multiple arguments (one of which being an enum). I couldn’t reproduce this issue with single argument initialisers.
The reproducibility appears to depend on whether the initialiser is 'complete'. I'm considering an initialiser to be 'complete' if it has all argument names and values (except the enum) defined. For example:
// Incomplete (foo is one argument of many)
let baz = Baz(foo: Foo.
// Semi-Complete (before you assign the second parameter a value)
let baz = Baz(foo: Foo., string: <String Placeholder>)
// Complete
let baz = Baz(foo: Foo., string: "")
// Complete (note the lack of the last bracket)
let baz = Baz(param: 0, foo: Foo.
Here is my test setup (Xcode 7.3, Swift 2.2):
enum Foo {
case Bar
}
class Baz {
var iReallyShouldntBeDisplayedHere = 0
init(foo:Foo, string:String) {}
init(foo: Foo) {}
}
And here is a list of cases where I've found the bug does & doesn't occur:
// Enum is the only argument
// CORRECT: accepting the initialiser's autocomplete (so it's 'complete'), then typing "Foo." brings up autocomplete options for enum cases
let baz = Baz(foo: Foo.)
// CORRECT: typing the initialiser yourself (so it's 'incomplete'), then typing "Foo." in the first parameter brings up autocomplete options for enum cases
let baz2 = Baz(foo: Foo.
// Enum is one argument of many
// INCORRECT: accepting the initialiser's autocomplete (so it's 'semi-complete'), then typing "Foo." in the first parameter brings up Baz's instance members ("iReallyShouldntBeDisplayedHere")
let baz3 = Baz(foo: Foo., string: <String Placeholder>)
// CORRECT: typing the initialiser yourself (so it's 'incomplete'), and typing "Foo." in the first parameter brings up enum cases
let baz4 = Baz(foo: Foo.
// Single dot syntax (where enum is one argument of many)
// CORRECT: typing the initialiser yourself (so it's 'incomplete'), and typing "." in the first parameter brings up enum cases
let baz5 = Baz(foo:.
// CORRECT: accepting the initialiser's autocomplete (so it's 'semi-complete'), then typing "." in the first parameter brings up enum cases
let baz6 = Baz(foo:., string: <String Placeholder>)
// INCORRECT: modifying the foo: argument once the initialiser is 'complete' by typing "." in the first parameter doesn't generate the autocomplete list
let baz7 = Baz(foo:., string: "")
I also tried this where foo: is the last argument, but the initialiser always has to be complete in that case, so it always fails. I did try with initialisers that take 3 arguments, but it appears to have the same behaviour as an initialiser with 2 arguments.
If anyone knows of any more cases where this bug can be reproduced, I'd love to know!
I'm new to Swift, and am trying to get my head around using .map. My understanding is that the behaviour is similar to that of Javascript, but maybe I'm not nailing the Swift syntax correctly?
I've created a public Array of my custom class CustomItem (which is a subclass of the type coming back in the response):
public var availableItems: [CustomItem] = []
public static func getAvailableItems(id: String, completion: (items: [CustomItem]) -> ()) -> Void {
DataConnector.getRelated(type: "users", id: id, relationship: "available") { (response) -> Void in
availableItems = (response.data?.map { return $0 as! CustomItem })!
completion(items: availableItems)
}
}
When I do a po response.data.map { return $0 } in the console with a breakpoint after the response is received, I get:
(lldb) po response.data.map { return $0 }
▿ Optional([<AnSDK.RemoteDataObject: 0x7f8a0b9c16b0>])
▿ Some : 1 elements
▿ [0] : < AnSDK.RemoteDataObject: 0x7f8a0b9c16b0>
So it definitely seems that part works, but when I try to cast the data object to CustomItem class, I get:
Could not cast value of type 'AnSDK.RemoteDataObject' (0x100abbda0) to 'Project.CustomItem' (0x100882c60).
Here's my CustomItem class just in case:
import AnSDK
public class CustomItem: RemoteDataObject {
var displayName: String = ""
var value: Float = 0.0
var owner: User?
}
If I don't use the ! to force downcast, I get:
RemoteDataObject is not convertible to CustomItem [...]
... in the compiler.
(I'm really just restating Ben Gottlieb's answer here, but hopefully a bit clearer since I believe some readers were confused by his attempt.)
The message seems fairly clear. You've received an array of AnSDK.RemoteDataObject. As best I can tell from your output, that is the actual class of the objects. You can't just say "it's really this subclass of RDO" unless it really is that subclass. Looking at your code, that seems unlikely. Somewhere in AnSDK it would have to construct a CustomItem and then just happen to return it as a RemoteDataObject. That doesn't appear to be what's happening inside of getRelated. Given your code, I doubt AnSDK knows anything about CustomItem, so how would it have constructed one?
There are numerous ways to fix this depending on what the types really are and how they interact. Ben's solution is one, which basically creates a copy of the object (though in that case, there's no particular reason for CustomItem to be a subclass of RDO, and probably shouldn't be.)
If you just want to add methods to RemoteDataObject, you can do that with extensions. You don't need to create a subclass.
I have a custom object called Field. I basically use it to define a single field in a form.
class Field {
var name: String
var value: Any?
// initializers here...
}
When the user submits the form, I validate each of the Field objects to make sure they contain valid values. Some fields aren't required so I sometimes deliberately set nil to the value property like this:
field.value = nil
This seems to pose a problem when I use an if-let to determine whether a field is nil or not.
if let value = field.value {
// The field has a value, ignore it...
} else {
// Add field.name to the missing fields array. Later, show the
// missing fields in a dialog.
}
I set breakpoints in the above if-else and when field.value has been deliberately set to nil, it goes through the if-let block, not the else. However, for the fields whose field.value I left uninitialized and unassigned, the program goes to the else block.
I tried printing out field.value and value inside the if-let block:
if let value = field.value {
NSLog("field.value: \(field.value), value: \(value)")
}
And this is what I get:
field.value: Optional(nil), value: nil
So I thought that maybe with optionals, it's one thing to be uninitialized and another to have the value of nil. But even adding another if inside the if-let won't make the compiler happy:
if let value = field.value {
if value == nil { // Cannot invoke '==' with an argument list of type '(Any, NilLiteralConvertible)'
}
}
How do I get around this? I just want to check if the field.value is nil.
I believe this is because Any? allows any value and Optional.None is being interpreted as just another value, since Optional is an enum!
AnyObject? should be unable to do this since it only can contain Optional.Some([any class object]), which does not allow for the case Optional.Some(Optional) with the value Optional.None.
This is deeply confusing to even talk about. The point is: try AnyObject? instead of Any? and see if that works.
More to the point, one of Matt's comment mentions that the reason he wants to use Any is for a selection that could be either a field for text input or a field intended to select a Core Data object.
The Swifty thing to do in this case is to use an enum with associated values, basically the same thing as a tagged/discriminated union. Here's how to declare, assign and use such an enum:
enum DataSelection {
case CoreDataObject(NSManagedObject)
case StringField(String)
}
var selection : DataSelection?
selection = .CoreDataObject(someManagedObject)
if let sel = selection { // if there's a selection at all
switch sel {
case .CoreDataObject(let coreDataObj):
// do what you will with coreDataObj
case .StringField(let string):
// do what you will with string
}
}
Using an enum like this, there's no need to worry about which things could be hiding inside that Any?. There are two cases and they are documented. And of course, the selection variable can be an optional without any worries.
There's a tip to replace my Any? type with an enum but I couldn't get this error out of my head. Changing my approach doesn't change the fact that something is wrong with my current one and I had to figure out how I arrived at an Optional(nil) output.
I was able to reproduce the error by writing the following view controller in a new single-view project. Notice the init signature.
import UIKit
class Field {
var name: String = "Name"
var value: Any?
init(_ name: String, _ value: Any) {
self.name = name
self.value = value
}
}
class AppState {
var currentValue: Field?
}
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let f = Field("Amount", AppState().currentValue)
NSLog("\(f.value)")
}
}
In short, I was passing a nil value (AppState().currentValue) to an initializer that accepts Any, and assigns it to a property whose type is Any?. The funny thing here is if I directly passed nil instead, the compiler will complain:
let f = Field("Amount", nil) // Type 'Any' does not conform to protocol 'NilLiteralConvertible'
It seems that somewhere along the way, Swift wraps the nil value of AppState().currentValue in an optional, hence Optional(nil).