I'm trying to get all signature of initializer from the class in Swift. Initializer can mirror, I can find the signatures like below code.
enum MessageType {
case say
case shout
case wisper
}
class Message {
var text = ""
var type : MessageType = .say
init(text: String, type: MessageType) {
self.type = type
self.text = text
}
init(text: String) {
self.text = text
}
}
let firstInit = Message.init(text:)
let secondInit = Message.init(text:type:)
let firstMirror = Mirror(reflecting: firstInit)
let secondMirror = Mirror(reflecting: secondInit)
print(firstMirror.subjectType)
// (String) -> Message
print(secondMirror.subjectType)
// ((String, MessageType)) -> Message
However, this code requires to specify init which I want to look it up. What I expected is something like below:
let mirror = Mirror(reflecting: Message)
let inits = mirror.initializers
// something like [Message.init(text:), Message.init(text:type:)] as [Any]
for method in inits {
let mirror = Mirror(reflecting: method)
print(method.subjectType)
}
How can I get all init initializers from class using Mirror?
The Mirror struct in Swift offers some runtime introspection features, but for the default case, these focus on the instance being reflected upon rather than the type of that instance. From the language reference for Mirror:
Mirror
Representation of the sub-structure and optional “display style” of any arbitrary subject instance.
Overview
Describes the parts—such as stored properties, collection elements, tuple elements, or the active enumeration case—that make up
a particular instance. May also supply a “display style” property that
suggests how this structure might be rendered.
You can implement a custom mirror for you Message type by conforming to the CustomReflectable protocol. Implementing a custom mirror with the single purpose of listing available initializers, however, would still require manually supplying the initializer's information to the implementation of the custom mirror.
E.g.:
extension Message: CustomReflectable {
var customMirror: Mirror {
let children = DictionaryLiteral<String, Any>(dictionaryLiteral:
("init(text:)", type(of: Message.init(text:))),
("init(text:type:)", type(of: Message.init(text:type:))))
return Mirror.init(Message.self, children: children,
displayStyle: .class)
}
}
// using your custom mirror
let myMessage = Message(text: "foo")
for case (let label?, let value) in Mirror(reflecting: myMessage).children {
print("\(label), \(value)")
} /* init(text:), (String) -> Message
init(text:type:), ((String, MessageType)) -> Message */
This manual implementation requirement possibly defeats the very purpose of the exercise though. Note also that reflection must still be performed upon an instance rather than the type itself (so possibly it's easier to simply implement a dictionary describing the initializers directly as a static type property; but the manual form of this implementation defeats much of its value).
Related
I am trying to practice "class with generic". I encountered 2 errors:
Generic parameter 'T' could not be inferred
Reference to generic type 'GenericObject' requires arguments in <...>
The 2 errors in GenericManager class. Please reference the following code. How do I solve this issue?
class User {
var name: String
init(name: String) {
self.name = name
}
}
class Employee {
var name: String
var position: String
init(name: String, position: String) {
self.name = name
self.position = position
}
}
class GenericObject<T> {
var items = [T]()
init(forType: T.Type) {}
func addObject(_ obj: T) {
self.items.append(obj)
}
}
class GenericManager {
//issue: Generic parameter 'T' could not be inferred
var objects = [GenericObject]()
//issue: Reference to generic type 'GenericObject' requires arguments in <...>
func addObject(_ obj: GenericObject) {
self.objects.append(obj)
}
}
let u = User(name: "User")
let uo = GenericObject(forType: User.self)
uo.addObject(u)
let e = Employee(name: "Employee", position: "session manager")
let eo = GenericObject(forType: Employee.self)
eo.addObject(e)
let manager = GenericManager()
manager.addObject(uo)
manager.addObject(eo)
The compiler needs to know the type of T, and in this case you haven't supplied it.
You can do it like this:
var objects = [GenericObject<YourTypeHere>]()
For example, if GenericObject will hold an array of Int, it would look like this:
var objects = [GenericObject<Int>]()
I noticed you updated your question. It would be helpful to know what you're trying to achieve, but I'll try to help you anyway.
When you have a generic object, you need to tell the compiler the type of the generic at compile time, that's why it's complaining that the type can't be inferred, it needs to know.
Since you want to be able to add objects to the GenericManager array, you need the generic in those two cases to be the same, so you can modify your class like this:
class GenericManager<T> {
var objects = [GenericObject<T>]()
func addObject(_ obj: GenericObject<T>) {
self.objects.append(obj)
}
}
However, since the objects have to be of the same generic, you can't add a GenericObject<User> and GenericObject<Employee> to the same manager, what you can do is to implement those as GenericObject<Any>, and do the same with the GenericManager, then it will look like this:
let u = User(name: "User")
let uo = GenericObject(forType: Any.self)
uo.addObject(u)
let e = Employee(name: "Employee", position: "session manager")
let eo = GenericObject(forType: Any.self)
eo.addObject(e)
let manager = GenericManager<Any>()
manager.addObject(uo)
manager.addObject(eo)
Keep in mind that this will lose you any advantage that generics would do, what you could do is to create a protocol or common superclass and use that instead of Any, but that depends on what you're trying to achieve.
If you have any further questions, please add a comment instead of silently updating your question.
The problem you are having is that you are trying to use generics, but want to ignore that in GenericManager and store references to objects of different types.
Consider this - when you call manager.objects[0] what would you expect to be returned?
You can solve this by type-erasure using Any as EmilioPelaez suggested. However this is often a codesmell which leads to casting hacks throughout your code.
One alternative would be to use an enum to specify the different types of data you want to represent:
enum GenericObject {
case users([User])
case employees([Employee])
}
...
let uo = GenericObject.users([ u ])
...
let eo = GenericObject.employees([ e ])
Now when you access the properties inside GenericManager you would be required to switch over the different supported types, and when you add a new type you would be required to implement code whenever you use a GenericObject
Let's say we have simple enum with message types:
enum MessageType {
case audio
case photo
case text
}
There is Handler class which handles messages with specific types only:
class Handler {
let allowed: [MessageType]
init(_ allowed: [MessageType]) { self.allowed = allowed }
func canHandle(_ messageType: MessageType) -> Bool {
return allowed.contains(messageType)
}
}
Basic usage example:
let handler = Handler([.audio, .photo])
print(handler.canHandle(.text)) // Prints false
I want to upgrade my MessageType and add associated value for some of message types.
class Audio {}
enum MessageType {
case audio(Audio)
case photo
case text
}
Problem is that I can't store enum's pattern in allowed array for the future check in canHandle:
// error: '_' can only appear in a pattern or on the left side of an assignment
let handler = Handler([.audio(_), .photo])
Is it possible to resolve such case in a "clean" way?
It's not possible to modify MessageType because it's in third-party library (e.g. making arguments optional and passing nil)
It's not possible to init MessageType.audio(Audio) with fake Audio because it could have private initializer
I hope to avoid switch, case let or other hard-coded checks in canHandle
Any suggestions? Thanks
If instantiating the enum with a default (or garbage) value isn't a big deal
enum MessageType {
case audio(String)
case photo
case text
}
protocol SneakyEquatableMessage {
func equals(message: MessageType) -> Bool
}
extension MessageType: SneakyEquatableMessage {
func equals(message: MessageType) -> Bool {
switch (self, message) {
case (.audio(_), .audio(_)),
(.photo, .photo),
(.text, .text):
return true
default:
return false
}
}
}
class Handler {
let allowed: [MessageType]
init(_ allowed: [MessageType]) { self.allowed = allowed }
func canHandle(_ messageType: MessageType) -> Bool {
return allowed.contains { $0.equals(message: messageType) }
}
}
Basic Usage
let handler = Handler([.audio(""), .photo])
print(handler.canHandle(.text)) // Prints false
print(handler.canHandle(.audio("abc")) //Prints true
Default (or garbage) values are unrealistic
This particular section is more specific in this context, but ultimately you're going to have breakdown your enum somehow as of Swift 4. So this is my suggestion: Dependency Injection of the Factory Pattern inside Handler. This solves all of your problems pretty cleanly without having to touch switch within Handler or optionals.
enum DisassembledMessage {
case audio
case photo
case text
}
protocol MessageTypeFactory {
func disassemble(message: MessageType) -> DisassembledMessage
func disassemble(messages: [MessageType]) -> [DisassembledMessage]
}
class Handler {
let allowed: [MessageType]
let factory: MessageTypeFactory
init(allowed: [MessageType], with factory: MessageTypeFactory) {
self.allowed = allowed
self.factory = factory
}
func canHandle(_ messageType: DisassembledMessage) -> Bool {
return factory
.disassemble(messages: allowed)
.contains { $0 == messageType }
}
}
Basic Usage
let audioValue: Audio = //...
let audioMessage = MessageType.audio(audioValue)
let factory: MessageTypeFactory = //...
let handler = Handler(allowed: [audioMessage, .photo], with: factory)
print(handler.canHandle(.text)) // Prints false
print(handler.canHandle(factory.disassemble(message: audioMessage))) //Prints true
You may be asking: wait... you just created another enum (also this is just an example, you could convert it to whatever you want in that protocol). Well I say: the enum you're using is from a library... see my notes section. Also, you can now use that factory anywhere you need to break down the library type to something, including inside Handler. You could easily expand the protocol MessageTypeFactory to convert your enum to other types (hopefully behaviors) you've created, and basically just distance yourself from the library type when you need to. I hope this helps clarify what I was getting at! I don't even think you should store MessageType in your class. You should store your own type which is some kind of mapped version of MessageType, like DisassembledType.
I hope this helps!
Notes
A few things:
I'm sorry your soul is owned by a library, really.
Use the Adapter Pattern. Clean C++ is one of many places you
can learn about it. Don't pollute your entire code base with one of
their types! That's just a tip.
I know you said you didn't want to use a switch... but you're working with enums... At least it's within the enum!
Use your own types! (Did I say that?)
Use the Adapter Pattern! (Stop it.)
Is there anyway to use conversion using a variable? I am using object stacking using type of "AnyObject" and I've been able to take the class type and populate a variable. Now I need to populate an array using conversion.
var myString = "Hello World"
var objectStack = [AnyObject]()
objectStack.append(myString)
let currentObject = String(describing: objectStack.last!)
var objectType = String()
let range: Range<String.Index> = currentObject.range(of: ":")!
objectType = currentObject.substring(to: range.lowerBound)
let range2: Range<String.Index> = objectType.range(of: ".")!
objectType = objectType.substring(from: range2.upperBound)
The code above will evaluate the class and set the value of "objectType" to "String". Now I'm trying to go the other way. Something like this:
for obj in objectStack{
obj = newObject as! objectType //this doesn't work
}
Is something like this possible?
There is a simpler, safer way to get the type:
let type = type(of: objectStack.last!) // String.Type
let typeString = String(describing: type) // "String"
The other way around is not possible because the type of the object is not known at compile time. Do you have a number of known types you want to try to cast to? In that case, use optional binding to check if the cast is successful:
let object = objectStack.last!
if let string = object as? String {
// do String stuff
}
else if let i = object as? Int {
// do Int stuff
}
// and so on
If you have a large number of possible types that share some common functionality: Use Protocols. See Swift Documentation for a nice introduction.
You define a protocol for some common functionality that different types can implement:
protocol Stackable {
func doStuff()
// (more methods or properties if necessary)
}
The protocol provides a contract that all types conforming to this protocol have to fulfill by providing implementations for all declared methods and properties. Let's create a struct that conforms to Stackable:
struct Foo: Stackable {
func doStuff() {
print("Foo is doing stuff.")
}
}
You can also extend existing types to make them conform to a protocol. Let's make String Stackable:
extension String: Stackable {
func doStuff() {
print("'\(self)' is pretending to do stuff.")
}
}
Let's try it out:
let stack: [Stackable] = [Foo(), "Cat"]
for item in stack {
item.doStuff()
}
/*
prints the following:
Foo is doing stuff.
'Cat' is pretending to do stuff.
*/
This worked for me:
var instance: AnyObject! = nil
let classInst = NSClassFromString(objectType) as! NSObject.Type
instance = classInst.init()
Consider the following code:
protocol JSONParserType {
associatedtype Element
}
// MARK: - Entities
struct Item {}
// MARK: - Parsers
struct OuterParser<T: JSONParserType where T.Element == Item>: JSONParserType {
typealias Element = Item
let innerParser: T
init(innerParser: T = InnerParser()) {
self.innerParser = innerParser
}
}
struct InnerParser: JSONParserType {
typealias Element = Item
}
The OuterParser has a child parser that should be constrained to a specific type. Unfortunately providing a default value in the initializer (or in the property definition itself) does lead to the compiler throwing a "Default argument value of type 'InnerParser' cannot be converted to type 'T'".
If I remove the default value assignment and just instantiate the OuterParser providing the InnerParser explicitly, everything is fine.
let outerParser = OuterParser(innerParser: InnerParser())
My question is what's the reason that the approach providing a default value that actually meets the constraints does not work.
The problem is that the actual type of T isn't defined by the class – it's defined by the code that uses the class. It will therefore be defined before you do anything in your class (at either instance or static level). You therefore can't assign InnerParser to T, as T has already been defined to be a given type by that point, which may well not be InnerParser.
For example, let's consider that you have another parser struct:
struct AnotherParser: JSONParserType {
typealias Element = Item
}
and let's assume that your current code compiles. Now consider what would happen when you do this:
let parser = OuterParser<AnotherParser>()
You've defined the generic type to be AnotherParser – but the initialiser will try to assign InnerParser to your property (now of type AnotherParser). These types don't match, therefore it cannot possibly work.
Following the same logic, this implementation also won't work:
struct OuterParser<T: JSONParserType where T.Element == Item>: JSONParserType {
typealias Element = Item
let innerParser: T
init() {
self.innerParser = InnerParser()
}
init(innerParser: T) {
self.innerParser = innerParser
}
}
As there's no guarantee that the generic type T will be the same type as InnerParser. Sure, you can force downcast to T – but that'll just make you code crash if the types aren't compatible.
Unfortunately, there's no real clean solution to this problem. I think the best your best option is probably to create two factory methods for creating your OuterParser instance.
enum Parser {
static func createParser() -> OuterParser<InnerParser> {
return OuterParser(innerParser:InnerParser())
}
static func createParser<T>(innerParser:T) -> OuterParser<T> {
return OuterParser(innerParser:innerParser)
}
}
let innerParser = Parser.createParser() // OuterParser<InnerParser>
let anotherParser = Parser.createParser(AnotherParser()) // OuterParser<AnotherParser>
We're using an caseless enum here to avoid polluting the global namespace with extra functions.
Although this isn't very Swifty, and for that reason I would also recommend maybe rethinking your logic for how you define your parsers.
type T more like a child protocol of JSONParserType you can convert it:
init(innerParser: T = InnerParser() as! T) {
self.innerParser = innerParser
}
One of the things that bugs me about Swift and Cocoa together is working with NSUserDefaults, because there is no type information and it is always necessary to cast the result of objectForKey to what you are expecting to get. It is unsafe and impractical. I decided to tackle this problem, making NSUserDefaults more practical in Swift-land, and hopefully learning something along the way. Here were my goals in the beginning:
Complete type safety: each key has one type associated with it. When setting a value, only a value of that type should be accepted and when getting a value the result should come out with the correct type
Global list of keys which are clear in meaning and content. The list should be easy to create, modify and extend
Clean syntax, using subscripts if possible. For example, this would
be perfect:
3.1. set: UserDefaults[.MyKey] = value
3.2. get: let value = UserDefaults[.MyKey]
Support for classes that conform to the NSCoding protocol by
automatically [un]archiving them
Support for all property list types accepted by NSUserDefaults
I started by creating this generic struct:
struct UDKey <T> {
init(_ n: String) { name = n }
let name: String
}
Then I created this other struct that serves as a container for all the keys in an application:
struct UDKeys {}
This can then be extended to add keys wherever needed:
extension UDKeys {
static let MyKey1 = UDKey<Int>("MyKey1")
static let MyKey2 = UDKey<[String]>("MyKey2")
}
Note how each key has a type associated with it. It represents the type of the information to be saved. Also, the name property is the string that is to be used as a key for NSUserDefaults.
The keys can be listed all in one constants file, or added using extensions on a per-file basis close to where they are being used for storing data.
Then I created an "UserDefaults" class responsible for handling the getting/setting of information:
class UserDefaultsClass {
let storage = NSUserDefaults.standardUserDefaults()
init(storage: NSUserDefaults) { self.storage = storage }
init() {}
// ...
}
let UserDefaults = UserDefaultsClass() // or UserDefaultsClass(storage: ...) for further customisation
The idea is that one instance for a particular domain is created and then every method is accessed in this way:
let value = UserDefaults.myMethod(...)
I prefer this approach to things like UserDefaults.sharedInstance.myMethod(...) (too long!) or using class methods for everything. Also, this allows interacting with various domains at the same time by using more than one UserDefaultsClass with different storage values.
So far, items 1 and 2 have been taken care of, but now the difficult part is starting: how to actually design the methods on UserDefaultsClass in order to comply with the rest.
For example, let's start with item 4. First I tried this (this code is inside UserDefaultsClass):
subscript<T: NSCoding>(key: UDKey<T>) -> T? {
set { storage.setObject(NSKeyedArchiver.archivedDataWithRootObject(newValue), forKey: key.name) }
get {
if let data = storage.objectForKey(key.name) as? NSData {
return NSKeyedUnarchiver.unarchiveObjectWithData(data) as? T
} else { return nil }
}
}
But then I find out that Swift doesn't allow generic subscripts!! Alright, then I guess I'll have to use functions then. There goes half of item 3...
func set <T: NSCoding>(key: UDKey<T>, _ value: T) {
storage.setObject(NSKeyedArchiver.archivedDataWithRootObject(value), forKey: key.name)
}
func get <T: NSCoding>(key: UDKey<T>) -> T? {
if let data = storage.objectForKey(key.name) as? NSData {
return NSKeyedUnarchiver.unarchiveObjectWithData(data) as? T
} else { return nil }
}
And that works just fine:
extension UDKeys { static let MyKey = UDKey<NSNotification>("MyKey") }
UserDefaults.set(UDKeys.MyKey, NSNotification(name: "Hello!", object: nil))
let n = UserDefaults.get(UDKeys.MyKey)
Note how I can't call UserDefaults.get(.MyKey). I have to use UDKeys.MyKey. And I can't do that because it's not yet possible to have static variables on a generic struct!!
Next, let's try number 5. Now that has been an headache and that's where I need lots of help.
Property list types are, as per the docs:
A default object must be a property list, that is, an instance of (or
for collections a combination of instances of): NSData, NSString,
NSNumber, NSDate, NSArray, or NSDictionary.
That in Swift means Int, [Int], [[String:Bool]], [[String:[Double]]], etc are all property list types. At first I thought that I could just write this and trust whoever is using this code to remember that only plist types are allowed:
func set <T: AnyObject>(key: UDKey<T>, _ value: T) {
storage.setObject(value, forKey: key.name)
}
func get <T: AnyObject>(key: UDKey<T>) -> T? {
return storage.objectForKey(key.name) as? T
}
But as you'll notice, while this works fine:
extension UDKeys { static let MyKey = UDKey<NSData>("MyKey") }
UserDefaults.set(UDKeys.MyKey, NSData())
let d = UserDefaults.get(UDKeys.MyKey)
This doesn't:
extension UDKeys { static let MyKey = UDKey<[NSData]>("MyKey") }
UserDefaults.set(UDKeys.MyKey, [NSData()])
And this doesn't either:
extension UDKeys { static let MyKey = UDKey<[Int]>("MyKey") }
UserDefaults.set(UDKeys.MyKey, [0])
Not even this:
extension UDKeys { static let MyKey = UDKey<Int>("MyKey") }
UserDefaults.set(UDKeys.MyKey, 1)
The problem is that they are all valid property list types yet Swift obviously interprets arrays and ints as structs, not as their Objective-C class counterparts. However:
func set <T: Any>(key: UDKey<T>, _ value: T)
won't work either, because then any value type, not just the ones that have a class cousin courtesy of Obj-C, is accepted, and storage.setObject(value, forKey: key.name) is no longer valid because value has to be a reference type.
If a protocol existed in Swift that accepted any reference type and any value type that can be converted to a reference type in objective-c (like [Int] and the other examples I mention) this problem would be solved:
func set <T: AnyObjectiveCObject>(key: UDKey<T>, _ value: T) {
storage.setObject(value, forKey: key.name)
}
func get <T: AnyObjectiveCObject>(key: UDKey<T>) -> T? {
return storage.objectForKey(key.name) as? T
}
AnyObjectiveCObject would accept any swift classes and swift arrays, dictionaries, numbers (ints, floats, bools, etc that convert to NSNumber), strings...
Unfortunately, AFAIK this doesn't exist.
Question:
How can I have write a generic function (or collection of overloaded generic functions) whose generic type T can be any reference type or any value type that Swift can convert to a reference type in Objective-C?
Solved: With the help of the answers I got, I arrived at what I wanted. In case anyone wants to take a look at my solution, here it is.
I don't mean to brag but ... oh who am I kidding, I totally do!
Preferences.set([NSData()], forKey: "MyKey1")
Preferences.get("MyKey1", type: type([NSData]))
Preferences.get("MyKey1") as [NSData]?
func crunch1(value: [NSData])
{
println("Om nom 1!")
}
crunch1(Preferences.get("MyKey1")!)
Preferences.set(NSArray(object: NSData()), forKey: "MyKey2")
Preferences.get("MyKey2", type: type(NSArray))
Preferences.get("MyKey2") as NSArray?
func crunch2(value: NSArray)
{
println("Om nom 2!")
}
crunch2(Preferences.get("MyKey2")!)
Preferences.set([[String:[Int]]](), forKey: "MyKey3")
Preferences.get("MyKey3", type: type([[String:[Int]]]))
Preferences.get("MyKey3") as [[String:[Int]]]?
func crunch3(value: [[String:[Int]]])
{
println("Om nom 3!")
}
crunch3(Preferences.get("MyKey3")!)
I'd like to introduce my idea. (Sorry for my poor English in advance.)
let plainKey = UDKey("Message", string)
let mixedKey
= UDKey("Mixed"
, array(dictionary(
string, tuple(
array(integer),
optional(date)))))
let ud = UserDefaults(NSUserDefaults.standardUserDefaults())
ud.set(plainKey, "Hello")
ud.set(plainKey, 2525) // <-- compile error
ud.set(mixedKey, [ [ "(^_^;)": ([1, 2, 3], .Some(NSDate()))] ])
ud.set(mixedKey, [ [ "(^_^;)": ([1, 2, 3], .Some(NSData()))] ]) // <-- compile error
The only difference is that UDKey() now requires #2 argument, a value of BiMap class. I've uncoupled the work originally of UDKey into BiMap which converts a value of a type to/from a value of another type.
public class BiMap<A, B> {
public func AtoB(a: A) -> B?
public func BtoA(b: B) -> A?
}
Consequently, types that set/get can accepts are conducted by BiMap, and no longer limited to types as can automatically cast
from/to AnyObject (more specifically, types NSUserDefaults can accepts.).
Because BiMap is a generic class, you can easily create subtypes of that, interchanging arbitrary two types you want.
Here is full source code. (But there are bugs yet to be fixed..)
https://gist.github.com/hisui/47f170a9e193168dc946