I have two classes ClassOne and ClassTwo. I want to initialize a different one depending on a variable value, i want to do something like:
if(a == "0") {
let b = ClassOne();
}else{
let b = ClassTwo();
}
without having to write it everytime I need it. Something like:
let b = MainClass()
and gets called ClassOne() or ClassTwo() depending on the case, a is a global variable.
In order for this to work the two types should be related either by extending a common base class or by implementing the same protocol. Further, subsequent operations on b would be restricted to these the two classes have in common.
If you are fine with that restriction, you can do it like this:
protocol CommonProtocol {
func foo() -> Double
var bar : Int { get }
}
class ClassOne : CommonProtocol {
...
}
class ClassTwo : CommonProtocol {
...
}
func MainClass() -> CommonProtocol {
if(a == "0") {
return ClassOne()
} else {
return ClassTwo()
}
}
...
let b = MainClass()
b.foo()
print(b.bar)
Note: You could forego all of the above in favor of a completely dynamic approach by following matt's advise.
You can do it, but it isn't going to be useful without further effort. Consider the following:
class ClassOne {}
class ClassTwo {}
Now we proceed to initialize a variable as an instance of one of these classes. To do so, we must type the variable as AnyObject:
var which : Bool { return true /* or false */}
let obj : AnyObject
switch which {
case true:
obj = ClassOne()
case false:
obj = ClassTwo()
}
You now have obj as either a ClassOne instance or a ClassTwo instance. But there's a problem. You don't know which it is. The AnyObject typing preserves the real underlying type (polymorphism), but it also hides the type. Every time you use obj, you will have to test whether it is a ClassOne or a ClassTwo and cast it to that type in order to use it.
if obj is ClassOne {
(obj as! ClassOne).doSomethingClassOneKnowsHowToDo()
}
The question is: is the pain worth the gain? I would suggest that your desire to do this in the first place is probably a Bad Smell and you should revise your intended architecture. Strict static typing is the point of Swift; you are wrong to want to throw it away.
You could use the ternary operator to do it quickly, however you do need to do it every time:
let b = (a == 0) ? ClassOne() : ClassTwo() //If a==0 let b = ClassOne if not let b= ClassTwo.
#dasblinkenlight solution is great but you can also do like this
protocol MyProto {
var x: Int { get }
}
class A: MyProto {
var x = 10
var y = 10
}
class B: MyProto {
var x = 20
}
class Demo {
var type: MyProto!
init(str: String) {
if str == "0" {
type = A()
} else {
type = B()
}
}
}
....
let obj = Demo(str: "0").type
print(obj.x)
Related
I am attempting to declare a linked list in Swift, with a finger type that is a reference to either a node, allowing to insert or remove beyond that node, or to the linked list itself, in which case inserting or removing at the top of the linked list.
I want to see if this can be made uniform down to the implementation, instead of having to special-case everything: Swift is object-oriented, after all.
I previously had a version which required forced casts, but again I'd like to see if this can be made to work without them (e.g. even if they never end up faulting they still imply runtime checks each time).
I currently have this code:
protocol ContainerNodeInterface: class {
associatedtype ContainedItem;
var contents: ContainedItem { get };
}
protocol ParentNodeInterface: class {
associatedtype LinkedItem: ContainerNodeInterface;
var next: LinkedItem? {get set};
}
class ProtoNode<Contents, NodeType: ParentNodeInterface>: ParentNodeInterface where NodeType.ContainedItem==Contents, NodeType.LinkedItem==NodeType { // not meant to be instantiated or directly referenced
typealias LinkedItem = NodeType;
var next: NodeType?;
init() {
next = nil;
}
final func insertThisAfterMe(_ node: NodeType) {
node.next = next;
next = .some(node);
}
final func removeNodeAfterMe() -> NodeType? {
guard let nextNode = next else {
return nil;
}
let result = nextNode;
next = result.next;
result.next = nil;
return nextNode;
}
}
class Node<Contents>: ProtoNode<Contents, Node<Contents>>, ContainerNodeInterface {
typealias ContainedItem = Contents;
typealias NextItem = Node<Contents>;
var contents: Contents;
init(withContents: Contents) {
contents = withContents;
super.init();
}
}
typealias ParentNode<Contents> = ProtoNode<Contents, Node<Contents>>;
But the Swift compiler, via Xcode, is complaining that Type 'Node<Contents>' does not conform to protocol 'ParentNodeInterface'. This makes no sense! And if I add explicit conformance to ParentNodeInterface to Node, then I get simultaneously that error and one of redundant conformance to the same protocol.
What is missing here?
Xcode Version 10.2 (10E125), Swift 5
I resolved it by splitting ProtoNode into an initial declaration and an extension:
protocol ContainerNodeInterface: class {
associatedtype ContainedItem;
var contents: ContainedItem { get };
}
protocol ParentNodeInterface: class {
associatedtype LinkedItem: ContainerNodeInterface;
var next: LinkedItem? {get set};
}
class ProtoNode<Contents, NodeType: ContainerNodeInterface>: ParentNodeInterface where NodeType.ContainedItem==Contents { // not meant to be instantiated or directly referenced
typealias LinkedItem = NodeType;
var next: NodeType?;
init() {
next = nil;
}
}
extension ProtoNode where NodeType: ParentNodeInterface, NodeType.LinkedItem==NodeType
{
final func insertThisAfterMe(_ node: NodeType) {
node.next = next;
next = .some(node);
}
final func removeNodeAfterMe() -> NodeType? {
guard let nextNode = next else {
return nil;
}
let result = nextNode;
next = result.next;
result.next = nil;
return nextNode;
}
}
class Node<Contents>: ProtoNode<Contents, Node<Contents>>, ContainerNodeInterface {
typealias ContainedItem = Contents;
typealias NextItem = Node<Contents>;
var contents: Contents;
init(withContents: Contents) {
contents = withContents;
super.init();
}
}
typealias ParentNode<Contents> = ProtoNode<Contents, Node<Contents>>;
I figure it helps the compiler break the dependency loop, where it has to determine whether Node, as a generic parameter, conforms to the protocol before it can determine the declaration is valid and consider the declared type, i.e. Node, as conforming to the protocol, but still it feels a bit silly for me to have to make this seemingly pointless extension declaration.
At the very least, the compiler could be slightly more helpful…
First, I would start with a simple linked-list Node type:
final class Node<Value> {
let value: Value
var next: Node<Value>?
init(_ value: Value) {
self.value = value
}
func insert(_ node: Node<Value>) {
node.next = next
next = node
}
func removeNext() -> Node<Value>? {
guard let removedNode = next else { return nil }
next = removedNode.next
removedNode.next = nil
return removedNode
}
}
Then, you can add the concept that you describe: a pointer to "either a node...or to the linked list itself." When you see "or" in a description, that implies a sum type, which in Swift is an enum, either a pointer to the head of a (possibly empty) list, or a pointer to a node. Each has slightly different behaviors, which you manage with switch.
enum NodePointer<Value> {
case head(Node<Value>?)
case node(Node<Value>)
mutating func insert(_ node: Node<Value>) {
switch self {
case .head(let n):
self = .head(node)
node.next = n
case .node(let n):
n.insert(node)
}
}
mutating func removeNext() -> Node<Value>? {
switch self {
case .head(let n):
self = .head(n?.next)
return n
case .node(let n):
return n.removeNext()
}
}
var pointee: Node<Value>? {
switch self {
case .head(let n): return n
case .node(let n): return n
}
}
}
With that you would have an interface like:
var list = Node(1)
list.insert(Node(2))
var ptr = NodePointer.head(list)
ptr.insert(Node(1))
ptr.pointee?.next?.next?.value // 2
Note that the specific problem you ran into (that the compiler couldn't work out the conformance) I believe is a compiler bug, though I also believe it's one that's fixed on master currently. I haven't tested that out though. But I don't believe the protocol-based approach is correct for this problem.
In order to reduce cut-and-paste code in this app, I'm trying to pass class names around in order to tell a method which way it should process some data. I have something like the following:
class MyClass : NSObject {
var name : String = ""
}
class OneClass : MyClass {
override init() {
super.init()
self.name = "One"
}
}
class TwoClass : MyClass {
override init() {
super.init()
self.name = "Two"
}
}
class Thing : NSObject {
func doStuff(withClass cls: AnyClass) -> String {
let x = cls.init()
return x.name
}
}
let z = Thing()
print(z.doStuff(withClass: OneClass))
print(z.doStuff(withClass: TwoClass))
Passing withClass cls: AnyClass the parser pushed me to change let x = cls() to let x = cls.init(). But I've got an Expected member name or constructor call after type name error for the last two lines. The recommended fixes both cause other problems.
The first suggestion, adding the () constructor after the class name, causes new errors on those lines: Cannot convert value of type 'OneClass' to expected argument type 'AnyClass' (aka 'AnyObject.Type')
Taking the second suggestion and changing them to OneClass.self and TwoClass.self gets rid of the parser errors, but when I execute the code it just runs forever.. never erroring out, and never completing.
I found a recommendation elsewhere that suggests I should change the Thing.doStuff() parameters to expect MyClass instead of AnyClass, but that causes another set of new problems.
First, the parser starts complaining about the cls.init() call, and the series of fixes it suggests eventually lead to something that makes no sense: let x = cls.type(of:;; init)(). The parser ends up in a suggestion loop where it keeps adding more semi-colons in the middle of the statement.
Second, I'm back to type mismatch errors on the calls to doStuff() in the last two lines: Cannot convert value of type 'OneClass.Type' to expected argument type 'MyClass'.
There's obviously something I'm not getting here about passing types as arguments, but none of the googling I've done has landed me on something that explains the problems I'm seeing.
How about the generic Swift way.
The code constrains the generic type T to MyClass since it must have a name property.
class MyClass : NSObject {
var name : String
override required init() {
self.name = ""
super.init()
}
}
class OneClass : MyClass {
required init() {
super.init()
self.name = "One"
}
}
class TwoClass : MyClass {
required init() {
super.init()
self.name = "Two"
}
}
class Thing : NSObject {
func doStuff<T : MyClass>(withClass cls: T.Type) -> String {
let x = cls.init()
return x.name
}
}
let z = Thing()
print(z.doStuff(withClass: OneClass.self))
print(z.doStuff(withClass: TwoClass.self))
Or use a protocol.
protocol Nameable {
var name : String { get }
init()
}
class MyClass : NSObject, Nameable { ...
...
class Thing : NSObject {
func doStuff<T : Nameable>(withClass cls: T.Type) -> String {
let x = cls.init()
return x.name
}
}
To get this working, you must call init on cls after typecasting it to NSObject.Type. Also, x.name only works if cls Class type contains that particular property. This is the reason x is then typecasted to MyClass.
class Thing : NSObject
{
func doStuff(withClass cls: AnyClass) -> String?
{
let x = (cls as? NSObject.Type)?.init()
if let x = x as? MyClass
{
return x.name
}
return nil
}
}
Call doStuff with ClassType.self
print(z.doStuff(withClass: OneClass.self))
print(z.doStuff(withClass: TwoClass.self))
Let me know if you still face any issues.
Is it possible to get the object type from an optional?
For example, if I have a class that has a property that is an optional string, can I somehow just get back the string type?
The exact use case I have is I have many custom classes all of which have a property that is storing another custom class as an optional value. I would like to write a generic function that will create an instance of the object class stored in the optional.
Here is an example of what I am looking for, although .dynamicType does not work since it is an optional:
class Class1 {
}
class Class2 {
var myOp: Class1?
}
var c = Class2()
c.myOp = c.myOp.dynamicType()
Since you wanted to use this with Generics I tried it for you. It works, but it may not be so useful.
First some setup:
This is a helper protocol to make sure our Generic type will have a known init method.
protocol ZeroParameterInit {
init()
}
This is an extension to get the type from an optional:
extension Optional {
var dynamicWrappedType : Wrapped.Type {
return Wrapped.self
}
}
Implemented in your code:
class Class1 : ZeroParameterInit {
required init() {}
}
class Class2 {
var myOp: Class1?
}
var c = Class2()
c.myOp = c.myOp.dynamicWrappedType.init()
Generic implementation:
class Class1 : ZeroParameterInit {
required init() {}
}
class Class2<T where T : ZeroParameterInit> {
var attribute: Optional<T>// used long syntax to remind you of : Optional<Wrapped>
init(attr:T) {
attribute = attr
attribute = nil
}
}
The function to create the instance:
func myFunc<T>(instance: Class2<T>) -> T {
return instance.attribute.dynamicWrappedType.init()
}
Some tests:
let alpha = Class1()
let beta = Class2(attr: alpha)
beta.attribute = myFunc(beta)
The issue:
You can't create an instance of Class2 without informing it about the type of it's generic attribute. So you need to pass it some object/type and that complicates things again.
Some extra methods that might improve how it all works:
init() {
}
let delta = Class2<Class1>()
delta.attribute = myFunc(delta)
init(type:T.Type) {
}
let epsilon = Class2(type: Class1.self)
epsilon.attribute = myFunc(epsilon)
You just need to check if the optional exist:
func myFunc(c: Class2) -> Class1? {
if let c1 = c.myOp{
return c1.dynamicType()
}
return nil
}
OR
func myFunc(c: Class2) -> Class1? {
if c.myOp != nil{
return c.myOp!.dynamicType()
}
return nil
}
Note the your return type need to be optional as well.
Tried this in simulator, seems like doing the right thing, if I understood you
class Class1 {
}
class Class2 {
var myOp: Class1?
}
func myFunc(c: Class2) -> AnyObject {
if let c1 = c.myOp{
return c1.self
}
return c
}
var object = Class2()
object.myOp = Class1()
myFunc(object) // Class1
I have some generic type class but no instance of object to test. What I would like to do is to alter the behavior of the function according to the runtime type.
class MyGenericUtility<SomeGenericClass> {
func myFunction() {
// so far I have tested "is", "==" and "==="
if SomeGenericClass is SomeRealClass {
println("some special stuff there")
}
println("some generic stuff as the name tells")
}
}
You can compare the class type, using SomeGenericClass.self == SomeRealClass.self as,
class MyGenericUtility<SomeGenericClass> {
func myFunction() {
if SomeGenericClass.self == SomeRealClass.self {
print("SomeRealClass stuffs")
} else if SomeGenericClass.self == String.self {
print("String stuffs")
}
}
}
let someRealUtility = MyGenericUtility<SomeRealClass>()
someRealUtility.myFunction()
let stringUtility = MyGenericUtility<String>()
stringUtility.myFunction()
Rather than testing at runtime, you should generally handle this at compile time with constrained extensions (this assumes Swift 2). Doing it this way avoids any need to do unsafe as! casting when you need to access type-specific parts of the instance.
class MyGenericUtility<SomeGenericClass> {
}
// Special handling for `SomeRealClass`
extension MyGenericUtility where SomeGenericClass: SomeRealClass {
func myFunction() {
print("SomeRealClass stuffs")
}
}
// Default handling for any unspecified class
extension MyGenericUtility {
func myFunction() {
print("Other stuffs")
}
}
let someRealUtility = MyGenericUtility<SomeRealClass>()
someRealUtility.myFunction()
let stringUtility = MyGenericUtility<String>()
stringUtility.myFunction()
Note that this is based on inheritance, not equality, so any subclass of SomeRealClass would get the SomeRealClass behavior.
You can't use the generic type directly, you need to use a property of that type when comparing with "is".
class MyGenericUtility<T> {
var a: T
func myFunction() {
if a is Int {
println("some special stuff there")
}
println("some generic stuff as the name tells")
}
init(value: T) {
a = value
}
}
let test = MyGenericUtility(value: 5)
test.myFunction()
// Output: some special stuff there
// some generic stuff as the name tells
let test2 = MyGenericUtility(value: "foo")
test2.myFunction()
// Output: some generic stuff as the name tells
This is a bit of a head banger (for me). Basically I want to have 2 different singletons that inherit from the same class. In either I want to use a certain class which itself is derived. So I have Utility and both AUtil:Utility and BUtil:Utility. And Singleton that is used as ASingleton using AUtility in its stomach and B respectively. I failed on all frontiers. The last attempt was a factory pattern which simply got Swift 1.2 to Segfault:
protocol Initializable { init() }
class A:Initializable {
var x = "A"
required init() {}
}
class B:Initializable {
var x = "B"
required init() {}
}
class C {
let t:Initializable
init(t:Initializable) {
self.t = t
println(t)
}
func factory() {
println(t.dynamicType())
}
}
As said I also tried to make the following pattern generic:
private let _SingletonSharedInstance = StaticClass()
class StaticClass {
class var sharedInstance : StaticClass {
return _SingletonSharedInstance
}
}
let s = StaticClass.sharedInstance
(This one isn't generic as you see. But all my attempts failed and so I show my starting point.)
Anyway I seem to be lost between doom and death.
Do you mean something like this?
protocol Initializable: class { init() }
private var instances = [String: Initializable]()
func singletonInstance<T: Initializable>(_ ty: T.Type = T.self) -> T {
let name = NSStringFromClass(ty)
if let o = (instances[name] as? T) {
return o
}
let o = ty()
instances[name] = o
return o
}
An use-side of it, for instance.
class Foo: Initializable { required init() {} }
class Bar: Initializable { required init() {} }
let foo1 = singletonInstance() as Foo // or `singletonInstance(Foo.self)`
let foo2 = singletonInstance() as Foo
assert(foo1 === foo2)
let bar1 = singletonInstance() as Bar
let bar2 = singletonInstance() as Bar
assert(bar1 === bar2)
(I've tested the code above and got it to work in Swift 1.2.)
Inspired by findalls implementation, I build my own singleton generator, which is a little more powerful.
You can create a singleton of any Class or Structure type in Swift. The only thing you have to do is to implement one of two different protocols to your type and use Swift 2.0 or newer.
public protocol SingletonType { init() }
private var singletonInstances = [String: SingletonType]()
extension SingletonType {
// this will crash Xcode atm. it's a Swift 2.0 beta bug. Bug-ID: 21850697
public static var singleton: Self { return singleton { $0 } }
public static func singleton(setter: (_: Self) -> Self) -> Self {
guard let instance = singletonInstances["\(self)"] as? Self else {
return setInstance(self.init(), withSetter: setter, overridable: true)
}
return setInstance(instance, withSetter: setter, overridable: false)
}
private static func setInstance(var instance: Self, withSetter setter: (_: Self) -> Self, overridable: Bool) -> Self {
instance = restoreInstanceIfNeeded(instance1: instance, instance2: setter(instance), overridable: overridable)
singletonInstances["\(self)"] = instance
return instance
}
private static func restoreInstanceIfNeeded(instance1 i1: Self, instance2 i2: Self, overridable: Bool) -> Self {
// will work if the bug in Swift 2.0 beta is fixed !!! Bug-ID: 21850627
guard i1.dynamicType is AnyClass else { return i2 }
return ((i1 as! AnyObject) !== (i2 as! AnyObject)) && !overridable ? i1 : i2
}
}
This may look a little scary, but don't be afraid of this code. The public function inside the protocol extension will create two access points for you.
For example you will be able to write code like this now:
// extend your type: as an example I will extend 'Int' here
extension Int : SingletonType {} // nothing else to do, because Int already has an 'init()' initializer by default
// let the magic happen
Int.singleton // this will generate a singleton Int with 0 as default value
Int.singleton { (_) -> Int in 100 } // should set your Int singleton to 100
Int.singleton { $0 - 55 } // your singleton should be 45 now
// I need to mention that Xcode will produce the setter like this and trow an error
Int.singleton { (yourCustomInstanceName) -> Self in // replace 'Self' with 'Int' and you should be fine
return yourCustomInstanceName
}
// btw. we just ignored the return value everywhere
print(Int.singleton) // will print 45 here
var singleton2 = Int.singleton { $0 + 5 }
singleton2 += 10
print(Int.singleton) // should print 50, because 'singleton2' is just a copy of an Int value type
class A : SingletonType {
var name = "no name"
required init() {}
}
A.singleton { $0; let i = A(); i.name = "hello world"; return i } // custom init on first singleton call for type A
print(A.singleton.name)
print(A.singleton { $0.name = "A"; return $0 }.name)
print(A.singleton.name)
// should print "hello world" and twice the string "A"
If you have any idea how to enhance this code and make it even safer, please let me know. I will push this code on GitHub (MIT License) soon, so everyone can benefit from it.
UPDATE: I modified the code a little so you can now pass a custom initialized instance of a class with the setter function when its called the first time.
UPDATE 2: I removed ClassInstance protocol and modified the private restore function. The Instance protocol is now called SingletonType. The setter function is not optional anymore. Right now Xcode 7 beta 3 will crash and provide an illegal instruction: 4 error when you will call the getter. But this is a confirmed beta bug.