CODEWITHSUNDEEP
swiftgenerics
Pop Flamingo
Pop Flamingo

Reputation: 2191

Use Swift `is` to check type of generic type

Lets say that I have a variable of type Any, and I would like to know wether this is an array or not, here is what I would like to do: 

if myVariable is Array { /* Do what I want */ }

But Swift requires to give the generic type of the array such as : 

if myVariable is Array<Int> { }

But I don't want to check the generic type, I just want to know wether this is an array or not, I tried : 

if myVariable is Array<Any> { }

Hoping that it would match every type of array, but that doesn't work either... (it doesn't match arrays of all types, so if my variable is an Int array, this code doesn't get called for instance)

What should I do ?

Thank you.

Edit with example of an approach solution that doesn't seem to work: 

struct Foo<T> {}

struct Bar {
    var property = Foo<String>()
}

var test = Bar()

let mirror = Mirror(reflecting: test)

// This code is trying to count the number of properties of type Foo
var inputCount = 0
for child in mirror.children {
    print(String(describing: type(of: child))) // Prints "(Optional<String>, Any)"
    if String(describing: type(of: child)) == "Foo" {
        inputCount += 1 // Never called
    }
}

print(inputCount) // "0"

Upvotes: 17

Views: 8169

Answers (3)

Aleks N.
Aleks N.

Reputation: 6247

In Java you would want to use Array<?>, in Swift it's not possible, but you can emulate it.

  1. Create a protocol protocol AnyArray {}

  2. Now let all arrays implement this protocol: extension Array: AnyArray {}

  3. Now you can easily do what you intended to do:

    if myVariable is AnyArray { ... }

Upvotes: 0

Leslie Godwin
Leslie Godwin

Reputation: 2661

This is how to test a generic type parameter for conformance:

let conforms = T.self is MyProtocol.Type

See this post: Swift: check if generic type conforms to protocol

Upvotes: 7

vacawama
vacawama

Reputation: 154513

Here's 2 things that might work for you.

Option 1:

Note that child is a tuple containing a String? with the name of the property ("property" in your example) and the item. So you need to look at child.1.

In this case, you should be checking:

if String(describing: type(of: child.1)).hasPrefix("Foo<")

Option 2:

If you create a protocol FooProtocol that is implemented by Foo<T>, you could check if child.1 is FooProtocol:

protocol FooProtocol { }

struct Foo<T>: FooProtocol {}

struct Bar {
    var property = Foo<String>()
}

var test = Bar()

let mirror = Mirror(reflecting: test)

// This code is trying to count the number of properties of type Foo
var inputCount = 0
for child in mirror.children {
    if child.1 is FooProtocol {
        inputCount += 1
    }
}

Upvotes: 6

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