Loop through Swift struct to get keys and values

Question

I'd like to loop trough every key of mystruct and print its key and its value for every property.

struct mystruct {
  var a = "11215"
  var b = "21212"
  var c = "39932"
}

func loopthrough {
    for (key, value) in mystruct {
        print("key: \(key), value: \(value)") // Type mystruct.Type does not conform to protocol 'Sequence'
    }
}

But using the few lines from above I always get this error message:

Type mystruct.Type does not conform to protocol 'Sequence'

How can I avoid getting this message?

You can not do this in swift, but may be in future as it is a proposed solution you can check it https://github.com/apple/swift-evolution/blob/master/proposals/0161-key-paths.md but you can get your solution here https://stackoverflow.com/questions/27292255/how-to-loop-over-struct-properties-in-swift — Vikky, Jan 27 '18 at 13:20
see this https://airspeedvelocity.net/2014/07/13/swift-structs-and-accessing-properties-by-name/ — Prashant Tukadiya, Jan 27 '18 at 13:22
Possible duplicate of https://stackoverflow.com/questions/24844681/list-of-classs-properties-in-swift. — Martin R, Jan 27 '18 at 13:23

Luca Angeletti · Accepted Answer · 2019-01-22T22:07:26.920

59

First of all let's use CamelCase for the struct name

struct MyStruct {
    var a = "11215"
    var b = "21212"
    var c = "39932"
}

Next we need to create a value of type MyStruct

let elm = MyStruct()

Now we can build a Mirror value based on the elm value.

let mirror = Mirror(reflecting: elm)

The Mirror value does allow us to access all the properties of elm, here's how

for child in mirror.children  {
    print("key: \(child.label), value: \(child.value)")
}

Result:

key: Optional("a"), value: 11215

key: Optional("b"), value: 21212

key: Optional("c"), value: 39932

edited Jan 22 '19 at 22:07

answered Jan 27 '18 at 13:23

Luca Angeletti

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Typo: You meant **Camel** Case – Santi Pérez Jan 22 '19 at 16:02
How can you change values ? – Hope Sep 05 '20 at 16:05
1

@SantiPérez Actually, it's title case. Camel case starts with a lower case. – Mykel Nov 02 '21 at 20:50

score 4 · Answer 2 · answered Jan 27 '18 at 13:25

use following code to get array of all the properties

protocol PropertyLoopable
{
    func allProperties() throws -> [String]
}

extension PropertyLoopable {
    func allProperties() throws -> [String] {

        var result: [String] = []

        let mirror = Mirror(reflecting: self)

        // Optional check to make sure we're iterating over a struct or class
        guard let style = mirror.displayStyle, style == .struct || style == .class else {
            throw NSError()
        }

        for (property,_) in mirror.children {
            guard let property = property else {
                continue
            }
            result.append(property)
         //   result[property] = value
        }

        return result
    }
}

Now just

let allKeys = try  self.allProperties()

Don't forgot to implement protocol

Hope it is helpful

dfrib · Answer 3 · 2018-01-27T13:36:43.327

You can use runtime introspection (on an instance of your type) combined with value-binding pattern matching to extract the property names and values; the latter used to unwrap the optional label property of the Mirror instance used to represent the sub-structure of your specific instance.

E.g.:

struct MyStruct {
    let a = "11215"
    let b = "21212"
    let c = "39932"
}

// Runtime introspection on an _instance_ of MyStruct
let m = MyStruct()
for case let (label?, value) in Mirror(reflecting: m)
    .children.map({ ($0.label, $0.value) }) {
    print("label: \(label), value: \(value)")
} /* label: a, value: 11215
     label: b, value: 21212
     label: c, value: 39932 */

Santi Pérez · Answer 4 · 2021-11-08T06:27:02.310

I hope it still helps someone: This is my version of the protocol for more complicated classes/structs (Objects within Objects within Objects ;-) ) I am sure there is a more elegant functional solution but this was a quick and dirty solution, as I only needed it for a temporary log.

protocol PropertyLoopable {
func allProperties() -> [String: Any]
}


extension PropertyLoopable {
func allProperties() -> [String: Any] {
    
    var result: [String: Any] = [:]
    let mirror = Mirror(reflecting: self)
    
    // make sure we're iterating over a struct or class
    guard let style = mirror.displayStyle, style == .struct || style == .class else {
        print("ERROR: NOT A CLASS OR STRUCT")
        return result
    }
    
    for (property, value) in mirror.children {
        guard let property = property else {
            continue
        }
        // It was a very complicated struct from a JSON with a 4 level deep structure. This is dirty dancing, remove unnecessary "for" loops for simpler structs/classes
        // if value from property is not directly a String, we need to keep iterating one level deeper
        if value is String {
            result.updateValue(value, forKey: property)
        } else {
            let mirror = Mirror(reflecting: value)
            
            for (property, value) in mirror.children {
                guard let property = property else {
                    continue
                }
                //let's go for a second level
                if value is String {
                    result.updateValue(value, forKey: property)
                } else {
                    let mirror = Mirror(reflecting: value)
                    
                    for (property, value) in mirror.children {
                        guard let property = property else {
                            continue
                        }
                        //3rd level
                        if value is String {
                            result.updateValue(value, forKey: property)
                        } else {
                            let mirror = Mirror(reflecting: value)
                            
                            for (property, value) in mirror.children {
                                guard let property = property else {
                                    continue
                                }
                                result.updateValue(value, forKey: property)
                            }
                        }
                    }
                }
            }
        }
    }
    return result
}

}

A cleaner solution that uses recursion here ;) https://stackoverflow.com/questions/27292255/how-to-loop-over-struct-properties-in-swift/56675017#56675017 — RPatel99, Jun 19 '19 at 20:00

Loop through Swift struct to get keys and values

4 Answers4