How can I change the textual representation displayed for a type in Swift?

swift

Solution

Swift 2 - 4

Summary

Conform to the `CustomStringConvertible` protocol and add `description`:

var description: String {
    return "description here"
}

Example

You can create some structs:

struct Animal : CustomStringConvertible {
    let type : String

    var description: String {
        return type
    }
}

struct Farm : CustomStringConvertible {
    let name : String
    let animals : [Animal]

    var description: String {
        return "\(name) is a \(self.dynamicType) with \(animals.count) animal(s)."
    }
}

If you initialize them:

let oldMajor = Animal(type: "Pig")
let boxer = Animal(type: "Horse")
let muriel = Animal(type: "Goat")

let orwellsFarm = Farm(name: "Animal Farm", animals: [oldMajor, boxer, muriel])

The custom descriptions will appear in your playground:

See also `CustomDebugStringConvertible`, which you can use for more verbose output during debugging.

Usage Note

You can initialize a `String` from any type without implementing this protocol. For example:

For this reason, the docs say:

Using `CustomStringConvertible` as a generic constraint, or accessing a conforming type's `description` directly, is therefore discouraged.

Problem

How can I modify the textual output that gets displayed in string interpolation? The `Printable` protocol looks the most obvious but it's ignored in both String Interpolation and when printing the instance, e.g: ``` struct Point : Printable { var x = 0 var y = 0 var description : String { return "(\(x), \(y))" } func toString() -> String { return description } } ``` Likewise the `toString()` convention has no effect either: ``` var p = Point(x: 10, y: 20) println(p) // V11lldb_expr_05Point (has 2 children) println("\(p)") // V11lldb_expr_05Point (has 2 children) println(p.description) // (10, 20) println("\(p.description)") // (10, 20) ``` The behavior is different again in PlayGround which uses its own String representation for structs, i.e: ``` p // {x 10, y 20} ``` Is there a way I can change how an instance is displayed?

Original source