How can I change the textual representation displayed for a type in Swift?
SwiftSwift Problem Overview
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?
Swift Solutions
Solution 1 - Swift
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.
Solution 2 - Swift
Apple provides this example:
struct MyType: Printable {
var name = "Untitled"
var description: String {
return "MyType: \(name)"
}
}
let value = MyType()
println("Created a \(value)")
// prints "Created a MyType: Untitled"
If you try this in playground, you will get the same issue that you're getting (V11lldb_expr...
). In playground, you get the description on the right hand side when you call the initializer, but the println
doesn't return something legible.
Out of playground, however, this code behaves as you would expect. Both your code and the sample code from Apple above print the correct description
when used in a non-playground context.
I don't think you can change this behavior in playground. It could also just be a bug.
EDIT: I'm pretty sure that this is a bug; I submitted a bug report to Apple.
UPDATE: In Swift 2, instead of Printable
, use CustomStringConvertible
(relevant doc link).
struct MyType: CustomStringConvertible {
var name = "Untitled"
var description: String {
return "MyType: \(name)"
}
}
let value = MyType()
println("Created a \(value)")
// prints "Created a MyType: Untitled"
Solution 3 - Swift
This appears to be a bug in the playground. If you actually compile and run the programme normally it prints:
(10, 20)
(10, 20)
(10, 20)
(10, 20)
as expected.
You should report this at https://bugreport.apple.com
Solution 4 - Swift
As an alternative in Swift 5+ you can extend the String.StringInterpolation
struct Point {
var x : Int
var y : Int
}
extension String.StringInterpolation {
mutating func appendInterpolation(_ value: Point) {
appendInterpolation("\(value.x):\(value.y)")
}
}
This will change the value for
print("\(p)")
but not for print(p)
- which will still use the description
Solution 5 - Swift
for swift 5 just implement CustomStringConvertible
(pls see https://developer.apple.com/documentation/swift/customstringconvertible/1539130-description)
Solution 6 - Swift
If you open the console view : View -> Assistant Editor -> Show Assistant Editor you can see expected print lines Checked in xCode 6.3.2 with Yosimite 10.10