2015-08-07 Swift Reflect example

Halfway last month, Erica Sadun wrote a blog entry where she used the reflect() function: Swift: Just because

She just gave the code and didn't comment any further so below, I've liberally sprinkled the code with comments:

	// Define a struct to represent a point
	struct Pt {
		let x: Int
		let y: Int
		init(_ x: Int, _ y: Int) {self.x = x; self.y = y}
		init(){self.init(0, 0)}
	// Declare an instance of Pt
	let p = Pt(2, 3)
	// Any means: any reference (i.e. class) or value type (i.e. struct, number, etc)
	// So this function will take an item that can be basically anything in Swift
	// and look through the names of its members to see if the string matches.
	func valueForKey(item : Any, key: String) -> Any? {
		// The built-in Swift reflect() function is used by Xcode to aid in debugging
		let mirror = reflect(item)
		// It returns an array, walk through it
		for index in 0..<mirror.count {
			// Each item in the array has a member called 0 and 1
			// This assignment is only there for readability's sake
			let child = mirror[index]
			// The .0 member is a string and it holds the name of our struct member
			if key == child.0 {
				// The .1 member holds information about our struct member
				// If it's not an optional, return its value with the .value property
				return child.1.value
		return nil
	// Try it out
	valueForKey(p, "x")

Instead of using the valueForKey() function, you can also open a Playground and try out a few things yourself:

	let mirror = reflect(p)

swift reflect.png