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How to use reflection to inspect type data

Written by Paul Hudson    @twostraws

Swift has a built-in Mirror struct that lets us query any kind of data in our code. It’s most commonly used to read through the list of properties that are available, but it’s also used in playgrounds to print out user-readable values inside types.

To get started, first create a custom type then an instance of that type:

struct Person {
    var name = "Taylor Swift"
    var age = 26
}

var taylor = Person()

You can now instantiate a Mirror object from taylor, like this:

var mirror = Mirror(reflecting: taylor)

That mirror isn’t a copy of taylor, a reflection of it – something you can inspect. For example, you can loop over all the properties inside taylor and print out their names and values like this:

for case let (label?, value) in mirror.children {
    print (label, value)
}

Available from iOS 8.0

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