In Swift, how can I declare a variable of a specific type that conforms to one or more protocols?

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In Swift 4 it is now possible to declare a variable that is a subclass of a type and implements one or more protocols at the same time.

var myVariable: MyClass & MyProtocol & MySecondProtocol

To do an optional variable:

var myVariable: (MyClass & MyProtocol & MySecondProtocol)?

or as the parameter of a method:

func shakeEm(controls: [UIControl & Shakeable]) {}

Apple announced this at WWDC 2017 in Session 402: Whats new in Swift

Second, I want to talk about composing classes and protocols. So, here
I’ve introduced this shakable protocol for a UI element that can give
a little shake effect to draw attention to itself. And I’ve gone ahead
and extended some of the UIKit classes to actually provide this shake
functionality. And now I want to write something that seems simple. I
just want to write a function that takes a bunch of controls that are
shakable and shakes the ones that are enabled to draw attention to
them. What type can I write here in this array? It’s actually
frustrating and tricky. So, I could try to use a UI control. But not
all UI controls are shakable in this game. I could try shakable, but
not all shakables are UI controls. And there’s actually no good way to
represent this in Swift 3. Swift 4 introduces the notion of composing
a class with any number of protocols.

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