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Swift customer design sample – The.Swift.Dev.


A fundamental customer instance

The customer design sample is likely one of the behavioral patterns, it’s used to increase an object with a given performance with out really modifying it. Sounds cool, proper? Truly this sample is what provides SwiftUI superpowers, let me present you the way it works.



open class View {}

closing class FirstView: View {}
closing class SecondView: View {}
closing class ThirdView: View {}

struct HeightVisitor {
    func go to(_ view: FirstView) -> Float { 16 }
    func go to(_ view: SecondView) -> Float { 32 }
    func go to(_ view: ThirdView) -> Float { 64 }
}

protocol AcceptsHeightVisitor {
    func settle for(_ customer: HeightVisitor) -> Float
}

extension FirstView: AcceptsHeightVisitor {
    func settle for(_ customer: HeightVisitor) -> Float { customer.go to(self) }
}

extension SecondView: AcceptsHeightVisitor {
    func settle for(_ customer: HeightVisitor) -> Float { customer.go to(self) }
}

extension ThirdView: AcceptsHeightVisitor {
    func settle for(_ customer: HeightVisitor) -> Float { customer.go to(self) }
}

let customer = HeightVisitor()
let view1: AcceptsHeightVisitor = FirstView()
let view2: AcceptsHeightVisitor = SecondView()
let view3: AcceptsHeightVisitor = ThirdView()


print(view1.settle for(customer))
print(view2.settle for(customer))
print(view3.settle for(customer))


First we outline our customized view courses, it will assist to visualise how the sample works. Subsequent we outline the precise HeightVisitor object, which can be utilized to calculate the peak for every view sort (FirstView, SecondView, ThirdView). This fashion we do not have to change these views, however we are able to outline a protocol AcceptsHeightVisitor, and prolong our courses to simply accept this customer object and calculate the end result utilizing a self pointer. 👈


On the decision aspect we are able to provoke a brand new customer occasion and easily outline the views utilizing the protocol sort, this manner it’s potential to name the settle for customer technique on the views and we are able to calculate the peak for every sort with out altering the interior construction of those courses.

A generic customer

We will additionally make this sample extra generic by making a Swift protocol with an related sort.


open class View {}

closing class FirstView: View {}
closing class SecondView: View {}
closing class ThirdView: View {}

struct HeightVisitor {
    func go to(_ view: FirstView) -> Float { 16 }
    func go to(_ view: SecondView) -> Float { 32 }
    func go to(_ view: ThirdView) -> Float { 64 }
}

protocol Customer {
    associatedtype R
    func go to<O>(_ object: O) -> R
}

protocol AcceptsVisitor {
    func settle for<V: Customer>(_ customer: V) -> V.R
}

extension AcceptsVisitor {
    func settle for<V: Customer>(_ customer: V) -> V.R { customer.go to(self) }
}

extension FirstView: AcceptsVisitor {}
extension SecondView: AcceptsVisitor {}
extension ThirdView: AcceptsVisitor {}

extension HeightVisitor: Customer {

    func go to<O>(_ object: O) -> Float {
        if let o = object as? FirstView {
            return go to(o)
        }
        if let o = object as? SecondView {
            return go to(o)
        }
        if let o = object as? ThirdView {
            return go to(o)
        }
        fatalError("Go to technique unimplemented for sort (O.self)")
    }
}

let customer = HeightVisitor()
let view1: AcceptsVisitor = FirstView()
let view2: AcceptsVisitor = SecondView()
let view3: AcceptsVisitor = ThirdView()

print(view1.settle for(customer))
print(view2.settle for(customer))
print(view3.settle for(customer))



You should utilize the generic Customer protocol to outline the customer and the AcceptsVisitor protocol to simply prolong your objects to simply accept a generic customer sort. Should you select this strategy you continue to need to implement the generic go to technique on the Customer, forged the item sort and name the sort particular go to technique. This fashion we moved the go to name logic into the customer. 🙃


For the reason that views already conforms to the AcceptsVisitor protocol, we are able to simply prolong them with different guests. For instance we are able to outline a shade customer like this:


struct ColorVisitor: Customer {
    func go to(_ view: FirstView) -> String { "pink" }
    func go to(_ view: SecondView) -> String { "inexperienced" }
    func go to(_ view: ThirdView) -> String { "blue" }
    
    func go to<O>(_ object: O) -> String {
        if let o = object as? FirstView {
            return go to(o)
        }
        if let o = object as? SecondView {
            return go to(o)
        }
        if let o = object as? ThirdView {
            return go to(o)
        }
        fatalError("Go to technique unimplemented for sort (O.self)")
    }
}

let customer = ColorVisitor()
let view1: AcceptsVisitor = FirstView()
let view2: AcceptsVisitor = SecondView()
let view3: AcceptsVisitor = ThirdView()

print(view1.settle for(customer))
print(view2.settle for(customer))
print(view3.settle for(customer))


As you’ll be able to see it is fairly good that we are able to obtain this type of dynamic object extension logic via guests. If you wish to see a sensible UIKit instance, be happy to try this text. Beneath the hood SwiftUI closely makes use of the customer sample to attain some magical TupleView & ViewBuilder associated stuff. This sample is so cool, I extremely advocate to study extra about it. 💪


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