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Swift customer design sample – The.Swift.Dev.


A primary 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 truly modifying it. Sounds cool, proper? Really 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 lessons, this may 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 manner we do not have to change these views, however we will outline a protocol AcceptsHeightVisitor, and lengthen our lessons to just accept this customer object and calculate the end result utilizing a self pointer. 👈


On the decision aspect we will provoke a brand new customer occasion and easily outline the views utilizing the protocol sort, this fashion it’s attainable to name the settle for customer technique on the views and we will calculate the peak for every sort with out altering the inner construction of those lessons.

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 lengthen your objects to just accept a generic customer sort. In case 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 kind particular go to technique. This manner we moved the go to name logic into the customer. 🙃


For the reason that views already conforms to the AcceptsVisitor protocol, we will simply lengthen them with different guests. For instance we will outline a colour customer like this:


struct ColorVisitor: Customer {
    func go to(_ view: FirstView) -> String { "purple" }
    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 may see it is fairly good that we will obtain this type of dynamic object extension logic by guests. If you wish to see a sensible UIKit instance, be happy to try this text. Underneath the hood SwiftUI closely makes use of the customer sample to realize some magical TupleView & ViewBuilder associated stuff. This sample is so cool, I extremely suggest to study extra about it. 💪


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