An example of simple implementation of Strategy Pattern to validators

StrategyPattern.swift

// for quick demonstation and run paste this code in here: http://online.swiftplayground.run

import Foundation 

class ValidatorContext {
    var strategy: ValidatorStrategy
    var isValid: Bool { 
        get {
            return self.validationResult?.valid ?? false
        }
        set {}
    }

    var msg: String {
        get {
            return self.validationResult?.msg ?? ""
        }
        set {}
    }

    private var validationResult: ValidationResult? {
        didSet {
            if let validationResult = validationResult {
                self.isValid = validationResult.valid
                self.msg = validationResult.msg
            }
        }
    }

    init() {
         self.strategy = DefaultValidatorStrategy()
    }
    
    init(strategy: ValidatorStrategy) { 
            self.strategy = strategy
    }

    func validate(value: Any) -> ValidatorContext {
        self.validationResult = self.strategy.validate(value: value)
        return self
    }
}

protocol ValidatorStrategy {
    func validate(value: Any) -> ValidationResult
}

class DefaultValidatorStrategy: ValidatorStrategy {
    func validate(value: Any) -> ValidationResult {
        return ValidationResult(valid: false, msg: "")
    }
}

struct ValidationResult {
    let valid: Bool
    let msg: String
}

class PasswordValidatorStrategy {
    
}

class EmailValidatorStrategy: ValidatorStrategy {
    func validate(value: Any) -> ValidationResult {
        var valid = true
        var msg = ""
        
        if let value = value as? String {
            if value.range(of:"@") == nil {
                valid = false
                msg = "Not valid email"
            }
        } else {
            valid = false
            msg = "Value is not of type String"   
        }

        return ValidationResult(valid: valid, msg: msg)
    }
}

// CLIENT AND TESTS
let myValidPassword = "Gasdb$%"
let myValidEmail = "test@example.com"

var context = ValidatorContext().validate(value: myValidPassword)

print("valid: ", context.isValid)
print("msg: ", context.msg)

context.strategy = EmailValidatorStrategy()
context.validate(value: myValidPassword)

print("valid: ", context.isValid)
print("msg: ", context.msg)

context.validate(value: myValidEmail)

print("valid: ", context.isValid)
print("msg: ", context.msg)

context.validate(value: 666)

print("valid: ", context.isValid)
print("msg: ", context.msg)
context.msg = "HELLO WORLD" // ignore setter
print("msg: ", context.msg)