以下用几个简单的例子来说明枚举在SwiftUI中的使用。

最常用的枚举

struct EnumUIView: View {
    var body: some View {
        VStack{
            Text("Hello, Swift!").padding()
            Button("Enum tests"){
                let testResult = TestResults.success
                runMyTests(results: testResult)
            }
        }
    }
    
    func runMyTests(results:TestResults){
        switch results{
        case .success:
            print("Success")
        case .failure:
            print("Failed")
        }
    }
}

enum TestResults{
    case success
    case failure
}

struct EnumUIView_Previews: PreviewProvider {
    static var previews: some View {
        EnumUIView()
    }
}

枚举增加额外的功能

import SwiftUI

struct EnumUIView: View {
    var body: some View {
        VStack{
            Text("Hello, Swift!").padding()
            Button("Enum tests"){
                let testResult01 = TestResults.success(["Execute Success", "Report Success"])
                runMyTests(results: testResult01)
                
                let testResult02 = TestResults.failure(401)
                runMyTests(results: testResult02)
            }
        }
    }
    
    func runMyTests(results:TestResults){
        switch results{
        case .success(let results):
            print("Success: \(results)")
        case .failure(let errorID):
            print("Failed, error code: \(errorID)")
        }
    }
}

enum TestResults{
    case success([String])
    case failure(Int)
}

struct EnumUIView_Previews: PreviewProvider {
    static var previews: some View {
        EnumUIView()
    }
}

将泛型应用到枚举

import SwiftUI

struct EnumUIView: View {
    var body: some View {
        VStack{
            Text("Hello, Swift!").padding()
            Button("Enum tests"){
                let testResult01 = TestResults<[String], Int>.success(["Execute Success", "Report Success"])
                runMyTests(results: testResult01)
                
                let testResult02 = TestResults<[String], Int>.failure(401)
                runMyTests(results: testResult02)
            }
        }
    }
    
    func runMyTests(results:TestResults<[String], Int>){
        switch results{
        case .success(let results):
            print("Success: \(results)")
        case .failure(let errorID):
            print("Failed, error code: \(errorID)")
        }
    }
}

enum TestResults<T1, T2>{
    case success(T1)
    case failure(T2)
}

struct EnumUIView_Previews: PreviewProvider {
    static var previews: some View {
        EnumUIView()
    }
}

大家可以看到，后面两个例子在枚举中增加了一些额外的功能。尤其是泛型到引入，会使到我们定义的枚举更加的灵活和强大。这个只是一个简单的例子，在实际的应用中，我们可能会在我们的数据中定义复杂的逻辑，这个时候泛型的灵活会给我们带来很大的方便。

最后看官方教材中的一个例子。我们可以看到“Result<Int, Error>”就是一个典型的泛型枚举的应用。

    static func save(employees: [Employee], completion: @escaping (Result<Int, Error>)->Void) {
        DispatchQueue.global(qos: .background).async {
            do {
                let data = try JSONEncoder().encode(employees)
                let outfile = try fileURL()
                try data.write(to: outfile)
                DispatchQueue.main.async {
                    completion(.success(employees.count))
                }
            } catch {
                DispatchQueue.main.async {
                    completion(.failure(error))
                }
            }
        }
    }

这个枚举的定义是：

@frozen public enum Result<Success, Failure> where Failure : Error