Use previewDevice with type-safe

ContentView.swift

//
//  ContentView.swift
//  SampleSwiftUI
//
//  Created by 中澤 郁斗 on 2019/06/23.
//  Copyright © 2019 中澤 郁斗. All rights reserved.
//

import SwiftUI
import UIKit

struct ContentView : View {
    let type: DeviceType

    init(type: DeviceType) {
        self.type = type
    }

    var body: some View {
        VStack {
            Text(type.rawValue)
        }
    }
}

struct ContentView_Previews : PreviewProvider {

    static var previews: some View {
        let type: DeviceType = .mac
        return ContentView(type: type)
            .previewDevice(PVDevice(type: type).rawValue)
    }
}

struct PVDevice {
    let rawValue: PreviewDevice

    init(type: DeviceType) {
        self.rawValue = PreviewDevice(stringLiteral: type.rawValue)
    }
}

enum DeviceType: String {
    case mac = "Mac"
    case iPhone7 = "iPhone 7"
    case iPhone7Plus = "iPhone 7 Plus"
    case iPhone8 = "iPhone 8"
    //.
    //.
    //.
}