orview.swift

//
//  ContentView.swift
//  OrView
//
//  Created by Chris Eidhof on 07.11.19.
//  Copyright © 2019 Chris Eidhof. All rights reserved.
//

import SwiftUI

struct SizePreference: PreferenceKey {
    static let defaultValue: CGSize? = nil
    static func reduce(value: inout CGSize?, nextValue: () -> CGSize?) {
        value = value ?? nextValue()
    }
}

extension View {
    func sizePreference() -> some View {
        return self.background(GeometryReader { proxy in
            Color.clear.preference(key: SizePreference.self, value: proxy.size)
        })
    }
}

struct OrView<L: View, R: View>: View {
    var left: L
    var right: R
    
    @State var leftWidth: CGFloat? = nil
    
    var body: some View {
        GeometryReader { proxy in
            self.left
                .sizePreference()
                .onPreferenceChange(SizePreference.self) { value in
                    self.leftWidth = value?.width
            }.hidden()
            if self.leftWidth != nil && self.leftWidth! > proxy.size.width {
                self.right
            } else {
                self.left
            }
        }.transformPreference(SizePreference.self, { $0 = nil })
    }
}

extension View {
    // Pick `self` (unless it doesn't fit, then pick `other`).
    func or<Other: View>(_ other: Other) -> some View {
        return OrView(left: self, right: other)
    }
}

struct ContentView: View {
    @State var fontSize: CGFloat = 20
    var body: some View {
        VStack {
            Text("Hello, World This Is Some Text!")
                .font(.system(size: fontSize))
                .fixedSize()
                .or(Text("HI"))
            Slider(value: $fontSize, in: 10...40)
        }
        
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}