Hi,

I bought Hacking with macOS from Apple Books as I wasn't aware that it doesn't come with the SwiftUI content, but when downloading the project files I got the swiftui folder as well. I wanted to at least look at the source code for the SwiftUI projects to see what it teaches, but when I opened and ran the 2nd SwiftUI project (Odd One Out) it didn't work for me. It seems like SwiftUI doesn't rebuild the UI when the layout array changes, resulting in a grid of empty rectangles. Is there a fix for this?

Xcode 11.5, macOS 10.15.5

Thanks! Jakub

Hi Jakub - I hit the same issue. The problem seems to be with the if statement inside a for loop - for some reason that stops the view from refreshing. I got it to work by deleting the if self.image(row, column) == "empty" block and the if/else statement, leaving the loop as just:

ForEach(0..<Self.gridSize) { column in Button(action: { self.processAnswer(at: row, column) }) { Image(self.image(row, column)) .renderingMode(.original) } .buttonStyle(BorderlessButtonStyle()) }

It looks like if/else inside a ForEach loop stops the view from refreshing properly. I'm guessing this is a SwiftUI bug, but I don't know for sure. I could reproduce the problem with this stripped down example (which I mailed to Paul Hudson but no reply so far):

import SwiftUI

struct ContentView: View {
    @State var layout = "Initial” 

    var body: some View {
        ForEach(0..<1) { _ in
            if (true) {
                    Text(self.layout)
            }
        }
        .onAppear(perform: { self.layout = "Updated" })
        .frame(width: 400, height: 100)
    }
} 

Mark Russell

Hey Mark,

Turns out this is actually a fully intentional performance optimization from the SwiftUI team. The ForEach(_:content:) initializer doesn't watch its data array for changes, so the view never rebuilds. The solution is to use the ForEach(_:id:content:) initializer, like so:

ForEach(0 ..< Self.gridSize, id: \.self) { row in
    HStack {
        ForEach(0 ..< Self.gridSize, id: \.self) { col in
            ...
        }
    }
}

--Jakub

The project runs perfectly with xcode 12 - I've just tried it with BigSur beta 8.