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SwiftUI: @State Variable Does Not Update View When Its Value Changes (within Same View)

- 1 answer

I am very new to SwiftUI. Maybe I am not completely understanding how @State annotated variables work. As far as I know, when you update their values, if they're within the same View, the screen will get updated too (in this case the background color of the screen). I think I followed exactly the video tutorial (SwiftUI Basics Tutorial) but nonetheless I did not get my code to work. By the way, I'm using Xcode Version 13.2.1, if it's worth mentioning. In my ContentView.swift this is what I have:

//
//  ContentView.swift
//


import SwiftUI

struct ContentView: View {
     
    @State private var isNight = false //Variable to be changed when pressing the button
    
    var body: some View {
        ZStack {
            //The BackgroundView color should change when pressing the button
            BackgroundView(topColor: isNight ? .black : .blue,
                           bottomColor: isNight ? .gray : Color("lightBlue"))
            VStack{
                CityTextView(cityName: "Cupertino, CA")
                MainWeatherStatusView(imageName: "cloud.sun.rain.fill",
                                      temperature: 72)
                HStack(spacing: 0){
                    WeatherView(dayOfWeek: "TUE",
                                imageName: "cloud.sun.fill",
                                temperature: 74)
                    WeatherView(dayOfWeek: "WED",
                                imageName: "sun.max.fill",
                                temperature: 30)
                    WeatherView(dayOfWeek: "THU",
                                imageName: "wind.snow",
                                temperature: 5)
                    WeatherView(dayOfWeek: "FRI",
                                imageName: "sunset.fill",
                                temperature: 60)
                    WeatherView(dayOfWeek: "SAT",
                                imageName: "snow",
                                temperature: 74)
                }
                Button{
                    isNight.toggle() //Changing this should update my View, right?
                } label: {
                    WeatherButton(title: "Change Day Time",
                                  textColor: .blue,
                                  backgroundColor: .white)
                }
                Spacer()
            }
        }
        
    }
    
}

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


struct WeatherView: View {
    
    var dayOfWeek: String
    var imageName: String
    var temperature: Int
    
    var body: some View {
        VStack{
            Text(dayOfWeek)
                .font(.system(size: 15, weight: .medium, design: .default))
                .foregroundColor(.white)
                .padding()
            
            Image(systemName: imageName)
                .renderingMode(.original)
                .resizable()
                .aspectRatio(contentMode: .fit)
                .frame(width: 40, height: 40)
            
            Text("\(temperature)°")
                .font(.system(size: 30, weight: .medium))
                .foregroundColor(.white)
            
            Spacer()
        }
    }
}

struct BackgroundView: View {
    
    var topColor: Color
    var bottomColor: Color
    
    var body: some View {
        LinearGradient(gradient: Gradient(colors: [.blue, Color("lightBlue")]), startPoint: .topLeading, endPoint: .bottomTrailing)
            .edgesIgnoringSafeArea(.all)
    }
}

struct CityTextView: View {
    
    var cityName: String
    
    var body: some View {
        Text(cityName)
            .font(.system(size: 32, weight: .medium, design: .default))
            .foregroundColor(.white)
            .padding()
    }
}

struct MainWeatherStatusView: View {
    
    var imageName: String
    var temperature: Int
    
    
    var body: some View {
        VStack(spacing: 8){
            Image(systemName: imageName)
                .renderingMode(.original)
                .resizable()
                .aspectRatio(contentMode: .fit)
                .frame(width: 180, height: 180)
            Text("\(temperature)°")
                .font(.system(size: 70, weight: .medium))
                .foregroundColor(.white)
        }
    }
}

In case you want to copy paste the code and want to get rid of the icons, here's the code without them:

    //
    //  ContentView.swift
    //
    
    
    import SwiftUI
    
    struct ContentView: View {
         
        @State private var isNight = false //VARIABLE TO BE CHANGED WHEN PRESSING BUTTON
        
        var body: some View {
            ZStack {
                //The BackgroundView color should change when pressing the button
                BackgroundView(topColor: isNight ? .black : .blue,
                               bottomColor: isNight ? .gray : Color("lightBlue"))
                VStack{
                    CityTextView(cityName: "Cupertino, CA")
                    Button{
                        isNight.toggle() //CHANGING THIS SHOULD UPDATE MY VIEW, RIGHT?
                    } label: {
                        WeatherButton(title: "Change Day Time",
                                      textColor: .blue,
                                      backgroundColor: .white)
                    }
                    Spacer()
                }
            }
            
        }
        
    }
    
    struct ContentView_Previews: PreviewProvider {
        static var previews: some View {
            ContentView()
        }
    }
    
    
    struct WeatherView: View {
        
        var dayOfWeek: String
        var imageName: String
        var temperature: Int
        
        var body: some View {
            VStack{
                Text(dayOfWeek)
                    .font(.system(size: 15, weight: .medium, design: .default))
                    .foregroundColor(.white)
                    .padding()
                
                Image(systemName: imageName)
                    .renderingMode(.original)
                    .resizable()
                    .aspectRatio(contentMode: .fit)
                    .frame(width: 40, height: 40)
                
                Text("\(temperature)°")
                    .font(.system(size: 30, weight: .medium))
                    .foregroundColor(.white)
                
                Spacer()
            }
        }
    }
    
    struct BackgroundView: View {
        
        var topColor: Color
        var bottomColor: Color
        
        var body: some View {
            LinearGradient(gradient: Gradient(colors: [.blue, Color("lightBlue")]), startPoint: .topLeading, endPoint: .bottomTrailing)
                .edgesIgnoringSafeArea(.all)
        }
    }
    
    struct CityTextView: View {
        
        var cityName: String
        
        var body: some View {
            Text(cityName)
                .font(.system(size: 32, weight: .medium, design: .default))
                .foregroundColor(.white)
                .padding()
        }
    }
    
    struct MainWeatherStatusView: View {
        
        var imageName: String
        var temperature: Int
        
        
        var body: some View {
            VStack(spacing: 8){
                Image(systemName: imageName)
                    .renderingMode(.original)
                    .resizable()
                    .aspectRatio(contentMode: .fit)
                    .frame(width: 180, height: 180)
                Text("\(temperature)°")
                    .font(.system(size: 70, weight: .medium))
                    .foregroundColor(.white)
            }
        }
    }
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Answer

In your current BackgroundView code, although you have topColor and bottomColor parameters, you don't actually use them in your view. You probably want to include them in your gradient:

struct BackgroundView: View {
    
    var topColor: Color
    var bottomColor: Color
    
    var body: some View {
        LinearGradient(gradient: Gradient(colors: [topColor, bottomColor]), startPoint: .topLeading, endPoint: .bottomTrailing)
            .edgesIgnoringSafeArea(.all)
    }
}

Also, make sure you have your custom colors (like Color("lightBlue")) in your asset catalogue.

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source: stackoverflow.com
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