ToolbarButton.swift

struct ContentView: View {
    
    @State var selected: Bool = false
    
    var body: some View {
        Color.clear
            .frame(width: 900, height: 400)
            .toolbar {
                ToolbarItem {
                    Button(action: {}, label: {
                        Image(systemName: "sidebar.leading")
                    })
                }
                ToolbarItem {
                    ToolbarButton(action: { print("action") }) {
                        Image(systemName: "sidebar.leading")
                    }
                }
            }
    }
}


struct ToolbarButton<Content: View>: View {
    
    @Environment(\.colorScheme) var colorScheme
    
    let action: (() -> Void)
    let content: () -> Content
    
    @State private var mouseDown = false
    @State private var isHovered = false
    
    var body: some View {
        content()
            .foregroundColor(Color(.controlTextColor))
            .font(.system(size: 17, weight: .regular))
            .opacity(foregroundOpacity())
            .padding(.bottom, 1)
            .padding([.leading, .trailing], 8)
            .frame(height: 28)
            .background(
                Color(.unemphasizedSelectedContentBackgroundColor).opacity(backgroundOpacity())
            )
            .blendMode(colorScheme == .dark ? .plusLighter : .plusDarker)
            .onHover {
                isHovered = $0
            }
            .onTapGesture {
                action()
            }
            .onLongPressGesture(minimumDuration: 2, pressing: {
                mouseDown = $0
            }, perform: { })
            .cornerRadius(6)
    }
    
    func foregroundOpacity() -> Double {
        if mouseDown {
            return 1
        }
        return colorScheme == .dark ? 0.45 : 0.65
    }
    
    func backgroundOpacity() -> Double {
        if mouseDown {
            return colorScheme == .dark ? 0.3 : 0.75
        } else if isHovered {
            return colorScheme == .dark ? 0.15 : 0.4
        }
        return 0
    }
    
}