Error Cannot use instance member 'xxx' within property initializer - swiftui

Question

26-07-19
I'll update my code as I'm making progress watching the WWDC video's. My data model is:
struct Egg: Identifiable {
var id = UUID()
var thumbnailImage: String
var day: String
var date: String
var text: String
var imageDetail: String
var weight: Double
}
#if DEBUG
let testData = [
Egg(thumbnailImage: "Dag-1", day: "1.circle", date: "7 augustus 2019", text: "Kippen leggen iedere dag een ei.", imageDetail: "Day-1", weight: 35.48),
Egg(thumbnailImage: "Dag-2", day: "2.circle", date: "8 augustus 2019", text: "Kippen leggen iedere dag een ei.", imageDetail: "Day-2", weight: 35.23),
Egg(thumbnailImage: "Dag-3", day: "3.circle", date: "9 augustus 2019", text: "Kippen leggen iedere dag een ei.", imageDetail: "Day-3", weight: 34.92)
Etc, etc
]
I've a TabbedView, a ContentView, a ContentDetail and a couple of other views (for settings etc). The code for the ContentView is:
struct ContentView : View {
var eggs : [Egg] = []
var body: some View {
NavigationView {
List(eggs) { egg in
EggCell(egg: egg)
}
.padding(.top, 10.0)
.navigationBarTitle(Text("Egg management"), displayMode: .inline)
}
}
}
#if DEBUG
struct ContentView_Previews : PreviewProvider {
static var previews: some View {
ContentView(eggs: testData)
}
}
#endif
struct EggCell : View {
let egg: Egg
var body: some View {
return NavigationLink(destination: ContentDetail(egg: egg)) {
ZStack {
HStack(spacing: 8.0) {
Image(egg.thumbnailImage)
.resizable()
.aspectRatio(contentMode: .fit)
.padding(.leading, -25)
.padding(.top, -15)
.padding(.bottom, -15)
.padding(.trailing, -25)
.frame(width: 85, height: 61)
VStack {
Image(systemName: egg.day)
.resizable()
.frame(width: 30, height: 22)
.padding(.leading, -82)
Spacer()
}
.padding(.leading)
VStack {
Text(egg.date)
.font(.headline)
.foregroundColor(Color.gray)
Text(egg.weight.clean)
.font(.title)
}
}
}
}
}
}
extension Double {
var clean: String {
return self.truncatingRemainder(dividingBy: 1) == 0 ? String(format: "%.0f", self) : String(format: "%.2f", self)
}
}
The code for the ContentDetail is:
struct ContentDetail : View {
let egg: Egg
#State private var photo = true
#State private var calculated = false
#Binding var weight: Double
var body: some View {
VStack (alignment: .center, spacing: 10) {
Text(egg.date)
.font(.title)
.fontWeight(.medium)
.navigationBarTitle(Text(egg.date), displayMode: .inline)
ZStack (alignment: .topLeading) {
Image(photo ? egg.imageDetail : egg.thumbnailImage)
.resizable()
.aspectRatio(contentMode: .fill)
.background(Color.black)
.padding(.trailing, 0)
.tapAction { self.photo.toggle() }
VStack {
HStack {
Image(systemName: egg.day)
.resizable()
.padding(.leading, 10)
.padding(.top, 10)
.frame(width: 50, height: 36)
.foregroundColor(.white)
Spacer()
Image(systemName: photo ? "photo" : "wand.and.stars")
.resizable()
.padding(.trailing, 10)
.padding(.top, 10)
.frame(width: 50, height: 36)
.foregroundColor(.white)
}
Spacer()
HStack {
Image(systemName: "arrow.left.circle")
.resizable()
.padding(.leading, 10)
.padding(.bottom, 10)
.frame(width: 50, height: 50)
.foregroundColor(.white)
Spacer()
Image(systemName: "arrow.right.circle")
.resizable()
.padding(.trailing, 10)
.padding(.bottom, 10)
.frame(width: 50, height: 50)
.foregroundColor(.white)
}
}
}
Text("the weight is: \(egg.weight) gram")
.font(.headline)
.fontWeight(.bold)
ZStack {
RoundedRectangle(cornerRadius: 10)
.padding(.top, 45)
.padding(.bottom, 45)
.border(Color.gray, width: 5)
.opacity(0.1)
HStack {
Spacer()
DigitPicker(digitName: "tens", digit: $weight.tens)
DigitPicker(digitName: "ones", digit: $weight.ones)
Text(".")
.font(.largeTitle)
.fontWeight(.black)
.padding(.bottom, 10)
DigitPicker(digitName: "tenths", digit: $weight.tenths)
DigitPicker(digitName: "hundredths", digit: $weight.hundredths)
Spacer()
}
}
Toggle(isOn: $calculated) {
Text(calculated ? "This weight is calculated." : "This weight is measured.")
}
Text(egg.text)
.lineLimit(2)
.frame(minWidth: 0, maxWidth: .infinity, alignment: .leading)
.padding(.leading, 6)
Spacer()
}
.padding(6)
}
}
#if DEBUG
struct ContentDetail_Previews : PreviewProvider {
static var previews: some View {
NavigationView { ContentDetail(egg: testData[0]) }
}
}
#endif
struct DigitPicker: View {
var digitName: String
#Binding var digit: Int
var body: some View {
VStack {
Picker(selection: $digit, label: Text(digitName)) {
ForEach(0 ... 9) {
Text("\($0)").tag($0)
}
}.frame(width: 60, height: 110).clipped()
}
}
}
fileprivate extension Double {
var tens: Int {
get { sigFigs / 1000 }
set { replace(tens: newValue) }
}
var ones: Int {
get { (sigFigs / 100) % 10 }
set { replace(ones: newValue) }
}
var tenths: Int {
get { (sigFigs / 10) % 10 }
set { replace(tenths: newValue) }
}
var hundredths: Int {
get { sigFigs % 10 }
set { replace(hundredths: newValue) }
}
private mutating func replace(tens: Int? = nil, ones: Int? = nil, tenths: Int? = nil, hundredths: Int? = nil) {
self = Double(0
+ 1000 * (tens ?? self.tens)
+ 100 * (ones ?? self.ones)
+ 10 * (tenths ?? self.tenths)
+ (hundredths ?? self.hundredths)) / 100.0
}
private var sigFigs: Int {
return Int((self * 100).rounded(.toNearestOrEven))
}
}
The compiler errors I'm still getting are:
in ContentView, beneath NavigationLink: Missing argument for
parameter 'weight' in call
in ContentDetail, at NavigationView: Missing argument for parameter
'weight' in call
in ContentDetail, after #endif: Missing argument for parameter
'weight' in call
25-07-19
The following code is part of a List detail view. The var 'weight' is coming from the List through a 'NavigationLink' statement. In this code I declare it as '35.48', but the NavigationLink fills in its real value.
I want to make an array [3, 5, 4, 8] with the compactMap statement. That works okay in Playground. The values go to 4 different pickers (within a HStack).
import SwiftUI
import Foundation
struct ContentDetail : View {
var weight : Double = 35.48
var weightArray = "\(weight)".compactMap { Int("\($0)") }
#State var digit1 = weightArray[0] // error
#State var digit2 = weightArray[1] // error
#State var digit3 = weightArray[2] // error
#State var digit4 = weightArray[3] // error
var body: some View {
VStack (alignment: .center, spacing: 10) {
Text(weight)
.font(.title)
.fontWeight(.medium)
etc etc
I get an error 'Cannot use instance member 'weightArray' within property initializer; property initializers run before 'self' is available'.
If I use the following code it works fine (for the first list element):
import SwiftUI
import Foundation
struct ContentDetail : View {
var weight : Double = 35.48
var weightArray = [3, 5, 4, 8]
#State var digit1 = 3
#State var digit2 = 5
#State var digit3 = 4
#State var digit4 = 8
var body: some View {
VStack (alignment: .center, spacing: 10) {
Text(weight)
.font(.title)
.fontWeight(.medium)
etc etc
What is the correct SwiftUI approach and why?

Answer 1

Indeed, a property initializer cannot refer to another property in the same container. You have to initialize your properties in an init instead.
struct ContentDetail: View {
var weight: Double
var weightArray: [Int]
#State var digit1: Int
#State var digit2: Int
#State var digit3: Int
#State var digit4: Int
init(weight: Double) {
self.weight = weight
weightArray = "\(weight)".compactMap { Int("\($0)") }
_digit1 = .init(initialValue: weightArray[0])
_digit2 = .init(initialValue: weightArray[1])
_digit3 = .init(initialValue: weightArray[2])
_digit4 = .init(initialValue: weightArray[3])
}
But I suspect you're breaking out the digits because you want to let the user edit them individually, like this:
If that's what you want, you should not have a separate #State property for each digit. Instead, weight should be a #Binding and it should have a separate mutable property for each digit.
First, extend Double to give you access to the digits:
fileprivate extension Double {
var tens: Int {
get { sigFigs / 1000 }
set { replace(tens: newValue) }
}
var ones: Int {
get { (sigFigs / 100) % 10 }
set { replace(ones: newValue) }
}
var tenths: Int {
get { (sigFigs / 10) % 10 }
set { replace(tenths: newValue) }
}
var hundredths: Int {
get { sigFigs % 10 }
set { replace(hundredths: newValue) }
}
private mutating func replace(tens: Int? = nil, ones: Int? = nil, tenths: Int? = nil, hundredths: Int? = nil) {
self = Double(0
+ 1000 * (tens ?? self.tens)
+ 100 * (ones ?? self.ones)
+ 10 * (tenths ?? self.tenths)
+ (hundredths ?? self.hundredths)) / 100.0
}
private var sigFigs: Int {
return Int((self * 100).rounded(.toNearestOrEven))
}
}
Then, change ContentDetail's weight property to be a #Binding and get rid of the other properties:
struct ContentDetail: View {
#Binding var weight: Double
var body: some View {
HStack {
DigitPicker(digitName: "tens", digit: $weight.tens)
DigitPicker(digitName: "ones", digit: $weight.ones)
DigitPicker(digitName: "tenths", digit: $weight.tenths)
DigitPicker(digitName: "hundredths", digit: $weight.hundredths)
}
}
}
struct DigitPicker: View {
var digitName: String
#Binding var digit: Int
var body: some View {
VStack {
Picker(selection: $digit, label: Text(digitName)) {
ForEach(0 ... 9) {
Text("\($0)").tag($0)
}
}.frame(width: 60, height: 110).clipped()
}
}
}
Here's the rest of the code needed to test this in a playground, which is how I generated that image above:
import PlaygroundSupport
struct TestView: View {
#State var weight: Double = 35.48
var body: some View {
VStack(spacing: 0) {
Text("Weight: \(weight)")
ContentDetail(weight: $weight)
.padding()
}
}
}
let host = UIHostingController(rootView: TestView())
host.preferredContentSize = .init(width: 320, height: 240)
PlaygroundPage.current.liveView = host