SwiftUI store View as a variable while passing it some bindings - swift

Question

I want to store a View as a variable for later use, while passing that View some Bindings.
Here's what I've tried:
struct Parent: View {
#State var title: String? = ""
var child: Child!
init() {
self.child = Child(title: self.$title)
}
var body: some View {
VStack {
child
//...
Button(action: {
self.child.f()
}) {
//...
}
}
}
}
struct Child: View {
#Binding var title: String?
func f() {
// complex work from which results a string
self.title = <that string>
}
var body: some View {
// ...
}
}
It compiles correctly and the View shows as expected, however when updating from the child the passed Binding from the parent, the variable never gets updated. You can even do something like this (from the child):
self.title = "something"
print(self.title) // prints the previous value, in this case nil
I don't know if this is a bug or not, but directly initializing the child in the body property does the trick. However, I need that child as a property to access its methods.

Answer 1

If you want to change something from Parent for the child, binding is the right way. If that's complicated, you have to use DataModel.
struct Parent: View {
#State var title: String? = ""
var body: some View {
VStack {
Child(title: $title)
Button(action: {
self.title = "something"
}) {
Text("click me")
}
}
}
}
struct Child: View {
#Binding var title: String?
var body: some View {
Text(title ?? "")
}
}

Answer 2

This is counter to the design of the SwiftUI framework. You should not have any persistent view around to call methods on. Instead, views are created and displayed as needed in response to your app's state changing.
Encapsulate your data in an ObservableObject model, and implement any methods you need to call on that model.
Update
It is fine to have such a function defined in Child, but you should only be calling it from within the Child struct definition. For instance, if your child view contains a button, that button can call the child's instance methods. For example,
struct Parent: View {
#State private var number = 1
var body: some View {
VStack {
Text("\(number)")
Child(number: $number)
}
}
}
struct Child: View {
#Binding var number: Int
func double() {
number *= 2
}
var body: some View {
HStack {
Button(action: {
self.double()
}) {
Text("Double")
}
}
}
}
But you wouldn't try to call double() from outside the child struct. If you wanted a function that can be called globally, put it in a data model. This is especially true if the function call is making network requests, as the model will stick around outside your child view, even if it is recreated due to layout changing.
class NumberModel: ObservableObject {
#Published var number = 1
func double() {
number *= 2
}
}
struct Parent: View {
#ObservedObject var model = NumberModel()
var body: some View {
VStack {
Text("\(model.number)")
Button(action: {
self.model.double()
}) {
Text("Double from Parent")
}
Child(model: model)
}
}
}
struct Child: View {
#ObservedObject var model: NumberModel
var body: some View {
HStack {
Button(action: {
self.model.double()
}) {
Text("Double from Child")
}
}
}
}