I want a if let statement inside a View.
@ObservedObject var person: Person?
var body: some View {
if person != nil {
// this works
}
if let p = person {
// Compiler error
}
}
Closure containing control flow statement cannot be used with function builder 'ViewBuilder'
2022 Update
You can just use normal Swift if let:
if let pieceOfData = pieceOfData {
// now it's guaranteed
} else {
// now it's not
}
Previous Answer
Or you can create your own IfLet view builder:
import SwiftUI
struct IfLet<Value, Content, NilContent>: View where Content: View, NilContent: View {
let value: Value?
let contentBuilder: (Value) -> Content
let nilContentBuilder: () -> NilContent
init(_ optionalValue: Value?, @ViewBuilder whenPresent contentBuilder: @escaping (Value) -> Content, @ViewBuilder whenNil nilContentBuilder: @escaping () -> NilContent) {
self.value = optionalValue
self.contentBuilder = contentBuilder
self.nilContentBuilder = nilContentBuilder
}
var body: some View {
Group {
if value != nil {
contentBuilder(value!)
} else {
nilContentBuilder()
}
}
}
}
extension IfLet where NilContent == EmptyView {
init(_ optionalValue: Value?, @ViewBuilder whenPresent contentBuilder: @escaping (Value) -> Content) {
self.init(optionalValue, whenPresent: contentBuilder, whenNil: { EmptyView() })
}
}
Using this, you can now do the following:
var body: some View {
IfLet(pieceOfData) { realData in
// realData is no longer optional
}
}
Want to respond if your optional is nil?
var body: some View {
IfLet(pieceOfData, whenPresent: { realData in
// realData is no longer optional
DataView(realData)
}, whenNil: {
EmptyDataView()
})
}
Yes, the notation requested in code snapshot is not allowed, at least for now, but the intended result is possible to achieve with the following very simple approach - extract control flow into function:
var person: Person? // actually @ObservedObject does not allowed optional
var body: some View {
VStack {
if person != nil {
// same as before
}
personViewIfExists() // << just extract it in helper function
}
}
private func personViewIfExists() -> some View { // generates view conditionally
if let p = person {
return ExistedPersonView(person: p) // << just for demo
}
}
on some conditions also the following variant of function might be required (although it produces the same result)
private func personViewIfExists() -> some View {
if let p = person {
return AnyView(ExistedPersonView(person: p))
}
return AnyView(EmptyView())
}
AnyView in SwiftUI, and instead use the @ViewBuilder wrapper.The simpler approach is to just use .map()
It will show a UserProfile if user has a value, and nothing if user is nil:
var user: User?
var body: some View {
user.map { UserProfile(user: $0) }
}
If you want to provide a default view for nil value, you can just use the nil-coalescing operator:
var user: User?
var body: some View {
user.map { UserProfile(user: $0) } ?? UserProfile(user: .default)
}
However this requires the two views to be of the same type, and in practice if they're of the same type you're better off just writing UserProfile(user: $0 ?? .default).
What's interesting is the case where the two views are not of the same type. You could just erase their type by wrapping them in AnyView, but it does get a bit cumbersome and difficult to read at this point:
var user: User?
var body: some View {
user.map { AnyView(UserProfile(user: $0)) } ?? AnyView(Text("Not logged in"))
}
So in this case, my preferred approach is to use a custom IfLet struct (same name as Procrastin8's, but a different implementation and syntax) that allows me to write something like this, which I find very clear in intent and easy to type.
var user: User?
var body: some View {
IfLet(user) { UserProfile(user: $0) }
.else { Text("Not logged in") }
}
The code for my IfLet component is as follows.
struct IfLet<Wrapped, IfContent> : View where IfContent: View {
let optionalValue: Wrapped?
let contentBuilder: (Wrapped) -> IfContent
init(_ optionalValue: Wrapped?, @ViewBuilder contentBuilder: @escaping (Wrapped) -> IfContent) {
self.optionalValue = optionalValue
self.contentBuilder = contentBuilder
}
var body: some View {
optionalValue.map { contentBuilder($0) }
}
func `else`<ElseContent:View>(@ViewBuilder contentBuilder: @escaping () -> ElseContent) -> some View {
if optionalValue == nil {
return AnyView(contentBuilder())
} else {
return AnyView(self)
}
}
}
Enjoy!
if...else.@ViewBuilderscope so you can use normal control flow likeif let.