[SR-13188] Please support @ViewBuilder for properties

[twostraws]

Previous ID	SR-13188
Radar	None
Original Reporter	@twostraws
Type	New Feature
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	New Feature, FunctionBuilder, TypeChecker
Assignee	None
Priority	Medium

md5: 7cb2c7fde2a7f991ee63ebf387d335e3

Issue Description:

I would like you consider adding support for @ViewBuilder to be used with properties, so that the memberwise initializer Swift generates for structs automatically gains that function builder.

As an example, if I wanted to implement a custom SwiftUI VStack right now, I might start with this:

struct CustomVStack<Content: View>: View {
    let content: () -> Content

    var body: some View {
        VStack {
            // custom stuff here
            content()
        }
    }
}

However, that doesn't support @ViewBuilder, and so this kind of code would not work:

CustomVStack {
    Text("Hello")
    Text("Hello")
}

To fix this, I need to define a custom initializer:

init(@ViewBuilder content: @escaping () -> Content) {
    self.content = content
}

In this simple example it isn't a massive problem, but often that initializer has to copy in lots of values – it does exactly what the memberwise initializer did now just with @ViewBuilder for the single content property.

Ideally I'd like to be able to write something along these lines:

struct CustomVStack<Content: View>: View {
    @ViewBuilder let content: () -> Content

    var body: some View {
        VStack {
            // do stuff
            content()
        }
    }
}

Thank you!

[theblixguy]

Maybe we can make this work by allowing users to add the function builder attribute on the type declaration i.e. let foo: @BarBuilder () -> T and then cloning it to the corresponding member-wise initializer parameter.

[DougGregor]

Implementation in #34097 which is going through CI now