1

I'm attempting to build a fairly simple macro:

public struct PreviewWithBindingsMacro: ExpressionMacro {
    public static func expansion(
        of node: some FreestandingMacroExpansionSyntax,
        in context: some MacroExpansionContext
    ) -> ExprSyntax {
        guard let builder = node.trailingClosure?.trimmed else {
            fatalError("trailing closure expected")
        }

        let prefix = context.createUniqueName("View")

        let previews = try! StructDeclSyntax("struct \(raw: prefix)_Previews: PreviewProvider") {
            try VariableDeclSyntax("static var previews: some View") {
              StmtSyntax("PreviewView \(raw: builder)")
            }
        }

        return "\(raw: previews.formatted())"
    }
}

I wonder if im holding this right.. there are a couple of problems
Firstly, The test input is

#PreviewWithBindings { bindings in
    Text("Hello")
}

and the output is:

struct __macro_local_4ViewfMu__Previews: PreviewProvider {
    static var previews: some View {
        PreviewView { bindings in
            Text("  Hello")
        }
    }
}

Note the additional spaces in the text String literal. Am I constructing the struct and outputting correctly?

Secondly, literal issue aside no matter what I do xcode will not recognize my preview and open the preview pane.. is there something hardcoded into xcode to only look for blessed macros? copy/pasting my macro output allows the pane to appear but xcode must not look at it otherwise?

2

The problem is that you declared PreviewWithBindingsMacro as an ExpressionMacro, but the macro is generating a declaration. This probably throws the parser off in multiple ways and makes it generate a completely invalid tree. Change the macro to a DeclarationMacro and you should be fine. That way you can also return previews as DecSyntax(previews), without the need for another string interpolation.

3

Good point on using the wrong type! but after updating the issue persists :frowning_face:

4

There's also a possible bug, the compiler is saying:

Use of protocol 'PreviewProvider' as a type must be written 'any PreviewProvider'

Even though copy/pasting the expanded code works without issue.

There's a feedback at FB12326868 for anyone able to peek into it.

5

Can you share the macro package with a test case that reproduces the issue? I suspect there’s some other issue where a node is parsed as a different type than it should be.

1 Like
6

Yup, no problem.

@freestanding(declaration, names: arbitrary)
public macro MyMacro<Content: View>(@ViewBuilder _ content: () -> Content) -> Void = #externalMacro(module: "MyMacros", type: "MyMacro")

public struct MyMacro: DeclarationMacro {
    public static func expansion<Node: FreestandingMacroExpansionSyntax, Context: MacroExpansionContext>(
        of node: Node,
        in context: Context
    ) throws -> [DeclSyntax] {
        guard let builder = node.trailingClosure?.trimmed else { fatalError("trailing closure expected") }

        let prefix = context.createUniqueName("View")

        let previews = try StructDeclSyntax("struct \(raw: prefix)_Previews: PreviewProvider") {
            try VariableDeclSyntax("static var previews: some View") {
              StmtSyntax("Wrapper \(raw: builder)")
            }
        }

        return [DeclSyntax(previews)]
    }
}```

```swift
assertMacroExpansion(
    """
    #MyMacro {
        Text("Hello")
    }
    """,
    expandedSource: """
    struct __macro_local_4ViewfMu__Previews: PreviewProvider {
        static var previews: some View {
            Wrapper {
                Text("Hello")
            }
        }
    }
    """,
    macros: ["MyMacro": MyMacro.self]
)

the diff provided with the test failure is:

 struct __macro_local_4ViewfMu__Previews: PreviewProvider {
     static var previews: some View {
         Wrapper {
–            Text("Hello")
+            Text(" Hello")
         }
     }
 }
7

The call to wrapper is a function call expression and so it needs to be

ExprSyntax("Wrapper \(raw: builder)")
8

ah that worked. thanks!

I dont suppose you might know if its possible to make xcode recognize my code and open the preview pane (which I can't even do manually)

9

actually nevermind.. I forgot im blocked on the compiler bug :+1: