SwiftUI when can i use if statement in result builder?-CodePudding

I notice that sometimes I am able to use if-else statement in SwiftUI's result builder just fine. For example, inside a body:

struct MyView: View {
  var body: some View {
    if foo {
      Text("foo")
    } 
  }
}

However, sometimes I can't, for example, inside view modifier:

  func body(content: Content) -> some View {
    content
      .toolbar {
        if foo { Button("foo") {} }
      }
    }
  }

This above gives me error saying:

Closure containing control flow statement cannot be used with result builder 'ToolbarContentBuilder'

This is so confusing. Why are they different? When can I use if-else, and when it should be avoided (and in that case, why is it bad? and what should I do? )

Edit:

I tried to put the button under a Group {} under the toolbar. It's not working either:

      .toolbar {
        Group {
          if true { // can't put if here
            Button("foo")

Got this error:

Closure containing control flow statement cannot be used with result builder 'CommandsBuilder'

CodePudding user response：

SwiftUI uses ResultBuilder to build views. John Sundell has a good deep-dive article on them.

Whether you can use an if depends on the result builders used by the view and, specifically, whether it implements buildIf/buildOptional methods.

An implication of supporting if in a result builder is that there can be 0 views in the result.

For whatever reason, the implementers of toolbar have decided that there must be some content in a toolbar and the so result builder they have used ToolBarContentBuilder does not implement the required methods to support conditional statements.

You are right, it is a little confusing. Unfortunately the only ways to find out whether conditionals are supported in a particular context is try or to read through the documentation to find the result builder that an element uses.

CodePudding user response：

ToolbarContentBuilder does not allow if statements, it's designed to use ToolbarItemGroup, where you can use if like a regular ViewBuilder:

var body: some View {
    NavigationView {
        Text("Hello, world!")
            .toolbar {
                ToolbarItemGroup(placement: .navigationBarTrailing) {
                    if foo { Button("foo") {} }
                }
            }
    }
}