As usually happens when I post a question to a forum, I more or less answered my own question. This problem is happening for any control which wraps NSButton or any class which extends NSButton, such as a Picker with a macOS default picker style which wraps NSPopUpButton.
SwiftUI Views which implement UI controls wrap the corresponding AppKit control -- Button wraps NSButton, Picker with default picker style wraps NSPopUpButton. The mechanism for this wrapping appears to be what's exposed to developers to wrap AppKit controls not covered by SwiftUI -- NSViewRepresentable. SwiftUI also appears to implement a default Coordinator instance to handle updates between the SwiftUI layer and the underlying AppKit control. This default Coordinator is mapped to the target field of the underlying NSButton.
When GarageBand loads the UI for an AudioUnit, it seems to try to add a selector to all of the NSButton instances in the UI to call a method called "lastClickedParameterView". Not surprisingly, this method is not implemented on the default SwiftUI Coordinator assigned to the target of any underlying NSButtons for a UI written using that framework. The selector refers to a method which doesn't exist, GarageBand doesn't handle the exception, and macOS aborts GarageBand. (GarageBand should handle this gracefully! But even more than that, GarageBand should not expect developers to implement methods which they couldn't possibly know about (it's not documented ANYWHERE), nor should it really try to attach functionality to code it knows nothing about!)
To mimic a Button, for the simplest cases, I was able to make my own out of a SwiftUI Group, but it's not that easy to do the same for a Picker with an underlying NSPopUpButton. It's a lot of work, which was frankly already done and provided by SwiftUI and AppKit, and I shouldn't have to reinvent the wheel.
Instead, I created a struct which implements NSViewRepresentable to wrap NSButton (and a separate struct to wrap NSPopUpButton), and of course, my own Coordinator classes to update SwiftUI bindings when the AppKit control updates. With my NSViewRepresentables in place, GarageBand does not crash!
The funny thing is, my Coordinators don't contain a method named "lastClickedParameterView"! Because I set the target of the underlying NSButton/NSPopUpButton to my Coordinator and the action to a selector referencing a method on my Coordinator, GarageBand either didn't or couldn't add its own selector to these NSButtons.
I'd really like to have any kind of hook in SwiftUI to the default Coordinator used in SwiftUI's wrappings of AppKit controls -- it would be great if I could extend the default Coordinator, add my own @objc public func's, and tell SwiftUI to use that class instead. Or pass a closure to a modifier, along with a "method name", so that any selector issued by an app hosting my UI would end up mapped to that closure.
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reindebock
Add a CommentThis actually works in GarageBand, by the way -- I defined my own View, IsleButton, to look and act exactly like a Button, and replaced my usages of Button with IsleButton:
import SwiftUIstruct IsleButton: View {var label: Stringvar action: () -> Void@State var backgroundColor = Color(red: 0.4, green: 0.4, blue: 0.4)var body: some View {Group {Text(label)}.padding(EdgeInsets(top: 2.0, leading: 10.0, bottom: 2.5, trailing: 10.0)).background(RoundedRectangle(cornerRadius: 5.0).fill(backgroundColor)).gesture(DragGesture(minimumDistance: 0).onChanged({ _ inbackgroundColor = Color.blueaction()}).onEnded({ _ inbackgroundColor = Color(red: 0.4, green: 0.4, blue: 0.4)}))}}import SwiftUIstruct SystemLevelView: View {@ObservedObject var patchBank: PatchBank@State var dialogDisplayed: Bool = falsevar body: some View {GeometryReader { parent inZStack {VStack {HStack {/Button("System-Wide Oscillators") {dialogDisplayed = true}/IsleButton(label: "System-Wide Oscillators", action: {dialogDisplayed = true})Spacer()/*Button("System-Wide Effects Send") {}*/IsleButton(label: "System-Wide Effects Send", action: {})}.padding(6.0)Spacer()}HStack {Spacer()VStack(alignment: .center) {ParameterSliderView(parameter: patchBank.gain, minMaxLabelToggle: .constant(false))ParameterSliderView(parameter: patchBank.tempo, minMaxLabelToggle: .constant(false))}.frame(width: 600.0)Spacer()}}}.sheet(isPresented: $dialogDisplayed) {SystemLevelPatchEditView(patchBank: patchBank, dialogDisplayed: $dialogDisplayed)}} `}Still, though, I should just be able to use Button, shouldn't I?