SwiftUI on Mac - How do I designate a button as being the primary?

macOS 11.0 / iOS 14:

As of Xcode 12 beta, new methods are exposed on Button() allowing assignment of keyEquivalent (either by enum case or explicit key and modifiers).

Setting as default:

Button( ... )
    .keyboardShortcut(.defaultAction)

Setting as cancel:

Button( ... )
    .keyboardShortcut(.cancelAction)

It's currently not possible. I have reported it to Apple.

However, for now, you can wrap NSButton.

Usage:

@available(macOS 10.15, *)
struct ContentView: View {
    var body: some View {
        NativeButton("Submit", keyEquivalent: .return) {
            // Some action
        }
            .padding()
    }
}

Implementation:

// MARK: - Action closure for controls

private var controlActionClosureProtocolAssociatedObjectKey: UInt8 = 0

protocol ControlActionClosureProtocol: NSObjectProtocol {
    var target: AnyObject? { get set }
    var action: Selector? { get set }
}

private final class ActionTrampoline<T>: NSObject {
    let action: (T) -> Void

    init(action: @escaping (T) -> Void) {
        self.action = action
    }

    @objc
    func action(sender: AnyObject) {
        action(sender as! T)
    }
}

extension ControlActionClosureProtocol {
    func onAction(_ action: @escaping (Self) -> Void) {
        let trampoline = ActionTrampoline(action: action)
        self.target = trampoline
        self.action = #selector(ActionTrampoline<Self>.action(sender:))
        objc_setAssociatedObject(self, &controlActionClosureProtocolAssociatedObjectKey, trampoline, .OBJC_ASSOCIATION_RETAIN)
    }
}

extension NSControl: ControlActionClosureProtocol {}

// MARK: -



@available(macOS 10.15, *)
struct NativeButton: NSViewRepresentable {
    enum KeyEquivalent: String {
        case escape = "\u{1b}"
        case `return` = "\r"
    }

    var title: String?
    var attributedTitle: NSAttributedString?
    var keyEquivalent: KeyEquivalent?
    let action: () -> Void

    init(
        _ title: String,
        keyEquivalent: KeyEquivalent? = nil,
        action: @escaping () -> Void
    ) {
        self.title = title
        self.keyEquivalent = keyEquivalent
        self.action = action
    }

    init(
        _ attributedTitle: NSAttributedString,
        keyEquivalent: KeyEquivalent? = nil,
        action: @escaping () -> Void
    ) {
        self.attributedTitle = attributedTitle
        self.keyEquivalent = keyEquivalent
        self.action = action
    }

    func makeNSView(context: NSViewRepresentableContext<Self>) -> NSButton {
        let button = NSButton(title: "", target: nil, action: nil)
        button.translatesAutoresizingMaskIntoConstraints = false
        button.setContentHuggingPriority(.defaultHigh, for: .vertical)
        button.setContentHuggingPriority(.defaultHigh, for: .horizontal)
        return button
    }

    func updateNSView(_ nsView: NSButton, context: NSViewRepresentableContext<Self>) {
        if attributedTitle == nil {
            nsView.title = title ?? ""
        }

        if title == nil {
            nsView.attributedTitle = attributedTitle ?? NSAttributedString(string: "")
        }

        nsView.keyEquivalent = keyEquivalent?.rawValue ?? ""

        nsView.onAction { _ in
            self.action()
        }
    }
}