BranchPickerButton.swift

import SwiftUI

#if os(iOS)
import UIKit

struct BranchPickerButton: UIViewRepresentable {
    public var title: String = ""
    public let producer: () async throws -> [(String, String)]
    @Binding public var value: String
    
    func makeUIView(context: Context) -> UIButton {
        var config = UIButton.Configuration.plain()
        config.title = value.isEmpty ? NSLocalizedString("Default", comment: "") : value
        config.image = UIImage(systemName: "chevron.up.chevron.down")
        config.imagePlacement = .trailing
        config.imagePadding = 5
        config.preferredSymbolConfigurationForImage = .init(pointSize: 12, weight: .regular)
        config.titleAlignment = .trailing
        config.contentInsets = .zero

        let button = UIButton(configuration: config, primaryAction: nil)
        button.contentHorizontalAlignment = .trailing
        
        let defererred = UIDeferredMenuElement { [weak button] callback in
            Task {
                let items: [(String, String)]
                do {
                    items = try await producer()
                } catch {
                    let warning = UIAction(title: error.localizedDescription,
                                                image: UIImage(systemName: "exclamationmark.triangle"), handler: { _ in })
                    warning.attributes = .disabled
                    callback([warning])
                    return
                }
                
                var elements = [UIMenuElement]()
                for (title, value) in items {
                    let element = UIAction(title: title, handler: { _ in
                        button?.setTitle(title, for: .normal)
                        self.value = value
                    })
                    if(self.value == value) {
                        element.state = .on
                    }
                    elements.append(element)
                }
                callback(elements)
            }
        }
        
        button.showsMenuAsPrimaryAction = true
        button.menu = UIMenu(title: self.title, children: [defererred])
        return button
    }
    
    func updateUIView(_ button: UIButton, context: Context) {
        button.configuration?.title = value.isEmpty ? NSLocalizedString("Default", comment: "") : value
    }
}
#endif

#if os(macOS)

import AppKit

fileprivate class BranchPickerPopupButton : NSPopUpButton {
    var callback: (String) -> Void = { _ in }
    var producer: () async throws -> [(String, String)] = { [] }
    var value = ""
    
    @objc func picked(_ sender: NSMenuItem) {
        if let string = sender.representedObject as? String {
            value = string
            callback(string)
        }
    }
    
    @objc func refreshMenu() {
        guard let menu = menu else { return }
        
        // Populate the menu asynchronously
        Task {
            do {
                let items = try await producer()
                menu.removeAllItems()

                var selected: NSMenuItem?
                for (title, itemValue) in items {
                    let menuItem = NSMenuItem(title: title,
                                              action: #selector(BranchPickerPopupButton.picked),
                                              keyEquivalent: "")
                    menuItem.target = self
                    menuItem.representedObject = itemValue
                    if value == itemValue {
                        selected = menuItem
                    }
                    menu.addItem(menuItem)
                }
                select(selected)
                
            } catch {
                menu.removeAllItems()

                let warningItem = NSMenuItem(title: error.localizedDescription, action: nil, keyEquivalent: "")
                warningItem.image = NSImage(systemSymbolName: "exclamationmark.triangle", accessibilityDescription: nil)
                warningItem.isEnabled = false
                menu.addItem(warningItem)
            }
        }
    }
}

struct BranchPickerButton: NSViewRepresentable {
    var title: String = ""
    let producer: () async throws -> [(String, String)]
    @Binding var value: String
    
    func makeNSView(context: Context) -> NSPopUpButton {
        let popUpButton = BranchPickerPopupButton()
        popUpButton.callback = { value = $0 }
        popUpButton.producer = producer
        
        NotificationCenter.default.addObserver(popUpButton, selector:   #selector(BranchPickerPopupButton.refreshMenu),
                                               name: NSPopUpButton.willPopUpNotification,
                                               object: popUpButton)
        
        let deferredMenuElement = NSMenuItem()
        deferredMenuElement.title = title
        deferredMenuElement.submenu = NSMenu(title: title)
        popUpButton.menu = deferredMenuElement.submenu

        let title = value.isEmpty ? NSLocalizedString("Default", comment: "") : value
        let menuItem = NSMenuItem(title: title, action: nil, keyEquivalent: "")
        popUpButton.menu?.addItem(menuItem)
        
        return popUpButton
    }
    
    func updateNSView(_ nsView: NSPopUpButton, context: Context) {
        let button = nsView as? BranchPickerPopupButton
        button?.value = value
        
        let title = value.isEmpty ? NSLocalizedString("Default", comment: "") : value
        let menuItem = NSMenuItem(title: title, action: nil, keyEquivalent: "")
        button?.menu?.removeAllItems()
        button?.menu?.addItem(menuItem)
    }
    
}
#endif