How can a macOS command line app that launches a GUI also have an icon participate in the Alt-Tab functionality?

I'm experimenting with some low(ish) level macOS coding for fun and curiosity.

I've been able to create a commandline app that can conditionally also launch a GUI with a window or two. (I'm sure other GUI app functionality is missing too.)

But despite having the regular macOS windows I can't alt-tab to the app when it's in GUI mode. I'm unable to find any discussion on what is needed for this.

Must I call some Foundation/AppKit APIs? Pass some flags when opening my windows? Add something to my Info.plist? If it's the last option, is there also a way to programmatically do all/most of what the Info.plist does declaratively?

(Additional info: My app is extremely minimal, consisting of just one .swift file and one XCode project file. No resource files or Info.plist or anything else.)

Solution

I found the solution thanks to this question.

You can set the app's "activation policy" to "regular" before launching the GUI:

    NSApplication.shared.setActivationPolicy(.regular)
    NSApplication.shared.run()

As well as working with Alt+Tab the app will now appear in the dock, will have a (default, empty) menu, respects Command+` to cycle through its windows, and it gets a default app icon that looks a bit like the Terminal or Console icon but with exec written in its top left corner.

An undesirable side-effect is that debug info for the GUI is now output to the terminal. I'll have to look into how to prevent that:

1   HIToolbox           0x00000001902e05c8 _ZN15MenuBarInstance22EnsureAutoShowObserverEv + 120
2   HIToolbox           0x00000001902e0188 _ZN15MenuBarInstance14EnableAutoShowEv + 60
3   HIToolbox           0x0000000190283310 SetMenuBarObscured + 372
4   HIToolbox           0x0000000190282ee8 _ZN13HIApplication15HandleActivatedEP14OpaqueEventRefhP15OpaqueWindowPtrh + 172
5   HIToolbox           0x000000019027cfcc _ZN13HIApplication13EventObserverEjP14OpaqueEventRefPv + 296
6   HIToolbox           0x0000000190243cd0 _NotifyEventLoopObservers + 176
7   HIToolbox           0x000000019027c96c AcquireEventFromQueue + 432
8   HIToolbox           0x000000019026be0c ReceiveNextEventCommon + 712
9   HIToolbox           0x000000019026bb2c _BlockUntilNextEventMatchingListInModeWithFilter + 72
10  AppKit              0x0000000189e18424 _DPSNextEvent + 632
11  AppKit              0x0000000189e175b4 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 728
12  AppKit              0x0000000189e0b9e4 -[NSApplication run] + 464
13  macos-cond-gui-2    0x0000000104cd0764 main + 2148
14  dyld                0x000000018678fe50 start + 2544