The `launch_passive` function launches the GUI viewer in a non-blocking manner, allowing the Python script or REPL to continue execution. The viewer is automatically kept up to date with any subsequent modifications to mjModel and mjData.
Note that when run inside a REPL (including IPython), `launch_passive` is functionally identical to `launch_repl`.
On Linux and Windows, this is achieved by spawning a new thread and launching the GUI there.
On macOS, this is not possible as all Cocoa API calls must be made on the "macOS main thread", which is always the first thread launched in a process and carries the `com.apple.main-thread` dispatch queue. We also cannot simply trampoline from a Python script on the main thread into the user's script on a side thread because CPython's signal handler can only be installed on the "Python main thread". Putting the user's script in a side thread means that it cannot e.g. gracefully handle SIGINT by catching a KeyboardInterrupt exception. To work around this, we ship a custom Python launcher on macOS called `mjpython`. This launcher is a native binary that spawns a pthread and initialize the Python interpreter on that thread, thus allowing "Python main thread" and "macOS main thread" to represent two distinct threads. From Python's point of view, the "macOS main thread" is a secondary thread that runs a loop that continuously empties a Queue of (mjModel, mjData) and launches a viewer.
PiperOrigin-RevId: 517167868
Change-Id: Icac9d2126bbb4760d47e0b9300e0a979cffa4338
Nimrod Gileadi