subprocess-async.vala

public class Updater.Application : GLib.Application
{
    public override int command_line (ApplicationCommandLine command_line) {
      var dnf = new Subprocess (SubprocessFlags.NONE, "dnf", "install", "glib-2.0-devel");

      hold (); // hold until the operation completes

      dnf.wait_check_async (() => {
        try {
          dnf.wait_check_async.end ();
        } finally {
          release ();
        }
      });
      
      return 0;
    }
}

Update

void main () {
    var proc = new Subprocess (SubprocessFlags.NONE, "sh", "-c", "sleep 2; true");

    proc.wait_check_async.begin (null, (obj, res) => {
        try {
            proc.wait_check_async.end (res);
            print ("sucessful\n");
        } catch (Error e) {
            print ("error: %s\n", e.message);
        }
    });

    var loop = new MainLoop ();
    loop.run ();
}