start to allow runtime gui frontend changes

src/guiloop/glutinloop.rs

@@ -83,6 +83,8 @@ pub struct GuiEventLoop {
 const TICK_INTERVAL: Duration = Duration::from_millis(50);
 const MAX_POLL_LOOP_DURATION: Duration = Duration::from_millis(500);
 
+impl super::GuiSystem for GuiEventLoop {}
+
 impl GuiEventLoop {
     pub fn new() -> Result<Self, Error> {
         let event_loop = glium::glutin::EventsLoop::new();

src/guiloop/mod.rs

@@ -1,6 +1,26 @@
 use super::ExitStatus;
 use failure::Error;
 
+#[derive(Debug, Deserialize, Clone, Copy)]
+pub enum GuiSelection {
+    Glutin,
+    X11,
+}
+
+impl Default for GuiSelection {
+    fn default() -> Self {
+        if cfg!(feature = "force-glutin") {
+            GuiSelection::Glutin
+        } else if cfg!(all(unix, not(target_os = "macos"))) {
+            GuiSelection::X11
+        } else {
+            GuiSelection::Glutin
+        }
+    }
+}
+
+pub trait GuiSystem {}
+
 #[cfg(any(windows, feature = "force-glutin", target_os = "macos"))]
 mod glutinloop;
 

src/guiloop/x11.rs

@@ -71,6 +71,8 @@ const TOK_CORE: usize = 0xffff_ffff;
 const TOK_CHLD: usize = 0xffff_fffd;
 const TOK_XCB: usize = 0xffff_fffc;
 
+impl super::GuiSystem for GuiEventLoop {}
+
 impl GuiEventLoop {
     pub fn new() -> Result<Self, Error> {
         let poll = Poll::new()?;