visibility: Centralize keyboard panel visibility policy and handling

With the policy being disentangled from application, it becomes testable.
This prepares for moving the entire visibility mechanism to the new class and taking away more pieces of ServerContextService.
In addition, this is a good warmup before trying to implement sizing policy.

src/ui_manager.rs

@@ -10,9 +10,49 @@
 use std::cmp::min;
 use ::outputs::c::OutputHandle;
 
-mod c {
+pub mod c {
     use super::*;
+    use std::os::raw::c_void;
     use ::util::c::Wrapped;
+    
+    /// ServerContextService*
+    #[repr(transparent)]
+    pub struct UIManager(*const c_void);
+
+    #[no_mangle]
+    extern "C" {
+        pub fn server_context_service_update_visible(imservice: *const UIManager, active: u32);
+        pub fn server_context_service_release_visibility(imservice: *const UIManager);
+    }
+
+    #[no_mangle]
+    pub extern "C"
+    fn squeek_visman_new() -> Wrapped<VisibilityManager> {
+        Wrapped::new(VisibilityManager {
+            ui_manager: None,
+            visibility_state: VisibilityFactors {
+                im_active: false,
+                physical_keyboard_present: false,
+            }
+        })
+    }
+    
+    /// Use to initialize the UI reference
+    #[no_mangle]
+    pub extern "C"
+    fn squeek_visman_set_ui(visman: Wrapped<VisibilityManager>, ui_manager: *const UIManager) {
+        let visman = visman.clone_ref();
+        let mut visman = visman.borrow_mut();
+        visman.set_ui_manager(Some(ui_manager))
+    }
+
+    #[no_mangle]
+    pub extern "C"
+    fn squeek_visman_set_keyboard_present(visman: Wrapped<VisibilityManager>, present: u32) {
+        let visman = visman.clone_ref();
+        let mut visman = visman.borrow_mut();
+        visman.set_keyboard_present(present != 0)
+    }
 
     #[no_mangle]
     pub extern "C"
@@ -79,3 +119,131 @@ impl Manager {
         }
     }
 }
+
+#[derive(PartialEq, Debug)]
+enum Visibility {
+    Hidden,
+    Visible,
+}
+
+#[derive(Debug)]
+enum VisibilityTransition {
+    /// Hide immediately
+    Hide,
+    /// Hide if no show request comes soon
+    Release,
+    /// Show instantly
+    Show,
+    /// Don't do anything
+    NoTransition,
+}
+
+/// Contains visibility policy
+#[derive(Clone, Debug)]
+struct VisibilityFactors {
+    im_active: bool,
+    physical_keyboard_present: bool,
+}
+
+impl VisibilityFactors {
+    /// Static policy.
+    /// Use when transitioning from an undefined state (e.g. no UI before).
+    fn desired(&self) -> Visibility {
+        match self {
+            VisibilityFactors {
+                im_active: true,
+                physical_keyboard_present: false,
+            } => Visibility::Visible,
+            _ => Visibility::Hidden,
+        }
+    }
+    /// Stateful policy
+    fn transition_to(&self, next: &Self) -> VisibilityTransition {
+        use self::Visibility::*;
+        let im_deactivation = self.im_active && !next.im_active;
+        match (self.desired(), next.desired(), im_deactivation) {
+            (Visible, Hidden, true) => VisibilityTransition::Release,
+            (Visible, Hidden, _) => VisibilityTransition::Hide,
+            (Hidden, Visible, _) => VisibilityTransition::Show,
+            _ => VisibilityTransition::NoTransition,
+        }
+    }
+}
+
+// Temporary struct for migration. Should be integrated with Manager eventually.
+pub struct VisibilityManager {
+    /// Owned reference. Be careful, it's shared with C at large
+    ui_manager: Option<*const c::UIManager>,
+    visibility_state: VisibilityFactors,
+}
+
+impl VisibilityManager {
+    fn set_ui_manager(&mut self, ui_manager: Option<*const c::UIManager>) {
+        let new = VisibilityManager {
+            ui_manager,
+            ..unsafe { self.clone() }
+        };
+        self.apply_changes(new);
+    }
+
+    fn apply_changes(&mut self, new: Self) {
+        if let Some(ui) = &new.ui_manager {
+            if self.ui_manager.is_none() {
+                // Previous state was never applied, so effectively undefined.
+                // Just apply the new one.
+                let new_state = new.visibility_state.desired();
+                unsafe {
+                    c::server_context_service_update_visible(
+                        *ui,
+                        (new_state == Visibility::Visible) as u32,
+                    );
+                }
+            } else {
+                match self.visibility_state.transition_to(&new.visibility_state) {
+                    VisibilityTransition::Hide => unsafe {
+                        c::server_context_service_update_visible(*ui, 0);
+                    },
+                    VisibilityTransition::Show => unsafe {
+                        c::server_context_service_update_visible(*ui, 1);
+                    },
+                    VisibilityTransition::Release => unsafe {
+                        c::server_context_service_release_visibility(*ui);
+                    },
+                    VisibilityTransition::NoTransition => {}
+                }
+            }
+        }
+        *self = new;
+    }
+
+    pub fn set_im_active(&mut self, im_active: bool) {
+        let new = VisibilityManager {
+            visibility_state: VisibilityFactors {
+                im_active,
+                ..self.visibility_state.clone()
+            },
+            ..unsafe { self.clone() }
+        };
+        self.apply_changes(new);
+    }
+
+    pub fn set_keyboard_present(&mut self, keyboard_present: bool) {
+        let new = VisibilityManager {
+            visibility_state: VisibilityFactors {
+                physical_keyboard_present: keyboard_present,
+                ..self.visibility_state.clone()
+            },
+            ..unsafe { self.clone() }
+        };
+        self.apply_changes(new);
+    }
+
+    /// The struct is not really safe to clone due to the ui_manager reference.
+    /// This is only a helper for getting desired visibility.
+    unsafe fn clone(&self) -> Self {
+        VisibilityManager {
+            ui_manager: self.ui_manager.clone(),
+            visibility_state: self.visibility_state.clone(),
+        }
+    }
+}