Distinguish ids that need to be unique and warn about name clashes

emigui/src/id.rs

@@ -1,10 +1,41 @@
+//! Emigui tracks widgets frame-to-frame using `Id`s.
+//!
+//! For instance, if you start dragging a slider one frame, emigui stores
+//! the sldiers Id as the current interact_id so that next frame when
+//! you move the mouse the same slider changes, even if the mouse has
+//! moved outside the slider.
+//!
+//! For some widgets `Id`s are also used to GUIpersist some state about the
+//! widgets, such as Window position or wether not a Foldable region is open.
+//!
+//! This implicated that the `Id`s must be unqiue.
+//!
+//! For simple things like sliders and buttons that don't have any memory and
+//! doesn't move we can use the location of the widget as a source of identity.
+//! For instance, a slider only needs a unique and persistent ID while you are
+//! dragging the sldier. As long as it is still while moving, that is fine.
+//!
+//! For things that need to persist state even after moving (windows, foldables)
+//! the location of the widgets is obviously not good enough. For instance,
+//! a fodlable region needs to remember wether or not it is open even
+//! if the layout next frame is different and the foldable is not lower down
+//! on the screen.
+//!
+//! Then there are widgets that need no identifiers at all, like labels,
+//! because they have no state nor are interacted with.
+//!
+//! So we have two type of Ids: PositionId and UniqueId.
+//! TODO: have separate types for PositionId and UniqueId.
+
 use std::{collections::hash_map::DefaultHasher, hash::Hash};
 
+use crate::math::Pos2;
+
 #[derive(Clone, Copy, Debug, Hash, Eq, PartialEq)]
 pub struct Id(u64);
 
 impl Id {
-    pub fn whole_screen() -> Self {
+    pub fn background() -> Self {
         Self(0)
     }
 
@@ -26,4 +57,10 @@ impl Id {
         child.hash(&mut hasher);
         Id(hasher.finish())
     }
+
+    pub fn from_pos(p: Pos2) -> Id {
+        let x = p.x.round() as i32;
+        let y = p.y.round() as i32;
+        Id::new(&x).with(&y)
+    }
 }