ecs: add relations, improve docs

lyra-ecs/src/archetype.rs

@@ -1,4 +1,4 @@
-use std::{ptr::{NonNull, self}, alloc::{self, Layout, alloc, dealloc}, mem, collections::HashMap, cell::{RefCell, Ref, RefMut, BorrowError, BorrowMutError}, ops::{DerefMut, Deref}};
+use std::{ptr::{NonNull, self}, alloc::{self, Layout, alloc, dealloc}, mem, collections::HashMap, cell::{RefCell, Ref, RefMut, BorrowError, BorrowMutError}, ops::DerefMut};
 
 use crate::{world::ArchetypeEntityId, bundle::Bundle, component_info::ComponentInfo, DynTypeId, Tick, Entity};
 
@@ -81,14 +81,15 @@ impl ComponentColumn {
     /// # Safety
     /// 
     /// This column MUST have the entity. If it does not, it WILL NOT panic and will cause UB.
-    pub unsafe fn get<T>(&self, entity_index: usize) -> &T {
+    pub unsafe fn get<T>(&self, entity_index: usize) -> Ref<T> {
         let data = self.data.borrow();
-        let data = data.deref();
 
-        let ptr = NonNull::new_unchecked(data.as_ptr()
+        Ref::map(data, |data| {
-            .add(entity_index * self.info.layout.size))
+            let ptr = NonNull::new_unchecked(data.as_ptr()
-            .cast();
+                .add(entity_index * self.info.layout.size))
-        &*ptr.as_ptr()
+                .cast();
+            &*ptr.as_ptr()
+        })
     }
 
     /// Get a component at an entities index.
@@ -411,11 +412,15 @@ impl Archetype {
     pub fn entity_of_index(&self, id: ArchetypeEntityId) -> Option<Entity> {
         self.ids_to_entity.get(&id).cloned()
     }
+
+    pub fn has_entity(&self, e: Entity) -> bool {
+        self.entities.contains_key(&e)
+    }
 }
 
 #[cfg(test)]
 mod tests {
-    use std::{alloc::Layout, ptr::NonNull};
+    use std::{alloc::Layout, cell::Ref, ptr::NonNull};
 
     use rand::Rng;
 
@@ -435,7 +440,7 @@ mod tests {
         let entity_arch_id = a.add_entity(entity, bundle, &Tick::default());
 
         let col = a.columns.get(0).unwrap();
-        let vec2: &Vec2 = unsafe { col.get(entity_arch_id.0 as usize) };
+        let vec2: Ref<Vec2> = unsafe { col.get(entity_arch_id.0 as usize) };
         assert_eq!(vec2.clone(), bundle.0);
     }
 
@@ -451,11 +456,11 @@ mod tests {
         let entity_arch_id = a.add_entity(entity, bundle, &Tick::default());
 
         let col = a.columns.get(0).unwrap();
-        let vec2: &Vec2 = unsafe { col.get(entity_arch_id.0 as usize) };
+        let vec2: Ref<Vec2> = unsafe { col.get(entity_arch_id.0 as usize) };
         assert_eq!(vec2.clone(), bundle.0);
 
         let col = a.columns.get(1).unwrap();
-        let vec3: &Vec3 = unsafe { col.get(entity_arch_id.0 as usize) };
+        let vec3: Ref<Vec3> = unsafe { col.get(entity_arch_id.0 as usize) };
         assert_eq!(vec3.clone(), bundle.1);
     }
 
@@ -477,9 +482,9 @@ mod tests {
         let earch2 = a.add_entity(e2, b2, &Tick::default());
 
         let col = a.columns.get(0).unwrap();
-        let vec2: &Vec2 = unsafe { col.get(earch1.0 as usize) };
+        let vec2: Ref<Vec2> = unsafe { col.get(earch1.0 as usize) };
         assert_eq!(vec2.clone(), b1.0);
-        let vec2: &Vec2 = unsafe { col.get(earch2.0 as usize) };
+        let vec2: Ref<Vec2> = unsafe { col.get(earch2.0 as usize) };
         assert_eq!(vec2.clone(), b2.0);
     }
 
@@ -501,15 +506,15 @@ mod tests {
         let earch2 = a.add_entity(e2, b2, &Tick::default());
 
         let col = a.columns.get(0).unwrap();
-        let vec2: &Vec2 = unsafe { col.get(earch1.0 as usize) };
+        let vec2: Ref<Vec2> = unsafe { col.get(earch1.0 as usize) };
         assert_eq!(vec2.clone(), b1.0);
-        let vec2: &Vec2 = unsafe { col.get(earch2.0 as usize) };
+        let vec2: Ref<Vec2> = unsafe { col.get(earch2.0 as usize) };
         assert_eq!(vec2.clone(), b2.0);
 
         let col = a.columns.get(1).unwrap();
-        let vec3: &Vec3 = unsafe { col.get(earch1.0 as usize) };
+        let vec3: Ref<Vec3> = unsafe { col.get(earch1.0 as usize) };
         assert_eq!(vec3.clone(), b1.1);
-        let vec3: &Vec3 = unsafe { col.get(earch2.0 as usize) };
+        let vec3: Ref<Vec3> = unsafe { col.get(earch2.0 as usize) };
         assert_eq!(vec3.clone(), b2.1);
     }
 
@@ -552,7 +557,7 @@ mod tests {
 
         let col = a.columns.get(0).unwrap();
         for i in 0..bundle_count {
-            let vec2: &Vec2 = unsafe { col.get(i) };
+            let vec2: Ref<Vec2> = unsafe { col.get(i) };
             assert_eq!(vec2.clone(), bundles[i].0);
         }
     }

lyra-ecs/src/command.rs

@@ -114,7 +114,7 @@ pub fn execute_deferred_commands(world: &mut World, mut commands: RefMut<Command
 
 #[cfg(test)]
 mod tests {
-    use std::ptr::NonNull;
+    use std::{cell::Ref, ptr::NonNull};
 
     use crate::{system::{GraphExecutor, IntoSystem}, tests::Vec2, Commands, DynTypeId, World};
 
@@ -144,7 +144,7 @@ mod tests {
         // there's only one archetype
         let arch = world.archetypes.values().next().unwrap();
         let col = arch.get_column(DynTypeId::of::<Vec2>()).unwrap();
-        let vec2: &Vec2 = unsafe { col.get(3) };
+        let vec2: Ref<Vec2> = unsafe { col.get(3) };
         assert_eq!(vec2.clone(), spawned_vec);
     }
 }

lyra-ecs/src/lib.rs

@@ -7,39 +7,42 @@ pub(crate) mod lyra_engine {
     }
 }
 
-pub mod archetype;
 use std::ops::BitOr;
 
+mod archetype;
 pub use archetype::*;
 
-pub mod entity;
+mod entity;
 pub use entity::*;
 
-pub mod world;
+mod world;
 pub use world::*;
 
-pub mod command;
+mod command;
 pub use command::*;
 
-pub mod bundle;
+mod bundle;
 pub use bundle::*;
 
-pub mod component;
+mod component;
 pub use component::*;
 
 pub mod query;
 //pub use query::*;
 
-pub mod component_info;
+mod relation;
+pub use relation::Relation;
+
+mod component_info;
 pub use component_info::*;
 
-pub mod resource;
+mod resource;
 pub use resource::*;
 
 pub mod system;
 //pub use system::*;
 
-pub mod tick;
+mod tick;
 pub use tick::*;
 
 /// Implements Component for glam math types

lyra-ecs/src/query/borrow.rs

@@ -221,7 +221,7 @@ impl<T: Component> AsQuery for &mut T {
 mod tests {
     use std::{mem::size_of, marker::PhantomData, ptr::NonNull};
 
-    use crate::{archetype::{Archetype, ArchetypeId}, bundle::Bundle, query::{Fetch, View}, tests::Vec2, world::World, DynTypeId, Entity, EntityId, Tick};
+    use crate::{archetype::{Archetype, ArchetypeId}, bundle::Bundle, query::{Fetch, ViewState}, tests::Vec2, world::World, DynTypeId, Entity, EntityId, Tick};
 
     use super::{QueryBorrow, QueryBorrowMut, FetchBorrowMut};
 
@@ -244,7 +244,7 @@ mod tests {
             _phantom: std::marker::PhantomData,
         };
         let archetypes: Vec<&Archetype> = world.archetypes.values().collect();
-        let v = View::<QueryBorrow<Vec2>>::new(&world, borrow, archetypes);
+        let v = ViewState::<QueryBorrow<Vec2>, ()>::new(&world, borrow, (), archetypes);
 
         for e in v.into_iter() {
             println!("Found entity at {:?}", e);
@@ -260,7 +260,7 @@ mod tests {
             _phantom: std::marker::PhantomData,
         };
         let archetypes: Vec<&Archetype> = world.archetypes.values().collect();
-        let v = View::<QueryBorrowMut<Vec2>>::new(&world, borrow, archetypes);
+        let v = ViewState::<QueryBorrowMut<Vec2>, ()>::new(&world, borrow, (), archetypes);
 
         let mut orig = vec![];
 
@@ -277,7 +277,7 @@ mod tests {
             _phantom: std::marker::PhantomData,
         };
         let archetypes: Vec<&Archetype> = world.archetypes.values().collect();
-        let v = View::<QueryBorrow<Vec2>>::new(&world, borrow, archetypes);
+        let v = ViewState::<QueryBorrow<Vec2>, ()>::new(&world, borrow, (), archetypes);
 
         for (new, orig) in v.into_iter().zip(orig.iter()) {
             assert!(new.x - orig.x == 10.0);

lyra-ecs/src/query/dynamic/mod.rs

@@ -2,7 +2,7 @@ use std::ptr::NonNull;
 
 use crate::{world::World, ComponentColumn, ComponentInfo};
 
-pub mod view;
+mod view;
 pub use view::*;
 
 use super::Fetch;

lyra-ecs/src/query/mod.rs

@@ -3,27 +3,27 @@ use crate::{archetype::Archetype, world::{ArchetypeEntityId, World}, Tick};
 pub mod view;
 pub use view::*;
 
-pub mod entities;
+mod entities;
 #[allow(unused_imports)]
 pub use entities::*;
 
-pub mod borrow;
+mod borrow;
 #[allow(unused_imports)]
 pub use borrow::*;
 
-pub mod tuple;
+mod tuple;
 #[allow(unused_imports)]
 pub use tuple::*;
 
-pub mod resource;
+mod resource;
 #[allow(unused_imports)]
 pub use resource::*;
 
-pub mod tick;
+mod tick;
 #[allow(unused_imports)]
 pub use tick::*;
 
-pub mod world;
+mod world;
 #[allow(unused_imports)]
 pub use world::*;
 
@@ -87,11 +87,45 @@ pub trait IntoQuery {
     fn into_query(self) -> Self;
 }
 
+impl<'a> Fetch<'a> for () {
+    type Item = ();
+
+    fn dangling() -> Self {
+        unreachable!()
+    }
+
+    unsafe fn get_item(&mut self, entity: ArchetypeEntityId) -> Self::Item {
+        ()
+    }
+}
+
+impl Query for () {
+    type Item<'a> = ();
+
+    type Fetch<'a> = ();
+
+    fn new() -> Self {
+        ()
+    }
+
+    fn can_visit_archetype(&self, archetype: &Archetype) -> bool {
+        true
+    }
+
+    unsafe fn fetch<'a>(&self, world: &'a World, archetype: &'a Archetype, tick: Tick) -> Self::Fetch<'a> {
+        ()
+    }
+}
+
+impl AsQuery for () {
+    type Query = ();
+}
+
 #[cfg(test)]
 mod tests {
     use crate::{world::World, archetype::Archetype, tests::Vec2};
 
-    use super::{View, Entities};
+    use super::{ViewState, Entities};
 
     #[test]
     fn simple_view() {
@@ -104,7 +138,7 @@ mod tests {
 
         let archetypes: Vec<&Archetype> = world.archetypes.values().collect();
 
-        let v = View::<Entities>::new(&world, entities, archetypes);
+        let v = ViewState::<Entities, ()>::new(&world, entities, (), archetypes);
 
         for e in v.into_iter() {
             println!("Got entity! {:?}", e);

lyra-ecs/src/query/tuple.rs

@@ -2,6 +2,50 @@ use crate::world::World;
 
 use super::{Query, Fetch, AsQuery};
 
+impl<'a, F1> Fetch<'a> for (F1,)
+where
+    F1: Fetch<'a>,
+{
+    type Item = (F1::Item,);
+
+    fn dangling() -> Self {
+        (F1::dangling(),)
+    }
+
+    fn can_visit_item(&mut self, entity: crate::world::ArchetypeEntityId) -> bool {
+        let (f1,) = self;
+        f1.can_visit_item(entity)
+    }
+    
+    unsafe fn get_item(&mut self, entity: crate::world::ArchetypeEntityId) -> Self::Item {
+        let (f1,) = self;
+        ( f1.get_item(entity), )
+    }
+}
+
+impl<Q1> Query for (Q1,)
+where
+    Q1: Query,
+{
+    type Item<'a> = (Q1::Item<'a>,);
+
+    type Fetch<'a> = (Q1::Fetch<'a>,);
+
+    fn new() -> Self {
+        (Q1::new(),)
+    }
+
+    fn can_visit_archetype(&self, archetype: &crate::archetype::Archetype) -> bool {
+        let (q1,) = self;
+        q1.can_visit_archetype(archetype)
+    }
+
+    unsafe fn fetch<'a>(&self, world: &'a World, archetype: &'a crate::archetype::Archetype, tick: crate::Tick) -> Self::Fetch<'a> {
+        let (q1,) = self;
+        ( q1.fetch(world, archetype, tick), )
+    }
+}
+
 // Technically all of these implementations for a 2-sized tuple
 // can be implemented by the macro near the end of the file, but
 // these are left here for development.

lyra-ecs/src/query/view.rs

@@ -4,41 +4,64 @@ use crate::{archetype::Archetype, world::{ArchetypeEntityId, World}, EntityId, T
 
 use super::{Query, Fetch, AsQuery};
 
-pub struct View<'a, Q: AsQuery> {
+pub type View<'a, Q, F = ()> = ViewState<'a, <Q as AsQuery>::Query, <F as AsQuery>::Query>;
+
+pub struct ViewState<'a, Q: Query, F: Query> {
     world: &'a World,
-    query: Q::Query,
+    query: Q,
+    filter: F,
     archetypes: Vec<&'a Archetype>,
 }
 
-impl<'a, Q> View<'a, Q>
+impl<'a, Q, F> ViewState<'a, Q, F>
 where
-    Q: AsQuery,
+    Q: Query,
+    F: Query,
 {
-    pub fn new(world: &'a World, query: Q::Query, archetypes: Vec<&'a Archetype>) -> Self {
+    pub fn new(world: &'a World, query: Q, filter: F, archetypes: Vec<&'a Archetype>) -> Self {
         Self {
             world,
             query,
+            filter,
             archetypes,
         }
     }
+
+    /// Converts self into an iterator
+    pub fn iter(self) -> ViewIter<'a, Q, F> {
+        self.into_iter()
+    }
+
+    /// Consumes `self`, adding a query to the view.
+    pub fn expand<U: AsQuery>(self, query: U::Query) -> ViewState<'a, (Q, U::Query), F> {
+        ViewState::new(self.world, (self.query, query), self.filter, self.archetypes)
+    }
+
+    /// Consumes `self`, adding a filter to the view.
+    pub fn with<U: AsQuery>(self, filter: U::Query) -> ViewState<'a, Q, (F, U::Query)> {
+        ViewState::new(self.world, self.query, (self.filter, filter), self.archetypes)
+    }
 }
 
-impl<'a, Q> IntoIterator for View<'a, Q>
+impl<'a, Q, F> IntoIterator for ViewState<'a, Q, F>
 where
-    Q: AsQuery,
+    Q: Query,
+    F: Query,
 {
-    type Item = <Q::Query as Query>::Item<'a>;
+    type Item = Q::Item<'a>;
 
-    type IntoIter = ViewIter<'a, Q::Query>;
+    type IntoIter = ViewIter<'a, Q, F>;
 
     fn into_iter(self) -> Self::IntoIter {
-        let tick = self.world.tick_tracker().tick_when(Q::Query::MUTATES);
+        let tick = self.world.tick_tracker().tick_when(Q::MUTATES);
 
         ViewIter {
             world: self.world,
             tick,
             query: self.query,
+            filter: self.filter,
             fetcher: None,
+            filter_fetcher: None,
             archetypes: self.archetypes,
             next_archetype: 0,
             component_indices: 0..0,
@@ -46,25 +69,28 @@ where
     }
 }
 
-pub struct ViewIter<'a, Q: Query> {
+pub struct ViewIter<'a, Q: Query, F: Query> {
     world: &'a World,
     tick: Tick,
     query: Q,
+    filter: F,
     fetcher: Option<Q::Fetch<'a>>,
+    filter_fetcher: Option<F::Fetch<'a>>,
     archetypes: Vec<&'a Archetype>,
     next_archetype: usize,
     component_indices: Range<u64>,
 }
 
-impl<'a, Q> Iterator for ViewIter<'a, Q>
+impl<'a, Q, F> Iterator for ViewIter<'a, Q, F>
 where
     Q: Query,
+    F: Query,
 {
     type Item = Q::Item<'a>;
 
     fn next(&mut self) -> Option<Self::Item> {
         loop {
-            if Q::ALWAYS_FETCHES {
+            if Q::ALWAYS_FETCHES && F::ALWAYS_FETCHES {
                 // only fetch this query once.
                 // fetcher gets set to Some after this `next` call.
                 if self.fetcher.is_none() {
@@ -80,10 +106,10 @@ where
 
             if let Some(entity_index) = self.component_indices.next() {
                 let fetcher = self.fetcher.as_mut().unwrap();
+                let filter_fetcher = self.filter_fetcher.as_mut().unwrap();
                 let entity_index = ArchetypeEntityId(entity_index);
-                if !fetcher.can_visit_item(entity_index) {
+
-                    continue;
+                if fetcher.can_visit_item(entity_index) && filter_fetcher.can_visit_item(entity_index) {
-                } else {
                     let i = unsafe { fetcher.get_item(entity_index) };
                     return Some(i);
                 }
@@ -100,12 +126,13 @@ where
                     continue;
                 }
 
-                if !self.query.can_visit_archetype(arch) {
+                if self.query.can_visit_archetype(arch) && self.filter.can_visit_archetype(arch) {
-                    continue;
+                    unsafe {
+                        self.fetcher = Some(self.query.fetch(self.world, arch, self.tick));
+                        self.filter_fetcher = Some(self.filter.fetch(self.world, arch, self.tick));
+                    }
+                    self.component_indices = 0..arch.entities.len() as u64;
                 }
-
-                self.fetcher = unsafe { Some(self.query.fetch(self.world, arch, self.tick)) };
-                self.component_indices = 0..arch.entities.len() as u64;
             }
         }
     }

lyra-ecs/src/relation/mod.rs

@@ -0,0 +1,175 @@
+use std::marker::PhantomData;
+
+use lyra_ecs_derive::Component;
+
+use crate::query::Query;
+use crate::query::ViewState;
+use crate::Entity;
+
+use crate::lyra_engine;
+use crate::World;
+
+mod relates_to;
+#[doc(hidden)]
+pub use relates_to::*;
+
+mod relate_pair;
+#[doc(hidden)]
+pub use relate_pair::*;
+
+pub trait Relation: 'static {
+    /// called when a relation of this type is set on a target
+    fn relation_add(&self, origin: Entity, target: Entity) { }
+    /// called when a relation is removed
+    fn relation_remove(&self, origin: Entity, target: Entity) { }
+}
+
+/// A component that stores the target of a relation.
+/// 
+/// This component is on the origin of the relation and can be used to find all 
+/// entities that the relation targets.
+#[derive(Component)]
+pub struct RelationOriginComponent<R: Relation> {
+    pub(crate) relation: R,
+    target: Entity,
+}
+
+/// A component that stores the origin of a relation.
+/// 
+/// This component is on the target of the relation and can be used to find the
+/// origin of the relation -- the entity that targets this entity.
+#[derive(Component)]
+pub struct RelationTargetComponent<R: Relation> {
+    origin: Entity,
+    _marker: PhantomData<R>,
+}
+
+impl World {
+    /// Creates a relation between two entities. 
+    /// 
+    /// ```compile_fail
+    /// struct ChildOf;
+    /// 
+    /// impl Relation for ChildOf { /* snip */ };
+    /// 
+    /// let a = world.spawn((Vec2 { x: 10.0, y: 20.0 },));
+    /// let b = world.spawn((Vec2 { x: 158.0, y: 65.0 },));
+    /// 
+    /// // Entity a is a child of entity b
+    /// world.add_relation(a, ChildOf, b);
+    /// 
+    /// // Construct a view for querying the entities that have a relation to entity `b`.
+    /// let v = world.view::<Entities>()
+    ///     .relates_to::<ChildOf>(b);
+    /// 
+    /// // Iterate through the entities. ChildOf has no members, so it will be unused.
+    /// for (e, _childof) in v.into_iter() {
+    ///     println!("{:?} is a child of {:?}", e, b);
+    /// }
+    /// ```
+    pub fn add_relation<R>(&mut self, origin: Entity, relation: R, target: Entity)
+    where
+        R: Relation
+    {
+        let comp = RelationTargetComponent {
+            origin,
+            _marker: PhantomData::<R>,
+        };
+        self.insert(target, comp);
+
+        let comp = RelationOriginComponent {
+            relation,
+            target,
+        };
+        
+        comp.relation.relation_add(origin, target);
+        self.insert(origin, comp);
+    }
+}
+impl<'a, Q, F> ViewState<'a, Q, F>
+where
+    Q: Query,
+    F: Query,
+{
+    /// Consumes `self` to return a view that fetches the relation to a specific target entity.
+    pub fn relates_to<R>(self, target: Entity) -> ViewState<'a, (Q, QueryRelatesTo<R>), F>
+    where
+        R: Relation,
+    {
+        let rel = QueryRelatesTo::new(Some(target));
+        self.expand::<RelatesTo<R>>(rel)
+    }
+
+    /// Consumes `self` to return a view that fetches the origin, target, and a reference to
+    /// the relation that the entities have together.
+    pub fn relate_pair<R>(self) -> ViewState<'a, (Q, QueryRelatePair<R>), F>
+    where
+        R: Relation,
+    {
+        let rel = QueryRelatePair::<R>::new();
+        self.expand::<RelatePair<R>>(rel)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{query::{Entities, ViewState}, relation::QueryRelatesTo, tests::Vec2, World};
+
+    use super::{RelatePair, Relation};
+
+    struct ChildOf;
+
+    impl Relation for ChildOf {
+
+    }
+
+    #[test]
+    fn simple_relates_to() {
+        let mut world = World::new();
+
+        let a = world.spawn((Vec2::new(10.0, 20.0),));
+        let b = world.spawn((Vec2::new(158.0, 65.0),));
+
+        world.add_relation(a, ChildOf, b);
+
+        let archetypes = world.archetypes.values().collect();
+        let v = ViewState::<(Entities, QueryRelatesTo<ChildOf>), ()>::new(&world, (Entities::default(), QueryRelatesTo::new(Some(b))), (), archetypes);
+
+        for (e, _relation) in v.into_iter() {
+            println!("{:?} is a child of {:?}", e, b);
+        };
+    }
+
+    #[test]
+    fn simple_relates_to_view() {
+        let mut world = World::new();
+
+        let a = world.spawn((Vec2::new(10.0, 20.0),));
+        let b = world.spawn((Vec2::new(158.0, 65.0),));
+
+        world.add_relation(a, ChildOf, b);
+
+        let v = world.view::<Entities>()
+            .relates_to::<ChildOf>(b);
+
+        for (e, _rel) in v.into_iter() {
+            println!("{:?} is a child of {:?}", e, b);
+        }
+    }
+
+    #[test]
+    fn simple_relate_pair_view() {
+        let mut world = World::new();
+
+        let a = world.spawn((Vec2::new(10.0, 20.0),));
+        let b = world.spawn((Vec2::new(158.0, 65.0),));
+
+        world.add_relation(a, ChildOf, b);
+
+        let v = world.view::<RelatePair<ChildOf>>();
+
+        for (origin, _childof, target) in v.into_iter() {
+            println!("{:?} is a child of {:?}", origin, target);
+        }
+    }
+}

lyra-ecs/src/relation/relate_pair.rs

@@ -0,0 +1,95 @@
+use std::{any::TypeId, cell::Ref, marker::PhantomData};
+
+use crate::{query::{AsQuery, Fetch, Query}, Archetype, ComponentColumn, Entity, World};
+
+use super::{Relation, RelationOriginComponent};
+
+pub struct FetchRelatePair<'a, T> {
+    col: &'a ComponentColumn,
+    arch: &'a Archetype,
+    _phantom: PhantomData<&'a T>
+}
+
+impl<'a, R> Fetch<'a> for FetchRelatePair<'a, R>
+where
+    R: Relation,
+{
+    type Item = (Entity, Ref<'a, R>, Entity);
+
+    fn dangling() -> Self {
+        unreachable!()
+    }
+
+    unsafe fn get_item(&mut self, entity: crate::world::ArchetypeEntityId) -> Self::Item {
+        let comp: Ref<RelationOriginComponent<R>> = self.col.get(entity.0 as usize);
+        let rel_target = comp.target;
+        let rel_origin = self.arch.entity_of_index(entity).unwrap();
+
+        let comp = Ref::map(comp, |r| &r.relation);
+        (rel_origin, comp, rel_target)
+    }
+}
+
+pub struct QueryRelatePair<R> {
+    _marker: PhantomData<R>,
+}
+
+impl<R> Copy for QueryRelatePair<R> {}
+
+impl<R> Clone for QueryRelatePair<R> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<R> QueryRelatePair<R> {
+    pub fn new() -> Self {
+        Self {
+            _marker: PhantomData
+        }
+    }
+}
+
+impl<R> Query for QueryRelatePair<R>
+where
+    R: Relation + 'static
+{
+    type Item<'a> = (Entity, Ref<'a, R>, Entity);
+
+    type Fetch<'a> = FetchRelatePair<'a, R>;
+
+    fn new() -> Self {
+        QueryRelatePair::<R>::new()
+    }
+
+    fn can_visit_archetype(&self, archetype: &crate::archetype::Archetype) -> bool {
+        let tyid = crate::DynTypeId::Rust(TypeId::of::<RelationOriginComponent<R>>());
+        archetype.has_column(tyid)
+    }
+
+    unsafe fn fetch<'a>(&self, _world: &'a World, archetype: &'a crate::archetype::Archetype, tick: crate::Tick) -> Self::Fetch<'a> {
+        let _ = tick;
+        let tyid = crate::DynTypeId::Rust(TypeId::of::<RelationOriginComponent<R>>());
+        let col = archetype.columns.iter().find(|c| c.info.type_id == tyid)
+            .expect("You ignored 'can_visit_archetype'!");
+
+        FetchRelatePair {
+            col,
+            arch: archetype,
+            _phantom: PhantomData,
+        }
+    }
+}
+
+/// A query that fetches the origin, and target of a relation of type `R`.
+/// 
+/// It provides it as a tuple in the following format: `(origin, relation, target)`.
+/// Similar to [`RelatesTo`](super::RelatesTo), you can use [`ViewState::relate_pair`] to get a view that fetches the
+/// pair, or unlike [`RelatesTo`](super::RelatesTo), you can do the common procedure of using [`World::view`].
+pub struct RelatePair<R: Relation> {
+    _marker: PhantomData<R>,
+}
+
+impl<R: Relation> AsQuery for RelatePair<R> {
+    type Query = QueryRelatePair<R>;
+}

lyra-ecs/src/relation/relates_to.rs

@@ -0,0 +1,111 @@
+use std::{any::TypeId, cell::Ref, marker::PhantomData};
+
+use crate::{query::{AsQuery, Fetch, Query}, ComponentColumn, Entity, World};
+
+use super::{Relation, RelationOriginComponent};
+
+pub struct FetchRelatesTo<'a, T> {
+    col: &'a ComponentColumn,
+    target: Entity,
+    _phantom: PhantomData<&'a T>
+}
+
+impl<'a, R> Fetch<'a> for FetchRelatesTo<'a, R>
+where
+    R: Relation,
+{
+    type Item = Ref<'a, R>;
+
+    fn dangling() -> Self {
+        unreachable!()
+    }
+
+    fn can_visit_item(&mut self, entity: crate::ArchetypeEntityId) -> bool {
+        unsafe {
+            let comp: Ref<RelationOriginComponent<R>> = self.col.get(entity.0 as usize);
+            comp.target == self.target
+        }
+    }
+
+    unsafe fn get_item(&mut self, entity: crate::world::ArchetypeEntityId) -> Self::Item {
+        let comp: Ref<RelationOriginComponent<R>> = self.col.get(entity.0 as usize);
+        let comp = Ref::map(comp, |r| &r.relation);
+        comp
+    }
+}
+
+pub struct QueryRelatesTo<R> {
+    target: Option<Entity>,
+    _marker: PhantomData<R>,
+}
+
+impl<R> Copy for QueryRelatesTo<R> {}
+
+impl<R> Clone for QueryRelatesTo<R> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<R> QueryRelatesTo<R> {
+    pub fn new(target: Option<Entity>) -> Self {
+        Self {
+            target,
+            _marker: PhantomData
+        }
+    }
+}
+
+impl<R> Query for QueryRelatesTo<R>
+where
+    R: Relation + 'static
+{
+    type Item<'a> = Ref<'a, R>;
+
+    type Fetch<'a> = FetchRelatesTo<'a, R>;
+
+    fn new() -> Self {
+        panic!("RelatesTo MUST be made with View::relates_to since it requires State provided by \
+            that function.")
+    }
+
+    fn can_visit_archetype(&self, archetype: &crate::archetype::Archetype) -> bool {
+        let tyid = crate::DynTypeId::Rust(TypeId::of::<RelationOriginComponent<R>>());
+        archetype.has_column(tyid)
+    }
+
+    unsafe fn fetch<'a>(&self, _world: &'a World, archetype: &'a crate::archetype::Archetype, tick: crate::Tick) -> Self::Fetch<'a> {
+        let _ = tick;
+        let tyid = crate::DynTypeId::Rust(TypeId::of::<RelationOriginComponent<R>>());
+        let col = archetype.columns.iter().find(|c| c.info.type_id == tyid)
+            .expect("You ignored 'can_visit_archetype'!");
+
+        FetchRelatesTo {
+            col,
+            target: self.target.expect("Filter not initialized"),
+            _phantom: PhantomData,
+        }
+    }
+}
+
+/// A query that fetches the relation to a specific target entity.
+/// 
+/// This can be combined with Entities, to query all entities that have a relation targeting
+/// the target entity.
+/// 
+/// ```compile_fail
+/// let v = world.view::<Entities>()
+///     .relates_to::<ChildOf>(b);
+/// 
+/// // Iterate through the entities. ChildOf has no members, so it will be unused.
+/// for (e, _childof) in v.into_iter() {
+///     println!("{:?} is a child of {:?}", e, b);
+/// }
+/// ```
+pub struct RelatesTo<R: Relation> {
+    _marker: PhantomData<R>,
+}
+
+impl<R: Relation> AsQuery for RelatesTo<R> {
+    type Query = QueryRelatesTo<R>;
+}

lyra-ecs/src/system/criteria.rs

@@ -2,14 +2,23 @@ use std::ptr::NonNull;
 
 use lyra_ecs::world::World;
 
+/// An enum that is used to control if the Criteria was met or not.
 #[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
 pub enum CriteriaSchedule {
+    /// The criteria was completely met and the system can continue to run.
     Yes,
+    /// The criteria was not met and must run next time the system batch is ran.
     No,
+    /// The criteria was met and the system can run.
+    /// After the system runs, the criteria should be checked again and may cause another
+    /// execution of the system.
     YesAndLoop,
+    /// The criteria was not met, but it should be checked again during this tick before giving
+    /// up. If the criteria returns `Yes` next check, the systems will run.
     NoAndLoop,
 }
 
+/// A Criteria can be used to conditionally execute [`BatchedSystems`](super::BatchedSystems).
 pub trait Criteria {
     /// Checks if this Criteria can run, and if it should check it again.
     /// 

lyra-ecs/src/system/fn_sys.rs

@@ -1,10 +1,11 @@
 use std::{any::Any, marker::PhantomData, ptr::NonNull};
 
 use paste::paste;
-use crate::{world::World, Access, ResourceObject, query::{Query, View, AsQuery, ResMut, Res}};
+use crate::{world::World, Access, ResourceObject, query::{Query, ViewState, ResMut, Res}};
 
 use super::{System, IntoSystem};
 
+/// A trait that is used for fetching an argument for a [`FnSystem`].
 pub trait FnArgFetcher {
     /// stores data that persists after an execution of a system
     type State: 'static;
@@ -30,14 +31,20 @@ pub trait FnArgFetcher {
     fn apply_deferred(state: Self::State, world: NonNull<World>);
 }
 
-pub trait FnArg {
+/// A system that is implemented as a function.
-    type Fetcher: FnArgFetcher;
+/// 
-}
+/// The arguments of the functions must implement `FnArgFetcher` so that the arguments can be
-
+/// fetched on the fly.
+/// 
+/// ```fail_compile
+/// fn enemy_movement_system(enemies: ViewState<(&Health, &EnemyStats, &Movement)>) -> anyhow::Result<()> {
+///     for (health, stats, movement) in enemies.iter() {
+///         // ...
+///     }
+/// }
+/// ```
 pub struct FnSystem<F, Args> {
     inner: F,
-    //#[allow(dead_code)]
-    //args: Args,
     arg_state: Option<Vec<Box<dyn Any>>>,
     _marker: PhantomData<Args>,
 }
@@ -129,22 +136,14 @@ impl_fn_system_tuple!{ A, B, C, D, E, F2, G, H, I, J, K, L, M, N }
 impl_fn_system_tuple!{ A, B, C, D, E, F2, G, H, I, J, K, L, M, N, O }
 impl_fn_system_tuple!{ A, B, C, D, E, F2, G, H, I, J, K, L, M, N, O, P }
 
-/// An ArgFetcher implementation for query [`View`]s
+/// An ArgFetcher implementation for query [`ViewState`]s
-/* pub struct ViewArgFetcher<Q: AsQuery> {
+impl<'c, Q, F> FnArgFetcher for ViewState<'c, Q, F>
-    query: Q::Query
-}
-
-impl<'a, Q: AsQuery> FnArg for View<'a, Q> {
-    type Fetcher = ViewArgFetcher<Q>;
-} */
-
-impl<'c, Q> FnArgFetcher for View<'c, Q>
 where
-    Q: AsQuery,
+    Q: Query + 'static,
-    <Q as AsQuery>::Query: 'static
+    F: Query + 'static,
 {
-    type State = Q::Query;
+    type State = (Q, F);
-    type Arg<'a, 'state> = View<'a, Q>;
+    type Arg<'a, 'state> = ViewState<'a, Q, F>;
 
     fn world_access(&self) -> Access {
         todo!()
@@ -153,7 +152,8 @@ where
     unsafe fn get<'a, 'state>(state: &'state mut Self::State, world: NonNull<World>) -> Self::Arg<'a, 'state> {
         let world = &*world.as_ptr();
         let arch = world.archetypes.values().collect();
-        let v = View::new(world, state.clone(), arch);
+        let (query, filter) = state.clone();
+        let v = ViewState::new(world, query, filter, arch);
 
         v
     }
@@ -161,17 +161,10 @@ where
     fn apply_deferred(_: Self::State, _: NonNull<World>) { }
     
     fn create_state(_: NonNull<World>) -> Self::State { 
-        <Q::Query as Query>::new()
+        (Q::new(), F::new())
     }
 }
 
-/// An ArgFetcher implementation for borrowing the [`World`].
-/* pub struct WorldArgFetcher;
-
-impl<'a> FnArg for &'a World {
-    type Fetcher = WorldArgFetcher;
-} */
-
 impl FnArgFetcher for &'_ World {
     type State = ();
     type Arg<'a, 'state> = &'a World;
@@ -246,7 +239,7 @@ impl<R: ResourceObject> FnArgFetcher for ResMut<'_, R> {
 mod tests {
     use std::ptr::NonNull;
 
-    use crate::{tests::{Vec2, Vec3}, world::World, query::{QueryBorrow, View, ResMut}};
+    use crate::{tests::{Vec2, Vec3}, world::World, query::{QueryBorrow, ViewState, ResMut}};
     use super::{System, IntoSystem};
 
     struct SomeCounter(u32);
@@ -262,7 +255,7 @@ mod tests {
 
         let mut count = 0;
 
-        let test_system = |view: View<QueryBorrow<Vec2>>| -> anyhow::Result<()> {
+        let test_system = |view: ViewState<QueryBorrow<Vec2>, ()>| -> anyhow::Result<()> {
             let mut vecs = vecs.to_vec();
             for v in view.into_iter() {
                 let pos = vecs.iter().position(|vec| *vec == *v)
@@ -291,7 +284,7 @@ mod tests {
 
         let mut count = 0;
 
-        let test_system = |view: View<(QueryBorrow<Vec2>, QueryBorrow<Vec3>)>| -> anyhow::Result<()> {
+        let test_system = |view: ViewState<(QueryBorrow<Vec2>, QueryBorrow<Vec3>), ()>| -> anyhow::Result<()> {
             for (v2, v3) in view.into_iter() {
                 println!("Got v2 at '{:?}' and v3 at: '{:?}'", v2, v3);
                 count += 1;
@@ -392,7 +385,7 @@ mod tests {
         world.spawn((Vec2::rand(), ));
         world.add_resource(SomeCounter(0));
 
-        let test_system = |mut counter: ResMut<SomeCounter>, view: View<QueryBorrow<Vec2>>| -> anyhow::Result<()> {
+        let test_system = |mut counter: ResMut<SomeCounter>, view: ViewState<QueryBorrow<Vec2>, ()>| -> anyhow::Result<()> {
             for v2 in view.into_iter() {
                 println!("Got v2 at '{:?}'", v2);
                 // .0 is twice here since ResMut's tuple field is pub(crate).

lyra-ecs/src/system/graph.rs

@@ -140,7 +140,7 @@ impl GraphExecutor {
 mod tests {
     use std::ptr::NonNull;
 
-    use crate::{world::World, query::{ResMut, View}, system::IntoSystem};
+    use crate::{query::{ResMut, View}, system::IntoSystem, world::World};
 
     use super::GraphExecutor;
 
@@ -152,7 +152,7 @@ mod tests {
 
         let mut exec = GraphExecutor::new();
 
-        let a_system = |view: View<ResMut<Vec<String>>>| -> anyhow::Result<()> {
+        let a_system = |view: View<ResMut<Vec<String>>, ()>| -> anyhow::Result<()> {
             println!("System 'a' ran!");
             
             let mut order = view.into_iter().next().unwrap();
@@ -161,7 +161,7 @@ mod tests {
             Ok(())
         };
 
-        let b_system = |view: View<ResMut<Vec<String>>>| -> anyhow::Result<()> {
+        let b_system = |view: View<ResMut<Vec<String>>, ()>| -> anyhow::Result<()> {
             println!("System 'b' ran!");
 
             let mut order = view.into_iter().next().unwrap();
@@ -170,7 +170,7 @@ mod tests {
             Ok(())
         };
 
-        let c_system = |view: View<ResMut<Vec<String>>>| -> anyhow::Result<()> {
+        let c_system = |view: View<ResMut<Vec<String>>, ()>| -> anyhow::Result<()> {
             println!("System 'c' ran!");
 
             let mut order = view.into_iter().next().unwrap();

lyra-ecs/src/system/mod.rs

@@ -2,16 +2,16 @@ use std::ptr::NonNull;
 
 use crate::{world::World, Access};
 
-pub mod graph;
+mod graph;
 pub use graph::*;
 
-pub mod criteria;
+mod criteria;
 pub use criteria::*;
 
-pub mod batched;
+mod batched;
 pub use batched::*;
 
-pub mod fn_sys;
+mod fn_sys;
 pub use fn_sys::*;
 
 /// A system that does not mutate the world

lyra-ecs/src/world.rs

@@ -1,9 +1,10 @@
 use std::{any::TypeId, cell::{Ref, RefMut}, collections::HashMap, ptr::NonNull};
 
-use crate::{archetype::{Archetype, ArchetypeId}, bundle::Bundle, query::{dynamic::DynamicView, AsQuery, Query, View, ViewIter, ViewOne}, resource::ResourceData, ComponentInfo, DynTypeId, Entities, Entity, ResourceObject, Tick, TickTracker};
+use crate::{archetype::{Archetype, ArchetypeId}, bundle::Bundle, query::{dynamic::DynamicView, AsQuery, Query, ViewState, ViewIter, ViewOne}, resource::ResourceData, ComponentInfo, DynTypeId, Entities, Entity, ResourceObject, Tick, TickTracker};
 
 /// The id of the entity for the Archetype.
-/// The Archetype struct uses this as the index in the component columns
+/// 
+/// The Archetype uses this as the index in the component columns
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub struct ArchetypeEntityId(pub u64);
 
@@ -207,9 +208,20 @@ impl World {
     }
 
     /// View into the world for a set of entities that satisfy the queries.
-    pub fn view_iter<T: 'static + AsQuery>(&self) -> ViewIter<T::Query> {
+    pub fn view<Q: AsQuery>(&self) -> ViewState<Q::Query, ()> {
+        self.filtered_view::<Q, ()>()
+    }
+
+    /// View into the world for a set of entities that satisfy the query and the filter.
+    pub fn filtered_view<Q: AsQuery, F: AsQuery>(&self) -> ViewState<Q::Query, F::Query> {
         let archetypes = self.archetypes.values().collect();
-        let v = View::<T>::new(self, T::Query::new(), archetypes);
+        ViewState::<Q::Query, F::Query>::new(self, Q::Query::new(), F::Query::new(), archetypes)
+    }
+
+    /// View into the world for a set of entities that satisfy the queries.
+    pub fn view_iter<Q: AsQuery>(&self) -> ViewIter<Q::Query, ()> {
+        let archetypes = self.archetypes.values().collect();
+        let v = ViewState::new(self, Q::Query::new(), (), archetypes);
         v.into_iter()
     }
 
@@ -217,7 +229,7 @@ impl World {
         DynamicView::new(self)
     }
 
-    pub fn view_one<T: 'static + AsQuery>(&self, entity: Entity) -> ViewOne<T::Query> {
+    pub fn view_one<T: AsQuery>(&self, entity: Entity) -> ViewOne<T::Query> {
         ViewOne::new(self, entity.id, T::Query::new())
     }