schedule: initial work

src/lib.rs

···
 pub mod resource;
 pub use resource::*;
 
+pub mod schedule;
+
 pub mod sqlite_ext;
 
 pub mod system;

src/schedule.rs

···
+use crate::{system, Entity, System};
+
+pub trait SchedulingMode: std::fmt::Debug {
+    fn should_run(&self, ecs: &crate::Ecs, system: Entity) -> bool;
+    fn did_run(&self, _ecs: &crate::Ecs, _system: Entity) {}
+}
+
+pub struct Schedule {
+    pub mode: Box<dyn SchedulingMode>,
+    pub systems: Vec<Box<dyn System>>,
+}
+
+#[derive(Debug)]
+pub struct Manually;
+
+impl SchedulingMode for Manually {
+    fn should_run(&self, _ecs: &crate::Ecs, _system: Entity) -> bool {
+        false
+    }
+}
+
+#[derive(Debug)]
+pub struct Always;
+
+impl SchedulingMode for Always {
+    fn should_run(&self, _ecs: &crate::Ecs, _system: Entity) -> bool {
+        true
+    }
+}
+
+#[derive(Debug)]
+pub struct Every(pub chrono::Duration);
+
+impl SchedulingMode for Every {
+    fn should_run(&self, _ecs: &crate::Ecs, system: Entity) -> bool {
+        system
+            .component::<system::LastRun>()
+            .map(|last_run| chrono::Utc::now().signed_duration_since(&last_run.0) > self.0)
+            .unwrap_or(true)
+    }
+}
+
+#[derive(Debug)]
+pub struct Oneshot;
+
+impl SchedulingMode for Oneshot {
+    fn should_run(&self, _ecs: &crate::Ecs, system: Entity) -> bool {
+        system.component::<system::LastRun>().is_none()
+    }
+}