Update

src/core/app/app.rs

@@ -0,0 +1,54 @@
+use std::error::Error;
+
+use bevy_ecs::{schedule::Schedules, world::World};
+
+pub enum AppExit {
+    Success,
+    Error(Box<dyn Error>),
+}
+
+pub type RunnerFn = Box<dyn FnOnce() -> AppExit>;
+
+pub struct App {
+    world: World,
+    runner: Option<RunnerFn>,
+}
+
+impl Default for App {
+    fn default() -> Self {
+        let mut world = World::new();
+        world.init_resource::<Schedules>();
+
+        Self {
+            world,
+            runner: None,
+        }
+    }
+}
+
+impl App {
+    pub fn world_mut(&mut self) -> &mut World {
+        &mut self.world
+    }
+
+    pub fn world(&self) -> &World {
+        &self.world
+    }
+
+    pub fn run(&mut self) -> Result<(), Box<dyn Error>> {
+        match self.runner.take() {
+            Some(runner) => match runner() {
+                AppExit::Success => Ok(()),
+                AppExit::Error(e) => Err(e),
+            },
+            None => Err(Box::new(std::io::Error::new(
+                std::io::ErrorKind::Other,
+                "runner is not set",
+            ))),
+        }
+    }
+
+    pub fn set_runner(&mut self, runner: RunnerFn) {
+        self.runner = Some(runner);
+    }
+}

src/core/app/mod.rs

@@ -0,0 +1,4 @@
+mod app;
+pub mod plugin;
+
+pub use app::App;

src/core/app/plugin.rs

@@ -0,0 +1,5 @@
+use super::app::App;
+
+pub trait Plugin {
+    fn build(&self, app: &mut App);
+}