new: Add AppExtension. (#54)

* Add ext.

* Add tests.

crates/framework/src/app.rs

@@ -25,6 +25,10 @@ pub enum Phase {
     Shutdown,
 }
 
+pub trait AppExtension {
+    fn extend(self, app: &mut App) -> AppResult<()>;
+}
+
 #[derive(Debug)]
 pub struct App {
     // Data
@@ -78,6 +82,11 @@ impl App {
         crate::tracing::setup_tracing(options);
     }
 
+    /// Extend the app with an extension that contains a set of systems.
+    pub fn extend(&mut self, extension: impl AppExtension) -> AppResult<()> {
+        extension.extend(self)
+    }
+
     /// Add a system function that runs during the startup phase.
     pub fn startup<S: SystemFunc + 'static>(&mut self, system: S) -> &mut Self {
         self.add_system(Phase::Startup, CallbackSystem::new(system))

crates/framework/tests/app_test.rs

@@ -1,7 +1,7 @@
 #![allow(dead_code)]
 
 use miette::{bail, IntoDiagnostic};
-use starbase::{App, AppPhase, Emitters, Resources, States, SystemResult};
+use starbase::{App, AppExtension, AppPhase, Emitters, Resources, States, SystemResult};
 use starbase_macros::*;
 use std::time::Duration;
 use tokio::task;
@@ -623,3 +623,30 @@ async fn tracks_app_state() {
         vec!["Startup", "Analyze", "Execute", "Shutdown"]
     );
 }
+
+struct TestExtension;
+
+impl AppExtension for TestExtension {
+    fn extend(self, app: &mut App) -> miette::Result<()> {
+        app.startup(extract_app_state);
+        app.analyze(extract_app_state);
+        app.execute(extract_app_state);
+        app.shutdown(extract_app_state);
+
+        Ok(())
+    }
+}
+
+#[tokio::test]
+async fn extension_can_register_systems() {
+    let mut app = App::new();
+    app.startup(setup_state);
+    app.extend(TestExtension).unwrap();
+
+    let states = app.run().await.unwrap();
+
+    assert_eq!(
+        states.get::<RunOrder>().0,
+        vec!["Startup", "Analyze", "Execute", "Shutdown"]
+    );
+}