docs: Add to AppContext book docs

book/examples/app-context/Cargo.toml

@@ -6,4 +6,6 @@ publish = false
 
 [dependencies]
 roadster = { path = "../../.." }
-axum = { workspace = true }
+axum = { workspace = true }
+async-trait = { workspace = true }
+anyhow = { workspace = true }

book/examples/app-context/src/app.rs

@@ -0,0 +1,57 @@
+use crate::state::CustomState;
+use anyhow::anyhow;
+use async_trait::async_trait;
+use axum::extract::FromRef;
+use roadster::app::context::{AppContext, AppContextWeak};
+use roadster::app::RoadsterApp;
+use roadster::error::RoadsterResult;
+use roadster::health::check::{CheckResponse, HealthCheck, Status};
+use roadster::util::empty::Empty;
+use std::time::Duration;
+
+pub type App = RoadsterApp<CustomState, Empty, Empty>;
+
+pub struct ExampleHealthCheck {
+    // Prevent reference cycle because the `ExampleHealthCheck` is also stored in the `AppContext`
+    context: AppContextWeak,
+}
+
+#[async_trait]
+impl HealthCheck for ExampleHealthCheck {
+    fn name(&self) -> String {
+        "example".to_string()
+    }
+
+    fn enabled(&self) -> bool {
+        true
+    }
+
+    async fn check(&self) -> RoadsterResult<CheckResponse> {
+        // Upgrade the `AppContext` in order to use it
+        let _context = self
+            .context
+            .upgrade()
+            .ok_or_else(|| anyhow!("Could not upgrade AppContextWeak"))?;
+
+        Ok(CheckResponse::builder()
+            .status(Status::Ok)
+            .latency(Duration::from_secs(0))
+            .build())
+    }
+}
+
+pub fn build_app() -> App {
+    RoadsterApp::builder()
+        .state_provider(|context| {
+            Ok(CustomState {
+                context,
+                custom_field: "Custom Field".to_string(),
+            })
+        })
+        .add_health_check_provider(|registry, state| {
+            // Downgrade the context before providing it to the `ExampleHealthCheck`
+            let context = AppContext::from_ref(state).downgrade();
+            registry.register(ExampleHealthCheck { context })
+        })
+        .build()
+}

book/examples/app-context/src/lib.rs

@@ -1,17 +1,2 @@
-use axum::extract::FromRef;
-use roadster::app::context::AppContext;
-
-#[derive(FromRef)]
-pub struct CustomState {
-    custom_field: String,
-    context: AppContext,
-}
-
-impl CustomState {
-    pub fn new(custom_field: String, context: AppContext) -> Self {
-        Self {
-            custom_field,
-            context,
-        }
-    }
-}
+pub mod app;
+pub mod state;

book/examples/app-context/src/state.rs

@@ -0,0 +1,8 @@
+use axum::extract::FromRef;
+use roadster::app::context::AppContext;
+
+#[derive(Clone, FromRef)]
+pub struct CustomState {
+    pub context: AppContext,
+    pub custom_field: String,
+}

book/src/features/app-context.md

@@ -6,7 +6,29 @@ directly, or you can define your own state type that can be used by both Roadste
 implementing [FromRef](https://docs.rs/axum-core/latest/axum_core/extract/trait.FromRef.html).
 
 ```rust,ignore
-{{#include ../../examples/app-context/src/lib.rs}}
+{{#include ../../examples/app-context/src/state.rs}}
+```
+
+## `Provide` and `ProvideRef`
+
+🛠️ todo 🛠️
+
+- https://docs.rs/roadster/latest/roadster/app/context/trait.Provide.html
+- https://docs.rs/roadster/latest/roadster/app/context/trait.ProvideRef.html
+
+## Weak reference
+
+In some cases, it can be useful to have a weak reference to the `AppContext` state in order to prevent reference cycles
+for things that are included in the `AppContext` but also need a reference to the `AppContext`. For example, the
+`AppContext` keeps a reference to the `HealthCheck`s, and most `HealthCheck`s need to use the `AppContext`.
+
+To get a weak reference to the `AppContext`'s state,
+use [AppContext#downgrade](https://docs.rs/roadster/latest/roadster/app/context/struct.AppContext.html#method.downgrade)
+to get a new instance
+of [AppContextWeak](https://docs.rs/roadster/latest/roadster/app/context/struct.AppContextWeak.html).
+
+```rust,ignore
+{{#include ../../examples/app-context/src/app.rs:12:}}
 ```
 
 ## Docs.rs links