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docs/router/src/reference/routing-update-callback.md

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+# Routing Update Callback
+
+In some cases we might want to run custom code when the current route changes.
+For this reason, the [`RouterConfiguration`] exposes an `on_update` field.
+
+## How does the callback behave?
+The `on_update` is called whenever the current routing information changes. It
+is called after the router updated its internal state, but before depended
+components and hooks are updated.
+
+If the callback returns a [`NavigationTarget`], the router will replace the
+current location with the specified target. It will not call the
+`on_update` again.
+
+If at any point the router encounters a
+[navigation failure](./failures/index.md), it will go to the appropriate state
+without calling the `on_update`. It doesn't matter if the invalid target
+initiated the navigation, was found as a redirect target or returned by the
+`on_update` itself.
+
+## Code Example
+```rust
+# // Hidden lines (like this one) make the documentation tests work.
+# extern crate dioxus;
+# extern crate dioxus_router;
+# extern crate dioxus_ssr;
+#
+use std::sync::Arc;
+
+use dioxus::prelude::*;
+use dioxus_router::prelude::*;
+
+fn App(cx: Scope) -> Element {
+    use_router(
+        cx,
+        &|| RouterConfiguration {
+            # synchronous: true,
+            on_update: Some(Arc::new(|state| -> Option<NavigationTarget> {
+                if state.path == "/" {
+                    return Some("/home".into());
+                }
+
+                None
+            })),
+            ..Default::default()
+        },
+        &|| Segment::empty().fixed("home", comp(Content))
+    );
+
+    render! {
+        Outlet { }
+    }
+}
+
+fn Content(cx: Scope) -> Element {
+    render! {
+        p { "Some content" }
+    }
+}
+#
+# let mut vdom = VirtualDom::new(App);
+# vdom.rebuild();
+# assert_eq!(dioxus_ssr::render(&mut vdom), "<p>Some content</p>");
+```
+
+[`NavigationTarget`]: https://docs.rs/dioxus-router-core/latest/dioxus_router_core/navigation/enum.NavigationTarget.html
+[`RouterConfiguration`]: https://docs.rs/dioxus-router/latest/dioxus_router/hooks/struct.RouterConfiguration.html