improve application data docs

src/app.rs

@@ -109,6 +109,7 @@ impl<T> App<T> {
     ///         .route("/", web::get().to(handler))
     /// })
     /// ```
+    #[doc(alias = "manage")]
     pub fn app_data<U: 'static>(mut self, ext: U) -> Self {
         self.extensions.insert(ext);
         self

src/data.rs

@@ -19,23 +19,32 @@ pub(crate) trait DataFactory {
 pub(crate) type FnDataFactory =
     Box<dyn Fn() -> LocalBoxFuture<'static, Result<Box<dyn DataFactory>, ()>>>;
 
-/// Application data.
+/// Application data wrapper and extractor.
 ///
-/// Application level data is a piece of arbitrary data attached to the app, scope, or resource.
+/// # Setting Data
-/// Application data is available to all routes and can be added during the application
+/// Data is set using the `app_data` methods on `App`, `Scope`, and `Resource`. If data is wrapped
-/// configuration process via `App::data()`.
+/// in this `Data` type for those calls, it can be used as an extractor.
 ///
-/// Application data can be accessed by using `Data<T>` extractor where `T` is data type.
+/// Note that `Data` should be constructed _outside_ the `HttpServer::new` closure if shared,
+/// potentially mutable state is desired. `Data` is cheap to clone; internally, it uses an `Arc`.
 ///
-/// **Note**: HTTP server accepts an application factory rather than an application instance. HTTP
+/// See also [`App::app_data`](crate::App::app_data), [`Scope::app_data`](crate::Scope::app_data),
-/// server constructs an application instance for each thread, thus application data must be
+/// and [`Resource::app_data`](crate::Resource::app_data).
-/// constructed multiple times. If you want to share data between different threads, a shareable
+///
-/// object should be used, e.g. `Send + Sync`. Application data does not need to be `Send`
+/// # Extracting `Data`
-/// or `Sync`. Internally `Data` contains an `Arc`.
+/// Since the Actix Web router layers application data, the returned object will reference the
+/// "closest" instance of the type. For example, if an `App` stores a `u32`, a nested `Scope`
+/// also stores a `u32`, and the delegated request handler falls within that `Scope`, then
+/// extracting a `web::<Data<u32>>` for that handler will return the `Scope`'s instance.
+/// However, using the same router set up and a request that does not get captured by the `Scope`,
+/// `web::<Data<u32>>` would return the `App`'s instance.
 ///
 /// If route data is not set for a handler, using `Data<T>` extractor would cause a `500 Internal
 /// Server Error` response.
 ///
+/// See also [`HttpRequest::app_data`]
+/// and [`ServiceRequest::app_data`](crate::dev::ServiceRequest::app_data).
+///
 /// # Unsized Data
 /// For types that are unsized, most commonly `dyn T`, `Data` can wrap these types by first
 /// constructing an `Arc<dyn T>` and using the `From` implementation to convert it.
@@ -79,6 +88,7 @@ pub(crate) type FnDataFactory =
 ///     .route("/index.html", web::get().to(index))
 ///     .route("/index-alt.html", web::get().to(index_alt));
 /// ```
+#[doc(alias = "state")]
 #[derive(Debug)]
 pub struct Data<T: ?Sized>(Arc<T>);
 
@@ -90,12 +100,12 @@ impl<T> Data<T> {
 }
 
 impl<T: ?Sized> Data<T> {
-    /// Get reference to inner app data.
+    /// Returns reference to inner `T`.
     pub fn get_ref(&self) -> &T {
         self.0.as_ref()
     }
 
-    /// Convert to the internal Arc<T>
+    /// Unwraps to the internal `Arc<T>`
     pub fn into_inner(self) -> Arc<T> {
         self.0
     }
@@ -143,13 +153,17 @@ impl<T: ?Sized + 'static> FromRequest for Data<T> {
             ok(st.clone())
         } else {
             log::debug!(
-                "Failed to construct App-level Data extractor. \
+                "Failed to construct Data extractor type: `{}`. For the Data extractor to work \
-                 Request path: {:?} (type: {})",
+                correctly, wrap the data with `Data::new()` and pass it to `App::app_data()`. \
-                req.path(),
+                Ensure that types align in both the set and retrieve calls. \
+                Request path: {}",
                 type_name::<T>(),
+                req.path()
             );
+
             err(ErrorInternalServerError(
-                "App data is not configured, to configure construct it with web::Data::new() and pass it to App::app_data()",
+                "Requested application data is not configured. \
+                View/enable debug logs for more details.",
             ))
         }
     }

src/request.rs

@@ -266,14 +266,34 @@ impl HttpRequest {
         self.app_state().config()
     }
 
-    /// Get an application data object stored with `App::data` or `App::app_data`
+    /// Retrieves a piece of application state.
-    /// methods during application configuration.
     ///
-    /// If `App::data` was used to store object, use `Data<T>`:
+    /// Extracts any object stored with [`App::app_data()`](crate::App::app_data) (or the
+    /// counterpart methods on [`Scope`](crate::Scope::app_data) and
+    /// [`Resource`](crate::Resource::app_data)) during application configuration.
     ///
-    /// ```ignore
+    /// Since the Actix Web router layers application data, the returned object will reference the
-    /// let opt_t = req.app_data::<Data<T>>();
+    /// "closest" instance of the type. For example, if an `App` stores a `u32`, a nested `Scope`
+    /// also stores a `u32`, and the delegated request handler falls within that `Scope`, then
+    /// calling `.app_data::<u32>()` on an `HttpRequest` within that handler will return the
+    /// `Scope`'s instance. However, using the same router set up and a request that does not get
+    /// captured by the `Scope`, `.app_data::<u32>()` would return the `App`'s instance.
+    ///
+    /// If the state was stored using the [`Data`] wrapper, then it must also be retrieved using
+    /// this same type.
+    ///
+    /// See also the [`Data`] extractor.
+    ///
+    /// # Examples
+    /// ```no_run
+    /// # use actix_web::{web, test::TestRequest};
+    /// # let req = TestRequest::default().to_http_request();
+    /// # type T = u32;
+    /// let opt_t: Option<&T> = req.app_data::<web::Data<T>>();
     /// ```
+    ///
+    /// [`Data`]: crate::web::Data
+    #[doc(alias = "state")]
     pub fn app_data<T: 'static>(&self) -> Option<&T> {
         for container in self.inner.app_data.iter().rev() {
             if let Some(data) = container.get::<T>() {

src/resource.rs

@@ -195,6 +195,7 @@ where
     ///         .route(web::get().to(handler))
     /// );
     /// ```
+    #[doc(alias = "manage")]
     pub fn app_data<U: 'static>(mut self, data: U) -> Self {
         self.app_data
             .get_or_insert_with(Extensions::new)

src/scope.rs

@@ -154,6 +154,7 @@ where
     ///         .route("/", web::get().to(handler))
     /// );
     /// ```
+    #[doc(alias = "manage")]
     pub fn app_data<U: 'static>(mut self, data: U) -> Self {
         self.app_data
             .get_or_insert_with(Extensions::new)