Update App State documentation

content/docs/application.md

@@ -43,30 +43,33 @@ as the first application, it would match all incoming requests.
 ## State
 
 Application state is shared with all routes and resources within the same scope. State
-can be accessed with the `web::Data<State>` extractor as read-only, but interior mutability with
-`Cell` can be used to achieve state mutability. State is also available for route
-matching guards and middlewares.
+can be accessed with the `web::Data<State>` extractor. State is also available for route matching guards and middlewares.
 
-Let's write a simple application that uses shared state. We are going to store request count
-in the state:
+Let's write a simple application and store the application name in the state:
 
 {{< include-example example="application" file="state.rs" section="setup" >}}
 
-When the app is initialized it needs to be passed the initial state:
-
-{{< include-example example="application" file="state.rs" section="make_app" >}}
-
-> **Note**: `HttpServer` accepts an application factory rather than an application
-> instance. `HttpServer` constructs an application instance for each thread, thus
-> application state must be constructed multiple times. If you want to share state between
-> different threads, a shared object should be used, e.g. `Arc`. There is also an
-> [Example][stateexample] using `Arc` for this. Application state does not need to be
-> `Send` and `Sync`, but the application factory must be `Send` + `Sync`.
-
-To start the previous app, create it into a closure:
+and pass in the state when initializing the App, and start the application:
 
 {{< include-example example="application" file="state.rs" section="start_app" >}}
 
+## Shared Mutable State
+
+`HttpServer` accepts an application factory rather than an application instance. 
+Http server constructs an application instance for each thread, thus application data must be 
+constructed multiple times. If you want to share data between different threads, a shareable 
+object should be used, e.g. Send + Sync. 
+
+Internally, `web::Data` uses Arc. Thus, in order to avoid double Arc, we should create our Data before registering it using `register_data()`.
+
+In the following example, we will write an application with mutable, shared state. First, we define our state and create our handler:
+
+{{< include-example example="application" file="state.rs" section="setup_mutable" >}}
+
+and initialize in in an App:
+
+{{< include-example example="application" file="state.rs" section="make_app_mutable" >}}
+
 ## Combining applications with different state
 
 Combining multiple applications with different state is possible as well.

examples/application/src/state.rs

@@ -1,42 +1,53 @@
 // <setup>
 use actix_web::{web, App, HttpServer};
-use std::cell::Cell;
 
 // This struct represents state
 struct AppState {
-    counter: Cell<i32>,
+    app_name: String,
 }
 
 fn index(data: web::Data<AppState>) -> String {
-    let count = data.counter.get() + 1; // <- get count
-    data.counter.set(count); // <- store new count in state
+    let app_name = data.app_name; // <- get app_name
 
-    format!("Request number: {}", count) // <- response with count
+    format!("Hello {}!", app_name) // <- response with app_name
 }
 // </setup>
 
-// <make_app>
+// <setup_mutable>
+struct AppStateWithCounter {
+    counter: Mutex<i32>, // <- Mutex is necessary to mutate safely across threads
+}
+
+fn _index(data: web::Data<AppStateWithCounter>) -> String {
+    let mut counter = state.counter.lock().unwrap(); // <- get counter's MutexGuard
+    *counter += 1; // <- access counter inside MutexGuard
+
+    format!("Request number: {}", counter) // <- response with count
+}
+// </setup_mutable>
+
+// <make_app_mutable>
 fn _main() {
+    let counter = web::Data::new(AppStateWithCounter { counter : Mutex::new(0)});
+
     App::new()
-        .data(AppState {
-            counter: Cell::new(0),
-        })
+        .register_data(counter.clone()) // <- register the created data
         .route("/", web::get().to(index));
 }
-// </make_app>
+// </make_app_mutable>
 
 // <start_app>
 pub fn main() {
     HttpServer::new(|| {
         App::new()
             .data(AppState {
-                counter: Cell::new(0),
+                app_name: String::from("Actix-web")
             })
             .route("/", web::get().to(index))
     })
-    .bind("127.0.0.1:8088")
-    .unwrap()
-    .run()
-    .unwrap();
+        .bind("127.0.0.1:8088")
+        .unwrap()
+        .run()
+        .unwrap();
 }
 // </start_app>