Web: usage through fetch, preferences in local storage (#238)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

apps/fieldcraft-web/src/usage.rs

@@ -0,0 +1,86 @@
+//! Anonymous usage in the browser (docs/privacy.md): the same events and the same consent as the
+//! desktop app, the choice kept in the browser's local storage with the other preferences. Queued
+//! events are posted with `fetch` every half minute and when the page is closed or left (`keepalive`, so
+//! a closing page still sends them); a failed post is dropped silently.
+//!
+//! The project key is a build setting, `FIELDCRAFT_POSTHOG_KEY`: a build without one asks nothing
+//! and sends nothing.
+
+use fieldcraft_engine::usage;
+use fieldcraft_ui_egui::files::PrefsStore;
+use fieldcraft_ui_egui::privacy::UsageHost;
+use wasm_bindgen::JsCast as _;
+
+const KEY: Option<&str> = option_env!("FIELDCRAFT_POSTHOG_KEY");
+/// Where the preferences are kept in local storage.
+const PREFS_KEY: &str = "fieldcraft-prefs";
+/// How often queued events are sent (seconds).
+pub const INTERVAL: f64 = 30.0;
+
+fn key() -> Option<&'static str> {
+    KEY.map(str::trim).filter(|k| !k.is_empty())
+}
+
+fn local_storage() -> Option<web_sys::Storage> {
+    web_sys::window()?.local_storage().ok().flatten()
+}
+
+/// The preferences in local storage (navigation, privacy).
+pub fn prefs_store() -> PrefsStore {
+    PrefsStore {
+        load: Box::new(|| local_storage()?.get_item(PREFS_KEY).ok().flatten().map(String::into_bytes)),
+        save: Box::new(|bytes| {
+            let text = std::str::from_utf8(bytes).map_err(|e| e.to_string())?;
+            local_storage().ok_or("browser storage is turned off")?.set_item(PREFS_KEY, text).map_err(crate::web::js_err)
+        }),
+    }
+}
+
+/// What the window needs to ask and record, when the build has a key.
+pub fn host() -> Option<UsageHost> {
+    // The browser's clock for command durations (the plain wasm target has none).
+    usage::set_clock(js_sys::Date::now);
+    key().is_some().then(|| UsageHost {
+        platform: "web",
+        os: "web",
+        arch: "wasm32",
+        new_id: Box::new(|| web_sys::window()?.crypto().ok().map(|c| c.random_uuid())),
+    })
+}
+
+/// Post what is queued.
+pub fn send() {
+    let (Some(key), Some(window)) = (key(), web_sys::window()) else { return };
+    let url = format!("{}/batch/", usage::DEFAULT_HOST);
+    while let Some(batch) = usage::take_batch() {
+        let init = web_sys::RequestInit::new();
+        init.set_method("POST");
+        init.set_body(&wasm_bindgen::JsValue::from_str(&batch.body(key)));
+        // The Fetch standard's `keepalive` (a closing page still sends it); web-sys has no setter.
+        let _ = js_sys::Reflect::set(&init, &"keepalive".into(), &true.into());
+        let headers = web_sys::Headers::new().ok();
+        if let Some(h) = &headers {
+            let _ = h.set("Content-Type", "application/json");
+            init.set_headers(h);
+        }
+        let promise = window.fetch_with_str_and_init(&url, &init);
+        wasm_bindgen_futures::spawn_local(async move {
+            // Offline or refused: the batch is dropped.
+            if let Err(e) = wasm_bindgen_futures::JsFuture::from(promise).await {
+                log::debug!("usage: not sent: {}", crate::web::js_err(e));
+            }
+        });
+    }
+}
+
+/// When the page is closed or left: the session's length, then send.
+pub fn send_when_hidden(window: &web_sys::Window) {
+    let on_hide = wasm_bindgen::closure::Closure::<dyn FnMut()>::new(|| {
+        usage::closing();
+        send();
+    });
+    if let Err(e) = window.add_event_listener_with_callback("pagehide", on_hide.as_ref().unchecked_ref()) {
+        log::warn!("usage: {}", crate::web::js_err(e));
+    }
+    on_hide.forget();
+}