android: the downloads say so in the shade

A Save still running with the app in the background now posts a progress
notice — how many files, the one it is on, a bar, Stop all and Open — and
takes it down when the queue empties. It sits on its own low-importance
channel so it can be silenced without silencing the player's card, and
rides its own dataSync foreground service rather than the media one: a
mediaPlayback service stops itself when the bar empties, a download is not
playback, and nothing else was keeping the process alive for a Save with
nothing playing — which is how the first real Save died, four tracks in.
Stop all goes through the same cancel(id) a row's tap takes, and Transfers
grew the matching button beside Clear finished. The walk pushes two rows
straight into the transfer store and reads the real notification back out
of `dumpsys notification`, without fetching anything.

crates/tauri-plugin-transfer-notice/src/lib.rs

@@ -0,0 +1,110 @@
+//! The downloads notice on the phone.
+//!
+//! The Transfers screen only exists inside the app: the desktop says what a
+//! Save is doing with its title-bar ring, the phone with a badge on the nav,
+//! and neither is visible once the app is in the background — which is
+//! exactly when a Save is left to run on its own. So a running Save says so
+//! where the phone says things, in the shade: how many files are left, the
+//! one it is on, a progress bar, Stop all and Open.
+//!
+//! Its own channel, low importance, so it can be silenced without silencing
+//! the player's card. Its own foreground service (`TransferService`,
+//! `dataSync`) rather than the media one, because a download is not
+//! playback: `MediaService` stops itself the moment the bar empties, and a
+//! `mediaPlayback` service with no session is a policy violation. That
+//! service is also what keeps the process alive for a Save with nothing
+//! playing — the way the first real Save died, four tracks in.
+//!
+//! Two commands, both fed by the phone UI (src/mobile/xfernote.svelte.js):
+//! `update` with what is still in flight, `clear` when the queue empties.
+//! The notice's buttons arrive as the plugin event `action`
+//! (`addPluginListener("transfer-notice", "action", …)`): `cancel` stops
+//! everything still going, `open` brings the app forward on Transfers.
+//!
+//! On the desktop the plugin is registered too, so the permission set
+//! resolves on every platform, but its commands answer "not here".
+
+use serde::{Deserialize, Serialize};
+use tauri::{
+    plugin::{Builder, TauriPlugin},
+    Runtime,
+};
+
+/// What the notice says: how many files are still in flight, the one it is
+/// on, and how far the burst has come. `indeterminate` is a queue whose
+/// sizes no peer has stated yet — a bar with no end to walk towards.
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+#[serde(rename_all = "camelCase")]
+pub struct TransferNotice {
+    #[serde(default)]
+    pub count: u32,
+    pub title: Option<String>,
+    pub line: Option<String>,
+    #[serde(default)]
+    pub percent: i32,
+    #[serde(default)]
+    pub indeterminate: bool,
+}
+
+#[cfg(mobile)]
+pub struct TransferNoticePlugin<R: Runtime>(tauri::plugin::PluginHandle<R>);
+
+#[tauri::command]
+fn update<R: Runtime>(app: tauri::AppHandle<R>, state: TransferNotice) -> Result<(), String> {
+    #[cfg(mobile)]
+    {
+        use tauri::Manager;
+        let notice = app.state::<TransferNoticePlugin<R>>();
+        notice
+            .0
+            .run_mobile_plugin::<serde_json::Value>("update", state)
+            .map(|_| ())
+            .map_err(|e| e.to_string())
+    }
+    #[cfg(not(mobile))]
+    {
+        let _ = (app, state);
+        Err("the downloads notice is a phone feature".into())
+    }
+}
+
+#[tauri::command]
+fn clear<R: Runtime>(app: tauri::AppHandle<R>) -> Result<(), String> {
+    #[cfg(mobile)]
+    {
+        use tauri::Manager;
+        let notice = app.state::<TransferNoticePlugin<R>>();
+        notice
+            .0
+            .run_mobile_plugin::<serde_json::Value>("clear", ())
+            .map(|_| ())
+            .map_err(|e| e.to_string())
+    }
+    #[cfg(not(mobile))]
+    {
+        let _ = app;
+        Err("the downloads notice is a phone feature".into())
+    }
+}
+
+pub fn init<R: Runtime>() -> TauriPlugin<R> {
+    Builder::new("transfer-notice")
+        .invoke_handler(tauri::generate_handler![update, clear])
+        .setup(|app, api| {
+            #[cfg(target_os = "android")]
+            {
+                use tauri::Manager;
+                let handle = api.register_android_plugin(
+                    "com.soulstream.transfernotice",
+                    "TransferNoticePlugin",
+                )?;
+                app.manage(TransferNoticePlugin(handle));
+            }
+            #[cfg(not(target_os = "android"))]
+            {
+                let _ = (app, api);
+            }
+            Ok(())
+        })
+        .build()
+}