feat(wallpapers-bing): refresh the feed in the background

wallpapers-bing/wallpapers_bing.vala

@@ -12,12 +12,20 @@ public void peas_register_types(TypeModule module) {
 
 namespace WallpapersBing {
 
+    private const uint REFRESH_SECONDS = 86400;
+    private const uint MAX_BACKOFF_SECONDS = 3600;
+
     public class Provider : WallpaperHelperProvider, WallpaperProvider {
         // Set by choices() from the helper's own answer, never re-derived
         // from the config file -- see WallpaperBing.CONSOLIDATED_ID. False
         // until the first choices() call, so the name starts as plain "Bing"
         // and the browser refreshes the label once the helper has replied.
         private bool combined = false;
+        private bool prefetching = false;
+        private int failures = 0;
+        private uint refresh_source = 0;
+        private ulong network_handler = 0;
+        private Cancellable cancel_all = new Cancellable();
         public string id { get { return WallpaperBing.PROVIDER_ID; } }
         // Matches the collection label the rotator helper writes for the
         // same de-duplicated view, so the online browser and the wallpaper
@@ -28,6 +36,54 @@ namespace WallpapersBing {
         public bool requires_credentials { get { return false; } }
         public bool supports_search { get { return false; } }
         public Provider() { base("singularity-wallpaper-bing"); }
+        public void start_prefetch() {
+            cancel_all = new Cancellable();
+            if (network_handler == 0)
+                network_handler = NetworkMonitor.get_default().network_changed.connect(() => {
+                    if (failures > 0 || refresh_source == 0) prefetch();
+                });
+            prefetch();
+        }
+
+        public void stop_prefetch() {
+            cancel_all.cancel();
+            if (refresh_source != 0) Source.remove(refresh_source);
+            refresh_source = 0;
+            if (network_handler != 0) NetworkMonitor.get_default().disconnect(network_handler);
+            network_handler = 0;
+        }
+
+        private void prefetch() {
+            var monitor = NetworkMonitor.get_default();
+            if (prefetching || cancel_all.is_cancelled() || !monitor.network_available || monitor.network_metered)
+                return;
+            prefetching = true;
+            run_prefetch.begin();
+        }
+
+        private async void run_prefetch() {
+            // Hold this run's own Cancellable: stop_prefetch() followed by
+            // start_prefetch() replaces cancel_all while a fetch is in flight.
+            var cancel = cancel_all;
+            bool ok = false;
+            try {
+                yield command({helper, "fetch", "--if-stale"}, cancel, 900);
+                ok = true;
+            } catch (Error e) {
+                if (!cancel.is_cancelled()) warning("wallpapers-bing: prefetch failed: %s", e.message);
+            }
+            prefetching = false;
+            if (cancel.is_cancelled()) return;
+            failures = ok ? 0 : failures + 1;
+            uint delay = ok ? REFRESH_SECONDS : uint.min(30U << uint.min(failures - 1, 7U), MAX_BACKOFF_SECONDS);
+            if (refresh_source != 0) Source.remove(refresh_source);
+            refresh_source = Timeout.add_seconds(delay, () => {
+                refresh_source = 0;
+                prefetch();
+                return Source.REMOVE;
+            });
+        }
+
         public async ArrayList<WallpaperProviderChoice> choices(string index, Cancellable? cancel) throws Error {
             var loaded = WallpaperBing.markets(yield command({helper, "markets"}, cancel, 30));
             // The helper is expected to always advertise the combined view
@@ -64,10 +120,12 @@ public class WallpapersBingPlugin : Object, Singularity.Plugin {
         this.context = ctx;
         provider = new WallpapersBing.Provider();
         context.add_wallpaper_provider(provider);
+        provider.start_prefetch();
     }
 
     public void deactivate() {
         if (provider != null) {
+            provider.stop_prefetch();
             context.remove_wallpaper_provider(provider);
             provider = null;
         }