Add pagination support for discover catalog browsing

Discover only ever fetched the first page (typically 20-50 items) of a
catalog and had no way to request more. Add a discoverPageRequested
action, a fetchDiscoverPage effect, and a dedicated DiscoverPaging
generation key so an in-flight "load more" fetch can't clobber a fresh
query (or vice versa) when the user changes catalog/genre mid-fetch.

src/headless_engine/contracts.rs

@@ -235,6 +235,14 @@ pub(super) enum AppAction {
         profile: Option<Value>,
         language: Option<String>,
     },
+    #[serde(rename = "discoverPageRequested")]
+    DiscoverPageRequested {
+        transport_url: Option<String>,
+        content_type: String,
+        catalog_id: String,
+        skip: Option<i32>,
+        genre: Option<String>,
+    },
     #[serde(rename = "catalogPageRequested")]
     CatalogPageRequested {
         category_id: String,

src/headless_engine/discover.rs

@@ -18,6 +18,25 @@ pub(super) struct DiscoverState {
     genres: Value,
     error: Value,
     generation: u64,
+    paging: DiscoverPaging,
+}
+
+#[derive(Clone, Debug, Default, PartialEq, Deserialize, Serialize)]
+#[serde(rename_all = "camelCase", default)]
+pub(super) struct DiscoverPaging {
+    is_loading: bool,
+    items: Value,
+    error: Value,
+}
+
+#[derive(Serialize)]
+#[serde(rename_all = "camelCase")]
+struct FetchDiscoverPagePayload {
+    transport_url: Option<String>,
+    content_type: String,
+    catalog_id: String,
+    skip: i32,
+    genre: Option<String>,
 }
 
 #[derive(Serialize)]
@@ -48,6 +67,7 @@ pub(super) fn dispatch_discover(
     language: Option<String>,
 ) -> Vec<EffectEnvelope> {
     let generation = engine.bump_generation(GenerationKey::Discover);
+    engine.bump_generation(GenerationKey::DiscoverPaging);
     let profile_value = profile.unwrap_or_else(|| engine.state.profile.active.clone());
     let profile_id = active_profile_id(&engine.state, &profile_value);
     let filters_value = filters.unwrap_or(Value::Null);
@@ -62,6 +82,7 @@ pub(super) fn dispatch_discover(
         genres: engine.state.discover.genres.clone(),
         error: Value::Null,
         generation,
+        paging: DiscoverPaging::default(),
     };
     vec![engine.effect(
         EffectKind::RunDiscover,
@@ -102,6 +123,33 @@ pub(super) fn dispatch_catalog_filters(
     )]
 }
 
+pub(super) fn dispatch_discover_page(
+    engine: &mut HeadlessEngine,
+    transport_url: Option<String>,
+    content_type: String,
+    catalog_id: String,
+    skip: Option<i32>,
+    genre: Option<String>,
+) -> Vec<EffectEnvelope> {
+    let generation = engine.bump_generation(GenerationKey::DiscoverPaging);
+    engine.state.discover.paging = DiscoverPaging {
+        is_loading: true,
+        items: Value::Null,
+        error: Value::Null,
+    };
+    vec![engine.effect(
+        EffectKind::FetchDiscoverPage,
+        generation,
+        FetchDiscoverPagePayload {
+            transport_url,
+            content_type,
+            catalog_id,
+            skip: skip.unwrap_or(0).max(0),
+            genre,
+        },
+    )]
+}
+
 pub(super) fn complete(
     engine: &mut HeadlessEngine,
     effect_type: &str,
@@ -149,6 +197,21 @@ pub(super) fn complete(
                 }
             }
         }
+        "fetchDiscoverPage" => {
+            if generation == engine.state.runtime.get(GenerationKey::DiscoverPaging) {
+                engine.state.discover.paging.is_loading = false;
+                if result.status.is_ok() {
+                    engine.state.discover.paging.items = result
+                        .value
+                        .get("items")
+                        .cloned()
+                        .unwrap_or_else(|| serde_json::json!([]));
+                    engine.state.discover.paging.error = Value::Null;
+                } else {
+                    engine.state.discover.paging.error = normalize_error(result.error.clone());
+                }
+            }
+        }
         _ => {}
     }
     vec![]

src/headless_engine/mod.rs

@@ -442,6 +442,20 @@ impl HeadlessEngine {
                 profile,
                 language,
             ),
+            AppAction::DiscoverPageRequested {
+                transport_url,
+                content_type,
+                catalog_id,
+                skip,
+                genre,
+            } => discover::dispatch_discover_page(
+                self,
+                transport_url,
+                content_type,
+                catalog_id,
+                skip,
+                genre,
+            ),
             AppAction::CatalogPageRequested {
                 category_id,
                 transport_url,
@@ -605,7 +619,9 @@ impl HeadlessEngine {
 
             EffectKind::RunSearch => search::complete(self, generation, &result),
 
-            EffectKind::RunDiscover | EffectKind::ReadDiscoverCatalogFilters => {
+            EffectKind::RunDiscover
+            | EffectKind::ReadDiscoverCatalogFilters
+            | EffectKind::FetchDiscoverPage => {
                 discover::complete(self, effect_type, generation, &result)
             }
 

src/headless_engine/state.rs

@@ -89,6 +89,7 @@ pub(super) enum GenerationKey {
     Addon,
     Search,
     Discover,
+    DiscoverPaging,
     Sync,
     Auth,
     Settings,
@@ -110,6 +111,7 @@ pub(super) struct RuntimeGenerations {
     addon_generation: u64,
     search_generation: u64,
     discover_generation: u64,
+    discover_paging_generation: u64,
     sync_generation: u64,
     auth_generation: u64,
     settings_generation: u64,
@@ -131,6 +133,7 @@ impl RuntimeGenerations {
             GenerationKey::Addon => self.addon_generation,
             GenerationKey::Search => self.search_generation,
             GenerationKey::Discover => self.discover_generation,
+            GenerationKey::DiscoverPaging => self.discover_paging_generation,
             GenerationKey::Sync => self.sync_generation,
             GenerationKey::Auth => self.auth_generation,
             GenerationKey::Settings => self.settings_generation,
@@ -152,6 +155,7 @@ impl RuntimeGenerations {
             GenerationKey::Addon => &mut self.addon_generation,
             GenerationKey::Search => &mut self.search_generation,
             GenerationKey::Discover => &mut self.discover_generation,
+            GenerationKey::DiscoverPaging => &mut self.discover_paging_generation,
             GenerationKey::Sync => &mut self.sync_generation,
             GenerationKey::Auth => &mut self.auth_generation,
             GenerationKey::Settings => &mut self.settings_generation,

src/runtime/effects.rs

@@ -15,6 +15,7 @@ pub enum EffectKind {
     FetchAddonManifest,
     FetchAddonResource,
     FetchCatalogPage,
+    FetchDiscoverPage,
     FetchDetailSecondary,
     FetchDetailStreams,
     FetchIntroSegments,
@@ -63,6 +64,7 @@ impl EffectKind {
             EffectKind::FetchAddonManifest => "fetchAddonManifest",
             EffectKind::FetchAddonResource => "fetchAddonResource",
             EffectKind::FetchCatalogPage => "fetchCatalogPage",
+            EffectKind::FetchDiscoverPage => "fetchDiscoverPage",
             EffectKind::FetchDetailSecondary => "fetchDetailSecondary",
             EffectKind::FetchDetailStreams => "fetchDetailStreams",
             EffectKind::FetchIntroSegments => "fetchIntroSegments",
@@ -112,6 +114,7 @@ impl EffectKind {
             "fetchAddonManifest" => EffectKind::FetchAddonManifest,
             "fetchAddonResource" => EffectKind::FetchAddonResource,
             "fetchCatalogPage" => EffectKind::FetchCatalogPage,
+            "fetchDiscoverPage" => EffectKind::FetchDiscoverPage,
             "fetchDetailSecondary" => EffectKind::FetchDetailSecondary,
             "fetchDetailStreams" => EffectKind::FetchDetailStreams,
             "fetchIntroSegments" => EffectKind::FetchIntroSegments,
@@ -212,6 +215,7 @@ mod tests {
             EffectKind::FetchAddonManifest,
             EffectKind::FetchAddonResource,
             EffectKind::FetchCatalogPage,
+            EffectKind::FetchDiscoverPage,
             EffectKind::FetchDetailSecondary,
             EffectKind::FetchDetailStreams,
             EffectKind::FetchIntroSegments,