feat(style): compose generated class states

Add CssClass/CssClasses with/with_if helpers so nested partial view data can compose generated CSS constants without ad hoc class strings.

req: style/003

req: component/003

REQUIREMENTS.md

@@ -860,7 +860,7 @@ All forms support returning `impl IntoEffect` and compose through tuples.
 002	Generated class constants are ergonomic references only: they do not create a CSS framework, require a framework project structure, or make dynamic class expressions compile-time facts. Unknown Rust class references fail by normal Rust name resolution when the generated constant is absent. [north_star]
 
 ### req: style/003
-003	When a hemplate dynamic class attribute needs more than one class token, Rust passes a displayable list of generated `CssClass` values as view data. Rust should not assemble ad hoc class strings for known style tokens. [north_star]
+003	When a hemplate dynamic class attribute needs more than one class token, Rust passes a displayable list of generated `CssClass` values as view data. Conditional state classes should compose from generated constants with boring Rust helpers such as `classes::card.with_if(selected, classes::is_selected)`. Rust should not assemble ad hoc class strings for known style tokens. [north_star]
 
 ---
 

examples/techdemo/src/main.rs

@@ -504,14 +504,7 @@ fn board_view(demo: &DemoState) -> BoardLanes {
 
 fn issue_card(item: &WorkItem, selected: bool) -> IssueCard {
     IssueCard {
-        class: if selected {
-            CssClasses::from([
-                classes::work_card,
-                classes::is_selected,
-            ])
-        } else {
-            CssClasses::from(classes::work_card)
-        },
+        class: classes::work_card.with_if(selected, classes::is_selected),
         id: item.id,
         title: item.title.clone(),
         stage: item.stage.label(),

slhx-core/src/lib.rs

@@ -186,6 +186,14 @@ impl CssClass {
     pub const fn as_str(self) -> &'static str {
         self.name
     }
+
+    pub fn with(self, class: CssClass) -> CssClasses {
+        CssClasses::from([self, class])
+    }
+
+    pub fn with_if(self, condition: bool, class: CssClass) -> CssClasses {
+        CssClasses::from(self).with_if(condition, class)
+    }
 }
 
 impl AsRef<str> for CssClass {
@@ -315,6 +323,22 @@ impl CssClasses {
     pub fn as_str(&self) -> &str {
         &self.names
     }
+
+    pub fn with(mut self, class: CssClass) -> Self {
+        if !self.names.is_empty() {
+            self.names.push(' ');
+        }
+        self.names.push_str(class.as_str());
+        self
+    }
+
+    pub fn with_if(self, condition: bool, class: CssClass) -> Self {
+        if condition {
+            self.with(class)
+        } else {
+            self
+        }
+    }
 }
 
 impl From<CssClass> for CssClasses {

slhx-core/tests/effect_batch.rs

@@ -141,6 +141,9 @@ fn generated_css_classes_join_for_hemplate_dynamic_class_attrs() {
 
     assert_eq!(classes.as_str(), "work-card is-selected");
     assert_eq!(classes.to_string(), "work-card is-selected");
+    assert_eq!(CARD.with(SELECTED).as_str(), "work-card is-selected");
+    assert_eq!(CARD.with_if(true, SELECTED).as_str(), "work-card is-selected");
+    assert_eq!(CARD.with_if(false, SELECTED).as_str(), "work-card");
 }
 
 #[test]