Engine: a typed prop.set can move follow components by each layer's own change (#2485)

crates/engine/src/cmd/prop.rs

@@ -300,7 +300,9 @@ pub(crate) fn proportional_share(index: usize, count: usize) -> f64 {
 /// `prop.set`; with `selected`, also on the other selected layers: the same value, or with
 /// `offset` (a scrub) the same change from each layer's own value. A typed value with `dims`
 /// (the components edited, e.g. `[2]` for Z) sets only those components on the other layers,
-/// which keep the rest of their own value (#1434). With `offset` and
+/// which keep the rest of their own value (#1434); `follow` components move on each layer by the
+/// change its first typed component takes (linked Audio Levels keep each layer's balance, #2485).
+/// With `offset` and
 /// `proportional` (Proportional Scrubbing) each layer takes [`proportional_share`] of the change
 /// by its place in the selection order, the scrubbed layer too. One undo step (with `merge`,
 /// one that a continuing scrub keeps folding into).
@@ -325,10 +327,18 @@ fn set_selected(s: &mut Session, p: &Value) -> Result<Value> {
         (None, Some(a), Some(_)) if !scalar => Some(a.iter().filter_map(Value::as_u64).filter_map(|d| usize::try_from(d).ok()).collect()),
         _ => None,
     };
+    let follow: Vec<usize> =
+        p.get("follow").and_then(Value::as_array).into_iter().flatten().filter_map(Value::as_u64).filter_map(|d| usize::try_from(d).ok()).collect();
     let typed = |s: &Session, q: &mut Value, olid: LayerId, ouid: Uid| {
         if let (Some(dims), Some(n)) = (&dims, &new)
             && let Some(mut c) = current_components(s, cid, olid, ouid)
         {
+            let change = dims.first().and_then(|&d| Some(n.get(d)? - c.get(d)?)).unwrap_or(0.0);
+            for &d in &follow {
+                if let Some(slot) = c.get_mut(d) {
+                    *slot += change;
+                }
+            }
             for &d in dims {
                 if let (Some(slot), Some(v)) = (c.get_mut(d), n.get(d)) {
                     *slot = *v;
@@ -1310,7 +1320,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Set Property Value",
             [],
             None,
-            "{layer?, path|prop, value, time? (layer s by default), timeBase?: layer|comp, merge?, selected? (also the other selected layers), offset? (them by the same change), proportional? (with offset: Proportional Scrubbing, each layer by its share in selection order, the first none, the last all), dims? (with selected, a typed value: the components edited, e.g. [2] for Z; the other layers keep the rest of their own value)}",
+            "{layer?, path|prop, value, time? (layer s by default), timeBase?: layer|comp, merge?, selected? (also the other selected layers), offset? (them by the same change), proportional? (with offset: Proportional Scrubbing, each layer by its share in selection order, the first none, the last all), dims? (with selected, a typed value: the components edited, e.g. [2] for Z; the other layers keep the rest of their own value), follow? (with dims: components that move on each other layer by the change its first typed component takes)}",
             has_layers,
             set_selected
         ),

crates/engine/src/tests/timeline.rs

@@ -1430,3 +1430,23 @@ fn a_typed_component_applies_only_that_component_to_the_selected_layers() {
     s.execute("prop.set", json!({"layer": a, "prop": uid, "value": [1.0, 2.0, 3.0], "selected": true})).unwrap();
     assert_eq!(pos(&s, c), vec![1.0, 2.0, 3.0]);
 }
+
+/// #2485: linked Audio Levels typed with several layers selected: each takes the typed Left, and
+/// its Right moves by its own change, so every layer keeps its own stereo balance.
+#[test]
+fn a_typed_component_moves_its_follow_components_by_each_layers_own_change() {
+    let mut s = Session::default();
+    s.execute("comp.new", json!({"name": "Sound", "width": 64, "height": 36, "duration": 4})).unwrap();
+    s.execute("comp.new", json!({"name": "Main", "width": 320, "height": 180, "duration": 4})).unwrap();
+    let a = s.execute("layer.addItem", json!({"item": "Sound"})).unwrap()["layer"].as_u64().unwrap();
+    let b = s.execute("layer.addItem", json!({"item": "Sound"})).unwrap()["layer"].as_u64().unwrap();
+    s.execute("prop.set", json!({"layer": a, "path": "audio/levels", "value": [-6.0, -12.0]})).unwrap();
+    s.execute("prop.set", json!({"layer": b, "path": "audio/levels", "value": [-20.0, -30.0]})).unwrap();
+    s.execute("layer.select", json!({"layers": [a, b]})).unwrap();
+    let uid = prop(&s, a, "audio/levels").uid;
+    let undo = s.history.undo.len();
+    s.execute_checked("prop.set", json!({"layer": a, "prop": uid, "value": [-3.0, -9.0], "selected": true, "dims": [0], "follow": [1]})).unwrap();
+    assert_eq!(prop(&s, a, "audio/levels").value.components(), vec![-3.0, -9.0]);
+    assert_eq!(prop(&s, b, "audio/levels").value.components(), vec![-3.0, -13.0], "B rises 17 dB on both channels");
+    assert_eq!(s.history.undo.len(), undo + 1, "one undo step");
+}