From ec95d2a63f7b1e8daf297717dfea7be5ca7db9ae Mon Sep 17 00:00:00 2001
From: Bart Louwers <bart@emeel.net>
Date: Sun, 24 Aug 2025 09:47:53 +0200
Subject: [PATCH 1/4] add failing tests for derive

---
 bau/test/derive.test.js | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/bau/test/derive.test.js b/bau/test/derive.test.js
index 66a77056..6cf8cb56 100644
--- a/bau/test/derive.test.js
+++ b/bau/test/derive.test.js
@@ -115,4 +115,35 @@ describe("derive", async () => {
     assert.equal(b.val, 1);
     expect(spys).toHaveBeenCalledTimes(2);
   });
+
+  it("updates incrementally", () => {
+    const a = bau.state(false);
+    const b = bau.state(false);
+    const derived = bau.derive(() => {
+      if (!a.val) return 1;
+      if (!b.val) return 2;
+      return 3;
+    });
+
+    assert.equal(derived.val, 1);
+    a.val = true;
+    assert.equal(derived.val, 2);
+    b.val = true;
+    assert.equal(derived.val, 3);
+  });
+
+  it("updates when first null", () => {
+    const a = bau.state(false);
+    const b = bau.state(false);
+    const derived = bau.derive(() => {
+      if (!a.val) return null;
+      if (!b.val) return null;
+      return true;
+    });
+
+    assert.equal(derived.val, null);
+    a.val = true;
+    b.val = true;
+    assert.equal(derived.val, true);
+  });
 });

From 25e09543d0bc74ecd44d0f2089f3dba67ce9fa0a Mon Sep 17 00:00:00 2001
From: Bart Louwers <bart@emeel.net>
Date: Sun, 24 Aug 2025 10:18:20 +0200
Subject: [PATCH 2/4] add potential fix

---
 bau/bau.js | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/bau/bau.js b/bau/bau.js
index 39560ae0..fce8e5b6 100644
--- a/bau/bau.js
+++ b/bau/bau.js
@@ -52,6 +52,7 @@ export default function Bau(input) {
   let updateDom = (state, op) => {
     !_stateOps.length && _window.requestAnimationFrame(processDom);
     _stateOps.push([state, op]);
+    for (let l of state.listeners) l.state.dirty = true;
   };
 
   const processDom = () => {
@@ -249,10 +250,14 @@ export default function Bau(input) {
     rawVal: initVal,
     bindings: [],
     listeners: [],
+    dirty: false,
     __isState: true,
     get val() {
       let _state = this;
       _curDeps?.g?.add(_state);
+      if (_state.computed && _state.dirty) {
+        deriveInternal(_state.computed, _state);
+      }
       return (
         _state.valProxy ??
         ((_state.valProxy = isArrayOrObject(initVal)
@@ -270,9 +275,11 @@ export default function Bau(input) {
       if (isArrayOrObject(value)) {
         state.valProxy = createProxy(state, value);
         updateDom(state, ["assign", value]);
+        state.dirty = false;
       } else if (value !== oldVal) {
         state.valProxy = value;
         state.bindings.length + state.listeners.length && updateDom(state);
+        state.dirty = false;
       }
     },
   });
@@ -304,12 +311,13 @@ export default function Bau(input) {
       )
     ))
       dep.listeners.push(listener);
+      state.dirty = false;
   };
 
   let derive = (computed, options) => {
     let state = createState(undefined, options);
+    state.computed = computed;
     deriveInternal(computed, state);
-    state.computed = true;
     return state;
   };
 

From 8d2e45ffc471ca763957ac590eb9baf86ce1884d Mon Sep 17 00:00:00 2001
From: Bart Louwers <bart@emeel.net>
Date: Sun, 24 Aug 2025 10:20:40 +0200
Subject: [PATCH 3/4] remove redundant state.dirty reset in value assignment

---
 bau/bau.js | 2 --
 1 file changed, 2 deletions(-)

diff --git a/bau/bau.js b/bau/bau.js
index fce8e5b6..bd72fbbe 100644
--- a/bau/bau.js
+++ b/bau/bau.js
@@ -275,11 +275,9 @@ export default function Bau(input) {
       if (isArrayOrObject(value)) {
         state.valProxy = createProxy(state, value);
         updateDom(state, ["assign", value]);
-        state.dirty = false;
       } else if (value !== oldVal) {
         state.valProxy = value;
         state.bindings.length + state.listeners.length && updateDom(state);
-        state.dirty = false;
       }
     },
   });

From fddf3fa72d21be0ac4f6e3ae8e42b06b52388a9d Mon Sep 17 00:00:00 2001
From: Bart Louwers <bart@emeel.net>
Date: Sun, 24 Aug 2025 12:33:19 +0200
Subject: [PATCH 4/4] add another test

---
 bau/test/derive.test.js | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/bau/test/derive.test.js b/bau/test/derive.test.js
index 6cf8cb56..f686792e 100644
--- a/bau/test/derive.test.js
+++ b/bau/test/derive.test.js
@@ -146,4 +146,24 @@ describe("derive", async () => {
     b.val = true;
     assert.equal(derived.val, true);
   });
+
+  it("does not call derive when non-dependencies change", () => {
+    const a = bau.state(0);
+    const b = bau.state(0);
+    let called = 0;
+    const derived = bau.derive(() => {
+      ++called;
+      return a.val;
+    });
+    assert.equal(derived.val, 0);
+    assert.equal(derived.val, 0);
+    assert.equal(called, 1);
+    ++a.val;
+    assert.equal(derived.val, 1);
+    assert.equal(derived.val, 1);
+    assert.equal(called, 2);
+    ++b.val;
+    assert.equal(derived.val, 1);
+    assert.equal(called, 2);
+  });
 });