From cbddda001b30dcd814a6431f9b246d19d225ad7b Mon Sep 17 00:00:00 2001
From: Greg Johnston <greg.johnston@gmail.com>
Date: Thu, 12 Dec 2024 18:55:39 -0500
Subject: [PATCH] fix: only mark a memo Check if it isn't already Dirty (closes
 #3339)

---
 reactive_graph/src/computed/inner.rs |  4 ++-
 reactive_graph/tests/memo.rs         | 37 ++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/reactive_graph/src/computed/inner.rs b/reactive_graph/src/computed/inner.rs
index 6512a960a6..4f6bb3c5a0 100644
--- a/reactive_graph/src/computed/inner.rs
+++ b/reactive_graph/src/computed/inner.rs
@@ -67,7 +67,9 @@ where
     fn mark_check(&self) {
         {
             let mut lock = self.write().or_poisoned();
-            lock.state = ReactiveNodeState::Check;
+            if lock.state != ReactiveNodeState::Dirty {
+                lock.state = ReactiveNodeState::Check;
+            }
         }
         for sub in (&self.read().or_poisoned().subscribers).into_iter() {
             sub.mark_check();
diff --git a/reactive_graph/tests/memo.rs b/reactive_graph/tests/memo.rs
index 2e6d3ae9aa..ec531e96b9 100644
--- a/reactive_graph/tests/memo.rs
+++ b/reactive_graph/tests/memo.rs
@@ -443,3 +443,40 @@ fn unsync_derived_signal_and_memo() {
     assert_eq!(f.with(|n| *n), 6);
     assert_eq!(f.get_untracked(), 6);
 }
+
+#[test]
+fn memo_updates_even_if_not_read_until_later() {
+    #![allow(clippy::bool_assert_comparison)]
+
+    // regression test for https://github.com/leptos-rs/leptos/issues/3339
+
+    let input = RwSignal::new(0);
+    let first_memo = Memo::new(move |_| input.get() == 1);
+    let second_memo = Memo::new(move |_| first_memo.get());
+
+    assert_eq!(input.get(), 0);
+    assert_eq!(first_memo.get(), false);
+
+    println!("update to 1");
+    input.set(1);
+    assert_eq!(input.get(), 1);
+    println!("read memo 1");
+    assert_eq!(first_memo.get(), true);
+    println!("read memo 2");
+    assert_eq!(second_memo.get(), true);
+
+    // this time, we don't read the memo
+    println!("\nupdate to 2");
+    input.set(2);
+    assert_eq!(input.get(), 2);
+    println!("read memo 1");
+    assert_eq!(first_memo.get(), false);
+
+    println!("\nupdate to 3");
+    input.set(3);
+    assert_eq!(input.get(), 3);
+    println!("read memo 1");
+    assert_eq!(first_memo.get(), false);
+    println!("read memo 2");
+    assert_eq!(second_memo.get(), false);
+}