moxie-dom: Attributes are not dynamic

[Christoph Herzog]

Attributes based on dynamic values from state are currently not functional.

A self-contained example can be found here: https://github.com/theduke/moxie-reproduction
The repo has compiled code committed, just run http-server or similar in the root .

The exact behaviour is:

• the attribute is present on the initial render
• on the first update, the attribute in question is removed from the dom completely

[Christoph Herzog]

Investigation shows that CachedNode::set_attribute is called correctly on each update.

The code that seems relevant is this: https://github.com/anp/moxie/blob/main/dom/src/cached_node.rs#L39

A scopeguard drop handler does node.remove_attribute(&name).

So it seems like the scopeguard is not functioning correctly.

[Christoph Herzog]

Futher finding of something that seems related: the id of the dom node that is used changes with each render. (determined with logging JsValue::into_abi()).

So something seems to be wrong with the dom node caching.

The builders (like div() etc) use moxie::cache to cache the node, so in theory they should remain the same nodes on each run as long as the dom structure does not change, correct?

[Adam Perry]

Looking into this on my end, I think the problem is that the scopeguard is firing after we've already set the updated attribute. The fix that works locally looks like this:

--- a/dom/src/cached_node.rs
+++ b/dom/src/cached_node.rs
@@ -31,15 +31,27 @@ impl CachedNode {
     // TODO accept PartialEq+ToString implementors
     #[topo::nested(slot = "&(self.id, name)")]
     pub(crate) fn set_attribute(&self, name: &'static str, value: &str) {
+        let mut should_set = false;
         cache_with(
             value,
-            |v| {
-                self.node.set_attribute(name, v);
+            |_| {
+                // when this isn't the first time the attribute is being set for this element,
+                // this closure executes while the previous attribute's guard is still live.
+                // if we actually set the attribute here, it will be removed when this closure exits
+                // which we definitely don't want. easiest fix is to set the attribute after our
+                // hypothetical cleanup has completed
+                should_set = true;
                 let name = name.to_owned();
-                scopeguard::guard(self.node.clone(), move |node| node.remove_attribute(&name))
+                scopeguard::guard(self.node.clone(), move |node| {
+                    node.remove_attribute(&name);
+                })
             },
             |_| {},
         );
+
+        if should_set {
+            self.node.set_attribute(name, value);
+        }
     }

[Adam Perry]

Need to write a regression test but will have a PR up shortly.