Add monitor node for graph introspection

Adds a `serialize` function on Node which can be implemented by nodes to
allow introspecting their content. This pr also adds a Monitor Node that
always caches the last value it was evaluated with.

Test Plan: -

Reviewers: 0HyperCube

Reviewed By: 0HyperCube

Pull Request: https://github.com/GraphiteEditor/Graphite/pull/1165

node-graph/gstd/src/memo.rs

@@ -1,9 +1,11 @@
 use graphene_core::Node;
+use serde::Serialize;
 
 use std::hash::{Hash, Hasher};
 use std::marker::PhantomData;
 use std::pin::Pin;
 use std::sync::atomic::AtomicBool;
+use std::sync::Mutex;
 use xxhash_rust::xxh3::Xxh3;
 
 /// Caches the output of a given Node and acts as a proxy
@@ -49,6 +51,37 @@ impl<T, CachedNode> CacheNode<T, CachedNode> {
 	}
 }
 
+/// Caches the output of the last graph evaluation for introspection
+#[derive(Default)]
+pub struct MonitorNode<T, CachedNode> {
+	output: Mutex<Option<T>>,
+	node: CachedNode,
+}
+impl<'i, T: 'i + Serialize + Clone, I: 'i + Hash, CachedNode: 'i> Node<'i, I> for MonitorNode<T, CachedNode>
+where
+	CachedNode: for<'any_input> Node<'any_input, I, Output = T>,
+{
+	type Output = T;
+	fn eval(&'i self, input: I) -> Self::Output {
+		let output = self.node.eval(input);
+		*self.output.lock().unwrap() = Some(output.clone());
+		output
+	}
+
+	fn serialize(&self) -> Option<String> {
+		let output = self.output.lock().unwrap();
+		(&*output).as_ref().map(|output| serde_json::to_string(output).ok()).flatten()
+	}
+}
+
+impl<T, CachedNode> std::marker::Unpin for MonitorNode<T, CachedNode> {}
+
+impl<T, CachedNode> MonitorNode<T, CachedNode> {
+	pub const fn new(node: CachedNode) -> MonitorNode<T, CachedNode> {
+		MonitorNode { output: Mutex::new(None), node }
+	}
+}
+
 /// Caches the output of a given Node and acts as a proxy
 /// It provides two modes of operation, it can either be set
 /// when calling the node with a `Some<T>` variant or the last