From 633611056e194dd0b935f825fcdf6534e8a837ec Mon Sep 17 00:00:00 2001
From: Tangent Wantwight <tangent128@gmail.com>
Date: Sat, 2 May 2020 21:09:38 -0400
Subject: [PATCH] create "catchTalkback" callbag operator

---
 src/Utilities/Callbag.ts | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/src/Utilities/Callbag.ts b/src/Utilities/Callbag.ts
index 61cc2c3..63328b9 100644
--- a/src/Utilities/Callbag.ts
+++ b/src/Utilities/Callbag.ts
@@ -189,3 +189,28 @@ export function map<I, O, TB>(f: (input: I) => O): (source: Callbag<TB, I>) => C
     };
 }
 
+/**
+ * Intercepts talkback data sent to a source and provides them to a closure instead.
+ * Does not disconnect signals; they are sent upstream as normal.
+ */
+export function catchTalkback<T, TB>(talkbackHandler: (data: TB) => void): (source: Source<T>) => Callbag<TB, T> {
+    return (source: Source<T>) => (type: START | DATA | END, data?: Callbag<T, TB> | TB) => {
+        if (type !== 0) return;
+        let sourceTalkback: Callbag<void, unknown> | undefined;
+        const sink = data as Callbag<T, TB>;
+        source(0, (type: START | DATA | END, data?: Callbag<void, unknown> | T) => {
+            if (type === 0) {
+                sourceTalkback = data as Callbag<void, unknown>;
+                sink(0, (type: START | DATA | END, data?: Callbag<T, TB> | TB) => {
+                    if (type === 1) {
+                        talkbackHandler(data as TB);
+                    } else {
+                        sourceTalkback?.(type as 0 & 1 & 2, data);
+                    }
+                });
+            } else {
+                sink(type as 1 & 2, data);
+            }
+        });
+    };
+}