Modify event forwarding to allow opting out

For events which we dispatch ourselves, we can use this to skip the forwarding. Otherwise the listeners don't fire.

svelte-material-ui uses a custom dispatch to get around this issue, I think.

src/lib/internal/forwardEventsBuilder.ts

@@ -10,7 +10,7 @@ import {
 const MODIFIER_DIVIDER = "!";
 const modifierRegex = new RegExp(`^[^${MODIFIER_DIVIDER}]+(?:${MODIFIER_DIVIDER}(?:preventDefault|stopPropagation|passive|nonpassive|capture|once|self))+$`);
 
-export function forwardEventsBuilder(component: SvelteComponent) {
+export function forwardEventsBuilder(component: SvelteComponent, except: string[] = []) {
   // This is our pseudo $on function. It is defined on component mount.
   let $on: (eventType: string, callback: (event: any) => void) => () => void;
   // This is a list of events bound before mount.
@@ -20,6 +20,16 @@ export function forwardEventsBuilder(component: SvelteComponent) {
   component.$on = (fullEventType: string, callback: (event: any) => void) => {
     let eventType = fullEventType;
     let destructor = () => { };
+    if (except.includes(eventType)) {
+      // Bail out of the event forwarding and run the normal Svelte $on() code
+      const callbacks = (component.$$.callbacks[eventType] || (component.$$.callbacks[eventType] = []));
+      callbacks.push(callback);
+      return () => {
+        const index = callbacks.indexOf(callback);
+        if (index !== -1)
+          callbacks.splice(index, 1);
+      };
+    }
     if ($on) {
       // The event was bound programmatically.
       destructor = $on(eventType, callback);