feat: add beforeSend hook to sub()

relay.js

@@ -119,7 +119,10 @@ export function relayConnect(url, onNotice = () => {}, onError = () => {}) {
     ws.send(msg)
   }
 
-  const sub = ({cb, filter}, channel = Math.random().toString().slice(2)) => {
+  const sub = (
+    {cb, filter, beforeSend},
+    channel = Math.random().toString().slice(2)
+  ) => {
     var filters = []
     if (Array.isArray(filter)) {
       filters = filter
@@ -127,16 +130,25 @@ export function relayConnect(url, onNotice = () => {}, onError = () => {}) {
       filters.push(filter)
     }
 
+    if (beforeSend) {
+      const beforeSendResult = beforeSend({filter, relay: url, channel})
+      filters = beforeSendResult.filter
+    }
+
     trySend(['REQ', channel, ...filters])
     channels[channel] = cb
     openSubs[channel] = filters
 
     const activeCallback = cb
     const activeFilters = filters
+    const activeBeforeSend = beforeSend
 
     return {
-      sub: ({cb = activeCallback, filter = activeFilters}) =>
-        sub({cb, filter}, channel),
+      sub: ({
+        cb = activeCallback,
+        filter = activeFilters,
+        beforeSend = activeBeforeSend
+      }) => sub({cb, filter, beforeSend}, channel),
       unsub: () => {
         delete openSubs[channel]
         delete channels[channel]