Loadable: Add registerCustomProtocol() for e.g. reading assets in mobile.

libs/loadable/loadable.nim

@@ -44,6 +44,7 @@
 
 {.hint[ConvFromXtoItselfNotNeeded]:off.}
 
+import std/tables
 import arr_ref # for SliceMem
 
 type LoadableResourceStatus* = enum
@@ -236,6 +237,15 @@ func escapeUTF8*(s: string): string =
 
 # TODO: automatically disable threads when not in main thread
 
+type ProtocolHandler* = proc(uri: string): proc(self: LoadableResource)
+var custom_protocol_handlers: Table[string, ProtocolHandler]
+
+proc registerCustomProtocol*(prefix: string, handler: ProtocolHandler) =
+    ## Registers a handler for a custom protocol. The function will be run for
+    ## each uri that is requested, and will return a start_func, which in turn
+    ## will call self.onload()
+    custom_protocol_handlers[prefix] = handler
+
 proc loadUri*(
         uri: string, 
         onload_func: proc(ok: bool, err: string, data: SliceMem[byte]) = nil,
@@ -245,6 +255,15 @@ proc loadUri*(
     ): Fetch {.discardable.} =
 
     echo "fetching ", uri
+    
+    for k,v in custom_protocol_handlers:
+        if uri.startswith k:
+            proc str(): string = uri
+            var self = newLoadableResource[Fetch](v(uri), str, false)
+            self.onload_func = onload_func
+            if auto_start:
+                start(self)
+            return self
 
     var start_func: proc(self: LoadableResource)
     var self: Fetch
@@ -318,3 +337,4 @@ proc loadUri*(
     if auto_start:
         start(self)
     return self
+