Handle binary data (buffers) transfer from client -> server

js/src/comm.ts

@@ -1,3 +1,4 @@
+import { encode } from "base64-arraybuffer";
 import { Throttler } from "./utils";
 
 // This class is a striped down version of Comm from @jupyter-widgets/base
@@ -29,11 +30,17 @@ export class ShinyComm {
     metadata?: any,
     buffers?: ArrayBuffer[] | ArrayBufferView[]
   ): string {
+
+    // Encode buffers as base64 before stringifying the message
+    const buffers_64 = (buffers || []).map((b: ArrayBuffer | ArrayBufferView) => {
+      const buffer = b instanceof ArrayBuffer ? b : b.buffer;
+      return encode(buffer);
+    });
+
     const msg = {
       content: {comm_id: this.comm_id, data: data},
       metadata: metadata,
-      // TODO: need to _encode_ any buffers into base64 (JSON.stringify just drops them)
-      buffers: buffers || [],
+      buffers: buffers_64,
       // this doesn't seem relevant to the widget?
       header: {}
     };

shinywidgets/_shinywidgets.py

@@ -125,6 +125,7 @@ def _():
         msg = json.loads(msg_txt)
         comm_id = msg["content"]["comm_id"]
         comm: ShinyComm = COMM_MANAGER.comms[comm_id]
+        # TODO: seems we probably need to transform (base64 encoded) buffers back into bytes/memoryviews?
         comm.handle_msg(msg)
 
     def _restore_state():
@@ -147,6 +148,7 @@ def _restore_state():
 # Reactivity
 # --------------------------------------
 
+
 def reactive_read(widget: Widget, names: Union[str, Sequence[str]]) -> Any:
     """
     Reactively read a widget trait