MSC4157: Futures (widget api)

proposals/4157-future-widget-api.md

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+# Futures for the widget api
+
+With [MSC4140](https://github.com/matrix-org/matrix-spec-proposals/pull/4140) a way to send Future events is introduced. Futures are events that are send **now** but will
+be inserted into the dag (and distributed to all clients and federating servers) a time after **now** (based on timeout or delegation conditions).
+
+This is an extension to the client server api to expose Futures to widgets.
+This can be useful for numerous widgets. It is required for widgets implementing MatrixRTC.
+Futures are needed for reliable MatrixRTC memberships that get cleaned up when a client looses network connection.
+
+Since ElementCall (EC) is a widget and based on MatrixRTC this widget api proposel is required for EC to work.
+
+## Proposal
+
+We extend the send
+`"send_event"` request:
+
+```
+{
+    state_key?: string;
+    type: string;
+    content: unknown;
+    room_id?: string;
+
+    future_timeout?: number;
+    future_group_id?: string;
+}
+```
+
+and the `"send_event"` response:
+
+```
+{
+    room_id: string;
+    event_id?: string;
+
+    future_group_id?: string;
+    send_token?: string;
+    cancel_token?: string;
+    refresh_token?: string;
+}
+```
+
+to provide the same properties needed for [MSC4140](https://github.com/matrix-org/matrix-spec-proposals/pull/4140). The client is responsible to check
+for the field `future_timeout` or `future_group_id` of the widget action and send a `/send` or `/send_future` http request
+if one of them is present.
+
+Additionally the response is extended with the tokens and the `future_group_id`.
+
+All other details about the future concept can be found in [MSC4140](https://github.com/matrix-org/matrix-spec-proposals/pull/4140) so here we intentionally don't mention
+any of the details about futures.