MSC3819: Allowing widgets to send/receive to-device messages

[turt2live]

Rendered

Implementations

• Implement MSC3819: Allowing widgets to send/receive to-device messages matrix-widget-api#57
• Indicate whether to-device events are / should be encrypted matrix-widget-api#63
• Implement MSC3819: Allowing widgets to send/receive to-device messages matrix-react-sdk#8885

Shepherd: @toger5

[robintown]

We need to include the event type somewhere in here. I'd suggest an interface of

{
  type: string;
  messages: { [userId: string]: { [deviceId: string]: unknown } };
}

[robintown]

We also need to control whether the event is encrypted or not

{
  type: string;
  encrypted: boolean;
  messages: { [userId: string]: { [deviceId: string]: unknown } };
}

[Fox32]

I noticed an issue with the current implementation. The payload (unknown above) varies between encrypted: true and encrypted: false in Element Web right now. For unencrypted messages it is directly the content of the message, like {"key": "value"}, but for encrypted events it has to be wrapped like {"type": "event type…", "content": {"key": "value"} }. If that is really desired and not just a mistake, we should make sure to document the behavior.

I created an issue for it: element-hq/element-web#24470

[Fox32]

While working with the current support of MSC3819 in Element we noticed a problem:

The send endpoint of the toDevice message API allows to target specific device
ids, but there is currently no way to know a specific device id for a widget.
Neither is it possible to embed a device id into the URL,
nor does a received toDevice message indicate the sender device to reply to it.
Sure, it is possible to target all devices via *, but using a specific device
id makes sending much more efficient.

Element Call, which this MSC initially was created for, is cheating a little bit.
As Element Call is a special widget anyway, it is created with
custom code instead of the normal widget API setup code.
This allows to embed additional parameters into the widget URL, including the device id.
From the perspective of bringing the product to the market quickly, I think it
is fine, but we should focus on getting this MSC into a complete state.

Therefore I suggest to amend small section here:

**Passing device id to widgets**

A new template variable is added to the available options for a widget URL:

* `device_id` - The device id of the client instance which is rendering the widget.

And extend the unstable prefix section with:

* `org.matrix.msc3819.device_id` in place of the proposed variable.

If this extension is desired, I'm open to add the implementation to the matrix-react-sdk and matrix-widget-api.

[Fox32]

I started working on implementing this addition over at matrix-org/matrix-widget-api#78

[dhenneke]

In matrix-org/matrix-widget-api#78, we decided to slightly rename the parameter. Thus the amendment should be as follows:

**Passing device id to widgets**

A new template variable is added to the available options for a widget URL:

* `matrix_device_id` - The device id of the client instance which is rendering the widget.

And extend the unstable prefix section with:

* `org.matrix.msc3819.matrix_device_id` in place of the proposed variable.