What’s new in relm 0.10.0?

The model is no longer a shared. In the previous versions of relm, it was a Rc<RefCell<Model>> for convenience: it was possible to share a model with a gtk+ callback for instance. However, when developing titanium, I’ve again got bitten by the infamous borrow mut error at runtime, while this was precisely one issue relm tries to solve. Now, there’s a single owner to a widget’s model and the only way to mutate the model is through its widget: if we want to update the model from another widget, we need to send a message to the other widget.

Also, the feature allowing to use synchronous callbacks in an asynchronous way were removed because they do not fit well within the model of relm, where everything is asynchronous. While this feature was convenient because it allowed to return a value to a gtk+ callback from within a widget’s update method, it was removed because it could cause slowdown if abused and mainly because it was unsound. Remember that the update method takes self by mutable reference, and with this feature, it was possible to block an event handling, while it was waiting for the widget to resolve the value to be returned to the gtk+ callback. So, if the event was triggered twice, it could be possible to have two &mut over the model. That’s a pretty bad thing that Rust prevents us from doing.

This release took a long time to be release. The reason is that I was waiting for tokio issues to be fixed. Since I’ve been waiting for months and they are still not resolved, I decided to release a version without the examples relying on tokio (the http and websockets examples). And they’re not likely to be fixed soon since tokio-proto is likely to be de-emphasize and it’s used both by hyper and twist (the websockets crate). That doesn’t mean it’s not possible to use tokio: titanium uses tokio, but it needs a special version of the tokio-based crate so that the futures can be spawned on the glib event loop. In order to be able to use tokio-based crates with relm, they need to be updated to use the new Executor trait instead of relying on the tokio-core Handle.