> There are all sorts of transient states that a parent's component shouldn't have to care about.
The problem with this way of thinking is that there are invariably things that you initially believe will be of no concern to the parent that later become concerns of the parent. It's tempting at that point to graft on some event system so the parent can watch the things that it now cares about, and then maybe you add some hooks to allow the parent to preempt or change certain behaviors. This evolution requires careful thinking every time, and it's easy to make mistakes.
The great thing about the elm architecture is that from the very beginning, it gives the parent all the hooks it will ever need to observe and modify the behavior of children. In the beginning, when the parent doesn't care, it just passes the actions up and passes the model back down without having to look at them. That might feel a little bit boilerplate-y at the beginning when you don't need it, but it's the same everywhere, so it isn't difficult at all. And once you've done this, evolution of the components requires much less thinking about architecture, and leaves much less room for mistakes.
If you make all events public, you end up sending all scroll events and mouse move events to the app's top-level signal just because some component needs them. The larger your app is, the more likely it is that some component somewhere will need an event sooner or later.
Traditional UI frameworks try to avoid sending events unnecessarily for performance reasons; they're designed so that each component can declare which events it's listening to and can start and stop listening as needed.
Perhaps Elm can make this work, so that unwanted events get filtered out so quickly that it's not a concern. Also, these days everything gets animated anyway based on things like scroll events, so apps are perhaps a bit more like video games that run at 60 frames/second regardless of what's going on, at least while they're in the foreground. And the browsers themselves perform a lot better.
But it shouldn't be hard to see why people who have experience building UI frameworks might be concerned about unnecessary event traffic, particularly on mobile where battery life is an issue.
I think that's a fair point. To be honest, I'm more concerned about the real limitations of the paradigm (the nice-to-have features I will have to pass because the architecture is a bit too closed)
The problem with this way of thinking is that there are invariably things that you initially believe will be of no concern to the parent that later become concerns of the parent. It's tempting at that point to graft on some event system so the parent can watch the things that it now cares about, and then maybe you add some hooks to allow the parent to preempt or change certain behaviors. This evolution requires careful thinking every time, and it's easy to make mistakes.
The great thing about the elm architecture is that from the very beginning, it gives the parent all the hooks it will ever need to observe and modify the behavior of children. In the beginning, when the parent doesn't care, it just passes the actions up and passes the model back down without having to look at them. That might feel a little bit boilerplate-y at the beginning when you don't need it, but it's the same everywhere, so it isn't difficult at all. And once you've done this, evolution of the components requires much less thinking about architecture, and leaves much less room for mistakes.