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The shell

We've looked at how the Weather app fits together, how it's tested, and if you were deveoping it along the way, you would now have a core with the important business logic, fully tested and rock solid. Time to build the UI.

(Okay sure, in practice, you would not build the whole core first, then the whole UI, you'd probably go feature by feature, but the point stands - we now know for a fact that the core does the right thing.)

The shell will have two responsibilities:

  1. Laying out the UI components, like we've already seen in Part I
  2. Supporting the app's capabilities. This will be new to us

Like in Part I, you can choose which Shell language you'd like to see this in, but first lets talk about what they all have in common.

Message interface between core and shell

In Part I, we learned to use the update and view APIs of the core. We also learned that in their raw form, they take serialized values as byte buffers.

What we skimmed the return value of update very quickly. In that case it only ever return a request for a RenderOperation - a signal that a new view model is available.

In the Weather's case, more options are possible. Recall the effect type:

#![allow(unused)]
fn main() {
#[effect(facet_typegen)]
pub enum Effect {
    Render(RenderOperation),
    KeyValue(KeyValueOperation),
    Http(HttpRequest),
    Location(LocationOperation),
}
}

Those are the four possible variants we'll see in the return from update. It is essentially telling us "I did the state update, and here are some side-effects for you to perform".

Lets say that the effect is a HTTP request. We execute it, get a response, and what do we do then? Well that's what the third core API, resolve is for:

#![allow(unused)]
fn main() {
pub fn update(data: &[u8]) -> Vec<u8>
pub fn resolve(id: u32, data: &[u8]) -> Vec<u8>
pub fn view() -> Vec<u8>
}

Each effect request comes with an identifier. We use resolve to return the output of the effect back to the app, alongside the identifier, so that it can be paired correctly.

Let's look at how this works in practice

Platforms

You can continue with you platform of choice: