Tour of the Composable Architecture: Stacks

Stephen: Maybe later we can come along and add more tests to this as we add more logic and behavior, or we can make an exhaustive version of this test to make sure we understand how everything in the feature evolves.

We now have yet another feature under our belts, that of seeing the details of a standup and even editing the standup.

Brandon: There’s still a lot more left to implement in the detail screen, but I think the most interesting thing to look at next is how can we navigate to this new detail screen from the standups list. There are actually two ways to go about this, and we’ve explored both ways in past episodes on Point-Free.

• The first style is what we like to call “tree-based navigation”. This is where you model navigation state with optionals, where nil represents you are not navigated to a feature, a non-nil represents an active navigation. We call this tree-based navigation because the optional states nest forming a tree-like structure. This is the style we have been using so far for our sheets, and it is the style we used when we built the Standups app in vanilla SwiftUI for our “Modern SwiftUI” series of episodes.

• But then, for drill-down navigation in particular, there’s a second style that we like to call “stack-based navigation”. This is where you model navigation stack as a flat array of states. The act of drilling down to a feature corresponds to appending a value to the stack, and popping a feature corresponds to removing a value from the stack. We re-factored the vanilla SwiftUI Standups app to use stack-based navigation during a live stream a few months ago.

Each style has their pros and cons in turns of power, expressiveness, decoupling, safety, ergonomics, and more. There is no single right answer of which to use.

But, for this application, since we have already shown off tree-based navigation, and we will even have more examples of tree-based navigation coming up soon, we will employ stack-based navigation in order to navigate from the standups list down to the detail screen.

Let’s get started.