The Per Rewrite Diary: Day 8

This post is part of a series about rewriting my iOS app, Per. Per is a price per unit comparison app with a bunch of neat convenience figures, but it hasn’t been updated in years, so I’m rewriting it from scratch to eliminate a bunch of technical debt. Just because it’s not an open-source app doesn’t mean I can’t share what I learn as I go!

See the rest of the series here.

Sorting order

In the original version of Per, you entered the details for two products, and hit a big red “Compare” button to see which option gave you the most value for your money. That isn’t going to be a great way to deal with more than two products — imagine having to initiate a comparison every time you add something?

Instead, the app can simply float the best option(s) to the top of the list every time you add a new product! That makes feedback really fast and really easy.

Turns out, it’s really easy to implement this, because of a couple of things working together.

First, because I made the Product protocol conform to Comparable based on the pricePerUnit computed property on day 3, all I really need to do is call _products.sort() whenever I add a new product to the ProductList. Remember that sort() sorts an array in place, whereas sorted returns a sorted array.

But I can’t just add _product.sort() at the end of the add(item: ProductItem) method I wrote on day 5 without creating a bug: if there’s already something in the _products array, I (unnecessarily) return after appending a new ProductItem, so those early returns have to be removed.

Furthermore, I’d be changing the semantics of add(item: ProductItem) as well — this method no longer just adds an item to the product list, it adds an item and then sorts the list. I am the only person working on this, so I could leave it as is, but I would rather be kind to my future self and create a new method that I call instead:

mutating func add(_ item: ProductItem, sort: Bool = false) {
    
    // The original add(item:) method implementation goes here

    if (sort) {
        _products.sort()
    }
}

Adding this new sort parameter with a default value of false to the add(item:) method means I don’t have to change calls to the method unless I want the list sorted.

I only call it in one place right now —the handleAddProductBarButtonItemTapped action in the product list context view controller, from yesterday— and it should sort the list when called there, because we want to sort the list before updating the product list table view:

self.contentViewController.productList.add(createRandomProduct(), sort: true)

And that’s it! We can now add (randomly-generated) items to the product list and they’ll sort themselves automatically.

Tomorrow feels like a good day to start working on the UI for adding an actual product’s details, so that I can get the app doing what it’s supposed to do — compare products.