The Per Rewrite Diary: Day 4

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.

Funny subtitle goes here

On day 2 of the rewrite, I put together a computed property for pricePerUnit in an extension to the Product protocol that looked like this:

var pricePerUnit: Double {
    get {
        let formatter = NumberFormatter()
        formatter.numberStyle = .currency
        let formattedPricePerUnit = formatter.string(for: NSNumber(value: price / quantity))
        return Double(truncating: formatter.number(from: formattedPricePerUnit!)!)
    }
}

As I mentioned at the time, I still don’t like the way I’m doing the necessary work of rounding digits using the NumberFormatter conversion to a String and back, but it works as a spike solution for now — we can come back to it another day.

I also added two static functions to the extension yesterday so that the protocol better conforms to Comparable — not only do we want to compare the pricePerUnit property, but we also want to compare the types of Units and make sure they’re comparable.

Here’s where I need to be careful. I can use quantity and units properties to initialize a Measurement, which gives me the ability to convert this to base units via the appropriately-named baseUnits() method on Measurement — this gives me a common unit to compare all items of the same unit type (UnitMass and UnitVolume are the two we care about). I can add the following logic to the pricePerUnit computed property getter and we’re good to go:

var pricePerUnit: Double {
    get {
        var amount: Double = 0
        let formatter = NumberFormatter()
        formatter.numberStyle = .currency
        
        if let unitType = units {
            if (unitType.isKind(of: UnitMass.self)) {
                // This product is sold by weight.
                let measurement = Measurement(value: quantity, unit: unitType as! UnitMass).converted(to: UnitMass.baseUnit())
                amount = measurement.value
            } else if (unitType.isKind(of: UnitVolume.self)) {
                // This product is sold by volumne.
                let measurement = Measurement(value: quantity, unit: unitType as! UnitVolume).converted(to: UnitVolume.baseUnit())
                amount = measurement.value
            }
        } else {
            amount = quantity
        }
        // We're force-unwrapping here but these variables should never be nil. 😬
        return Double(truncating: formatter.number(from: formatter.string(for: NSNumber(value: price / amount))!)!)
    }
}

With that, we can set up a simple ProductList model and get it hooked up as a UITableViewController data source tomorrow!