An Android SkuDetails property extension that calculates price-per-month.

SkuDetails.PricePerMonth.kt

import com.android.billingclient.api.SkuDetails
import java.lang.Exception
import java.text.NumberFormat
import java.util.*

/**
 * Currency code for the U.S. dollar
 */
private const val USD = "USD"

/**
 * Uses [SkuDetails.getSubscriptionPeriod], [SkuDetails.getOriginalPriceAmountMicros],
 * [SkuDetails.getPriceCurrencyCode], and the device's default locale to calculate and format the
 * price per month from the given subscription's "original" price.
 *
 * "Original" price is defined in the SDK as "before any applicable sales have been applied."
 *
 * @return Null if [SkuDetails.getSubscriptionPeriod] does not match a supported ISO 8601 formatted
 * string (i.e. the value is absent/null).
 * @see SkuDetails
 */
val SkuDetails.pricePerMonth: String?
    get() {
        // Bail early if we know that we can't derive a subscription from the SkuDetails. This value
        // is expected to be absent for non-subscription products.
        if (this.subscriptionPeriod.isNullOrBlank()) {
            return null
        }

        val originalPriceAmountMicros: Float = this.originalPriceAmountMicros.toFloat()

        // `subscriptionPeriod` produces an ISO 8601 formatted string. Only check against the
        // relevant strings specified by the `BillingClient` API.
        val rawPricePerMonth: Float = when (this.subscriptionPeriod) {
            "P1Y" -> originalPriceAmountMicros / 12_000_000 // 12 month period
            "P6M" -> originalPriceAmountMicros / 6_000_000 // 6 month period
            "P3M" -> originalPriceAmountMicros / 3_000_000 // 3 month period
            "P1M" -> originalPriceAmountMicros / 1_000_000 // 1 month period
            else -> return null
        }

        // Use the device's locale and the SkuDetail's currency code to produce a string that
        // represents the calculated monthly price.
        val numberFormat = NumberFormat.getCurrencyInstance(Locale.getDefault())
        numberFormat.currency = try {
            Currency.getInstance(this.priceCurrencyCode)
        } catch (ex: Exception) {
            // Protect against null currency codes as well as a codes that aren't a valid ISO 4217
            // string. Fallback to USD in this case.
            Currency.getInstance(USD)
        }

        return numberFormat.format(rawPricePerMonth)
    }