Apple's (NASDAQ:AAPL) most recent quarter marked the end of a reasonably successful year for the company . Net revenue grew to $229.2 billion, up from $215.6 billion in the year prior, thanks to growth in its iPhone, Mac, services, and other-products businesses, partially offset by another decline in its iPad business.

Let's look at how Apple's largest and most important business -- its iPhone business -- did during the year, what drove that performance, and what investors can expect from this business in the years ahead.

Apple exec Phil Schiller on stage with an image of both the iPhone 8 and iPhone 8 Plus projected on the screen behind him.

Image source: Apple.

A return to growth

Apple sold a record 231.2 million iPhones, generating $155 billion in revenue during its fiscal 2015, which is when the iPhone 6 and 6 Plus "super cycle" took place. After that, the company saw iPhone unit shipments drop to just 211.9 million and revenue dip to $136.7 billion in the following year.

In Apple's most recent annual filing, the company attributed that decline to "a lower rate of iPhone upgrades during 2016 compared to 2015 and challenging macroeconomic conditions in a number of major markets in 2016."

Clearly, Apple's iPhone 6s and iPhone SE products just weren't enough for Apple to sustain the success it enjoyed in fiscal 2015.

In fiscal 2017, the year of the iPhone 7 and 7 Plus, Apple saw iPhone unit shipments tick up 2% year over year to 216.8 million units. iPhone revenue growth outpaced unit shipment growth, coming in at 3%, good for $141.3 billion in iPhone revenue.

Apple saw neither iPhone revenue nor unit shipments return to the levels seen during fiscal 2015, but it's encouraging that the company didn't see further year-over-year declines in its iPhone business this year.

In addition, Apple says that "the weakness in foreign currencies relative to the U.S. dollar had an unfavorable impact on iPhone net sales during 2017 compared to 2016." Had currency exchange rates remained at the same levels in fiscal year 2017 that they were at in fiscal 2016, iPhone revenue could've been greater, though Apple doesn't specify by how much.

What drove this performance?

Apple saw both iPhone unit shipments grow and iPhone average selling prices move up. The company attributes the latter to customers who bought a "different mix of iPhones with higher average selling prices."

That doesn't tell us much, but it's widely believed that Apple's iPhone 7 Plus did quite well over the last year relative to the standard iPhone 7, thanks to the inclusion of a dual-lens camera and customer interest in larger-screen displays.

I wouldn't be surprised if a mix shift toward the iPhone 7 Plus from the iPhone 7, coupled with a potential decline in low-cost iPhone SE sales, were responsible for the favorable product mix Apple sold during the year.

The unit increase is perhaps more interesting. Research firm IDC predicted back in March that the overall smartphone market would see unit shipment growth of about 4.2% in calendar year 2017. Apple's fiscal 2017 doesn't quite align with calendar year 2017, but I would imagine that the unit growth Apple enjoyed was primarily due to overall smartphone market growth.

This coming year

In fiscal 2018, Apple's current fiscal year, the iPhone business will see revenue from three new flagship products: the iPhone 8, iPhone 8 Plus, and iPhone X (pronounced "iPhone ten"). The iPhone 8 and 8 Plus are straightforward improvements of last year's iPhone 7 and 7 Plus, while the iPhone X represents a radically redesigned new phone with a lot of interesting and unique technologies. The iPhone X is also more expensive than prior-generation iPhones have been, with the entry-level version starting at $999.

Over the next year, then, I think it's fair to assume that Apple will see an acceleration in both unit shipment sales driven by the iPhone X and substantial average selling price growth, also driven by the iPhone X.

All told, I expect that during this product cycle, Apple will see iPhone unit shipments and revenue surpass the levels seen in fiscal 2016.