The world's fastest-growing smartphone maker manufactures Alphabet's Nexus 6P.

A great deal is written about the intense smartphone competition between Apple (AAPL 1.30%) and Samsung (NASDAQOTH: SSNLF). When it comes to high-end units, Samsung's Galaxy and Apple's iPhone lines are the high-end standard, taking the vast majority of market share.

However, the two have been on different growth trajectories. Samsung's bifurcated phone strategy has been slowing, and the company is under threat as Apple has been winning high-end market share and a host of Android-based manufacturers are stealing market share in the low end. More recently, it appears Apple is not immune to greater smartphone weakness, with its last fiscal quarter registering a year-on-year iPhone shipment growth of an anemic 0.4%.

As a result, neither company could claim the title as the world's fastest-growing major smartphone vendor in 2015, according to data from market research firm IDC. That title goes to Chinese manufacturer Huawei.

Samsung lost market share in 2015 but ended on a strong note
Although Samsung remains the top smartphone vendor, the company lost market share last year. Overall, the firm shipped 325 million smartphones in 2015, up 2.1% from the 318 million units the firm sold in the prior year, so the firm narrowly avoided a year-on-year decrease in unit shipments. Unfortunately for Samsung, the overall smartphone market grew 10.1%, shipping 1.43 billion units, up from 1.3 billion in the prior year. As a result of growing at a slower rate than the overall market, Samsung's market share decreased from 24.4% in 2014 to 22.7% last year, according to IDC.

If there was one positive for Samsung in IDC's data, it was that the company grew above the greater market's clip in the fourth calendar quarter. The company shipped 14% more units in the last quarter of 2015 versus its 2014 total, whereas the overall smartphone market cooled to a 5.7% growth clip. Samsung is hoping its next-gen Galaxy S7 unit will continue the strong performance the company exhibited in the last quarter and reverse 2015's market-share loss.

Apple gained market share but ended on a sour note
Per IDC's data, Apple's year is the exact opposite of Samsung's. On a full-year basis, the company outperformed the total market. Apple doubled the greater smartphone market's growth rate of 10.1%, by shipping 20.2% more units in 2015 than it did in the prior year. The company shipped 231.5 million iPhones in 2015 versus 193 million in 2014. As a result of growth outpacing the market, the company increased its market share 1.4 percentage points, taking 16.2% share versus 14.8% in 2014.

Apple didn't end the year on a high note, however. IDC's shipment-volume data matches Apple's own iPhone figures, so there's no new info uncovered in the totals. But Apple's fourth-quarter growth of 0.4% versus the greater market's fourth-quarter shipment gains of 5.7% is a rare episode of underperformance for Cupertino. Investors are left asking will slowing growth continue and lead to year-on-year shipment decreases, or was this quarter a one-off issue?

Huawei's overperformance is notable
Although 2015 was characterized by a slowing smartphone market, there were a few standout performers. Most notable was Chinese manufacturer Huawei, which grew its units shipped figure an amazing 44.3%, joining Apple and Samsung as the only 100-million-plus-unit companies by moving 106.6 million smartphones. Huawei went from strength to strength during 2015, doing well in its home market of China and aggressively expanding in the United States through its partnership with Alphabet to manufacture the high-end Nexus 6P.

In the process, Huawei grew its market share 1.7 percentage points to report 7.4% market share in 2015. Its year-on-year growth slowed a little in the fourth quarter under pressure from Lenovo, but its fourth-quarter growth clip of 37% was still much higher than the overall market's rate of 5.7%.

In a slowing market, it is imperative for smartphone makers to consolidate market share to keep the top lines growing. Therefore, carefully watching market-share projections is of critical importance. Anybody investing in the smartphone ecosystem should pay careful attention to IDC's data.