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Apple's growth in China continues.

The world's most populous nation has now surpassed Japan in iOS app spending, according to App Annie data cited by TechCrunch. China is now second to the U.S. in iOS app revenue after it doubled from 2014 to 2015 to hit $3.4 billion. App revenue in China is expected to grow 35% in 2016 to reach $4.6 billion.

A surge in iPhone adoption and app downloads drove the revenue growth. Apple's total revenue in Greater China soared 85% year-over-year in 2015, thanks in large part to iPhone sales. And China supplanted the U.S. as the top iOS app downloader in the first quarter of 2015.

Mobile game popularity also helped, as in-app spending in mobile games drove most of the iOS revenue growth. Games such as Fantasy Westward Journey, Westward Journey Online, Hero Moba, and The Legend of Mir 2 represented the most spending by iOS users in China.

Analysts expect China to surpass the U.S. in gross app store spend by the end of this year. Smartphone ownership in major cities such as Biejing and Harbin are close to saturation, but the smaller urban areas throughout China are still rife with potential for adoption, which in turn creates potential for app developers.

China already represents more than half (51.9%) of all app downloads in the entire world.

Apple's success also shows that Google needs to re-enter China's market after it abandoned the nation in 2010. But Google is reportedly planning to open a Google Play store in China later this year, which should let the tech giant leverage its enormous installed base within the nation. Android represented 77.4% of all smartphone sales in the nation in the third quarter of 2015.

But Apple has an advantage that Google does not. Data from iiMedia notes than 54.4% of China's app users go through third-party app stores, such as 360 Mobile Guard and Tencent's Myapp, to download Android apps. But iOS users typically cannot download third-party apps onto Apple devices without jailbreaking their phones, which gives Apple a great deal of immunity from an issue that can hurt Google.

But one challenge both Apple and Google will both face is the growth, or lack thereof, of smartphone sales.

The global smartphone market is expected to slow considerably over the next few years. Despite a record-setting holiday quarter, 2015 was likely the last year of double-digit growth for smartphone shipments.

Mature markets were at the heart of this year’s deceleration. Adoption has reached new highs in key markets in the United States, Europe, and China. The pool of first-time buyers in these countries is shrinking rapidly, and sales are now primarily coming from phone upgrades.

Meanwhile, emerging markets will continue to see robust shipment growth. India and Indonesia, in particular, will help fuel a large share of the shipments growth within the global smartphone market over the next few years.

Will McKitterick, senior research analyst at BI Intelligence, Business Insider's premium research service, has compiled a detailed report on smartphones by country that forecasts the market through 2021 to reflect slower, stabilizing growth in the long term.

BI Intelligence

Here are some key points from the report:

The global smartphone market is still growing at a steady pace due to more widespread adoption in emerging markets. We estimate the global market will hit about 2.1 billion units shipped in 2021.

Shipments growth over the past few years has been driven by the falling price of smartphones, which has made handsets more accessible in emerging markets. The average selling price of a smartphone in India nearly halved between 2010 and 2015.

With relatively low smartphone penetration, we forecast Indian smartphone shipments to grow rapidly over the next five years. Nevertheless, India has a long way to go before it surpasses China as the world’s leading market for smart handsets. India is estimated to account for roughly 10% of the global smartphone market in 2016, considerably less than China’s 30% share.

The global platform wars are over, even as smartphone adoption continues to rise across various markets worldwide. Android and iOS are estimated to account for 97.3% of global platform market share in 2015, compared to 96.3% last year.

Apple closed the year with another strong quarter on the back of its iPhone 6s and iPhone 6s Plus launches. Still, the vendor saw a slight decline in YoY growth of its share of the market in the face of stiff competition from Samsung and Chinese vendors such as Huawei.

In full, the report:

Forecasts global smartphone shipments through 2021.

Explores why India is the next high-growth smartphone market.

Breaks down the global smartphone platform wars.

Discusses smartphone vendor performance market share.

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