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For all the new products, services, and technologies, investors are really focused on two metrics as it relates to Apple (NASDAQ:AAPL): iPhone units and China sales. And that makes sense, over the past four quarters, the company's signature product has now been responsible for over 60% of its total revenue for over the past four quarters. The company went as far as listing its reliance to a single product as a potential risk in the company's third-quarter report.

And when it comes to iPhone growth, the company is doing well. In the recently reported fourth fiscal quarter, Apple reported $32.21 billion in iPhone revenue with unit sales of 48 million. As opposed to last year's corresponding quarter, those figures are up 36% and 22%, respectively, leading the company to an earnings-per-share figure of $1.96, up 38% from last-year's figure.

For China, however, Apple continues to execute. Taking advantage of perhaps the last easy year-on-year comparison, Apple reported yet another huge year-on-year growth figure in the Middle Kingdom.

China keeps growing
On a year-on-year basis, China keeps enriching Apple investors. Here's a representation of China's increasing reliance to Apple's top line:

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Source: Apple's quarterly report. Revenue figures in millions. 

As you can see, Apple continues to register excellent year-on-year growth from China. For the past four quarters, Apple has grown its top line in China over 70%, this quarter barely missing its second triple-digit year-on-year growth rate. Per Tim Cook's conference call, the company grew iPhone unit sales an aggressive 120% in developing Mainland China (a designation that specifically excludes developed regions Hong Kong and Macau).
 
Overall, Apple tremendous growth is mostly powered by China. On a raw revenue basis, Apple grew its top line nearly $9.4 billion, growing from total revenue of $42.1 billion last year to $51.5 billion this year. Of that $9.4 billion revenue, $6.2 billion -- or nearly two-thirds -- came directly from Greater China. On a growth basis, Apple is increasingly becoming tethered to China. On the other hand, other smartphone makers like Samsung (NASDAQOTH:SSNLF) and Xiaomi are feeling the pinch from China's slowing smartphone markets. As such, investors should follow its moves in the country closely.
 
A canary in the coal mine?
If there was one slightly negative factor in Apple's China result, however, it has to be the sequential figure. Oddly enough, and uncharacteristic of Apple's overall result, Cupertino saw its Greater China revenue drop when compared with last quarter -- although it was only a 5% drop. There doesn't appear to be a difference in seasonality or culture that would justify a sequential revenue drop. Recently Samsung reported its first year-on-year operating profit in roughly two years during its third-quarter earnings report -- but the growth wasn't due to strong performance in its mobile division, but rather from the company's semiconductors business as competition from Apple and Huawei have hit Samsung's mobile business in the Middle Kingdom. 
 
As far as Apple's China operations are concerned, Cook has gone to great lengths to convince nervous investors that the company is still on solid footing, even going as far as emailing Mad Money host Jim Cramer, numerous data points may be hinting this isn't totally true going forward. So while Apple's been known as a company that seemingly is unaffected by the greater economy, as it continued to grow revenue during the Great Recession, this theory could be put to the test if China's economy continues to slow.
 
Perhaps Apple's preparing for a Chinese slowdown: For its seasonally heavy first fiscal quarter ended in December, the company estimates it will recognize between $75.5 billion and $77.5 billion in revenue. At the midpoint, this is year-on-year revenue growth of 2.5%. Apple investors would be wise to watch China with a hopeful, but sober, eye in regard to its overall economy. 

Jamal Carnette owns shares of Apple. The Motley Fool owns shares of and recommends Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.