https://www.cnbc.com/2019/01/29/apple-china-revenue-q1-2019.html

Apple's revenue in China came in at $13.17 billion during the December quarter, a drop of nearly $5 billion from a year ago, when Apple reported $17.96 billion in China revenue. Apple's revenue is typically highest during this quarter, the first of its fiscal year.

Apple saw a similar slowdown between the December 2015 and December 2016 quarters, when China revenue fell 12 percent.