BEIJING — China’s leaders have been markedly reticent about what kind of leader they think Donald J. Trump will be. A pragmatic dealmaker, as his business background might indicate? Or a provocateur who tests the ways of statecraft?

By talking on Friday with Taiwan’s president, Tsai Ing-wen, Mr. Trump answered that question in stark terms, Chinese analysts said Saturday. Breaking decades of American diplomatic practice, he caught the Chinese government off guard by lunging into the most sensitive of its so-called core interests, the “One China” policy agreed to by President Richard M. Nixon more than four decades ago.

“This is a wake-up call for Beijing — we should buckle up for a pretty rocky six months or year in the China-U.S. relationship,” Wang Dong, an associate professor at the School of International Studies at Peking University, said Saturday. “There was a sort of delusion based on overly optimistic ideas about Trump. That should stop.”

Chinese leaders covet stability in their relationship with Washington, and perhaps for that reason, they have allowed fairly rosy assessments of Mr. Trump to appear in the state-run news media. Many of those accounts have depicted the president-elect as a practical operator devoid of ideology, the kind of person China might find common ground with despite his threats of a trade war.