There were two reasons China was and might still be consider buying Su-35, however these two reasons are all dead now.
1. In 2012, there is a chance that China might go to war with Japan and China thought buying some Su-35 (24-48) will secure Russia's alliance with China also getting Russia's best fighters.
2. 117S engine. In 2012 just one year after J-20's first flight China thought that 117S is the best engine China can get for J-20 for the next 5-6 years, however, Russia is not giving up 117S until China buys some Su-35.
Reason 1 is dead because Japan does not have the balls to go to war with China on its own, and after USA brutally turn its back on Japan, Japan realized that there is no chance they can go anywhere, China saw that too and realized that it was just a bluff.
Reason 2 is also dead because China decided to use the AL-31FM1 as the interim engine for J-20, and WS-10B as back up. Until WS-15 is ready. China can get AL-31FM1 without buying any Russian planes because the contract is secure way back in 2008, when Russia went to war with Georgia and Russia went to China to seek political support and China got it in exchange. The deal is reinforced by Putin in 2012, when he visited China. China figure that it is a bloody mess to use 117S because it will for one harm China's reputation, and two it will be useless to get a different fighter with systems not compatible with Chinese air force. It is simply not worth it to get 117S with all the extra pain.