Now, a source has confirmed to US Weekly magazine the couple are engaged.
The pair first met on the set of male stripper movie ‘Magic Mike’, in which they both star alongside ‘The Vow’ hunk Channing Tatum.
In addition to Dianna, Alex – who plays a young man who is mentored in the world of adult entertainment in ‘Magic Mike’ - has also previously been linked to up-and-coming actress Emma Roberts.
Riley has formerly dated Jonah Hill.
Although neither Alex nor Riley have commented about their relationship, an eyewitness in October said the pair looked particularly cosy at LAX airport in Los Angeles.
The onlooker said: "They looked really cute together, cuddling and kissing each other before boarding their flight. They seem to be enjoying their new relationship."