Small aircraft pilot reported a NMAC while beginning a descent to a non-towered airport and took evasive action to avoid a collision.

Narrative

Enroute ZZZ to 7S5 via visual with aid of ForeFlight on iPad and Stratus ADS-B in via overflight of MMV at 4402 MSL... started gradual descent in anticipation of arrival 7S5... keeping in mind need to detour around a UAS area just to west and do a teardrop entry to the 45 for 7S5 RWY 34. I was monitoring MMV and my iPad for traffic as well as looking visually. I had no traffic spotted as I switched frequencies to pick up SLE ATIS for the wind information and BARO. When I looked again at the iPad traffic I saw an immediate conflict at 1NM and 300 ft above at 1 o'clock but never saw the actual aircraft. I saw it was climbing... now at 400 ft above my altitude (again ADS-B iPad)I started a steeper descent and a slight turn to the west but the traffic passed overhead without a visual sighting. I neglected during the event to capture the ADS-B tail identifier. I was not on Flight Following for this short flight. This was not my normal route to Independence as I'm usually further to the west to avoid the majority of the traffic in that corridor; but; this day a UAS event up to 5000ft was in the usual route to the west around COGOK.In retrospect; I should have at least monitor PDX departure on 126 and/or contacted them for traffic alerts or flew around the UAS event to the west. I may have seen the traffic on my iPad during my last scan just after MMV but obviously did not perceive it at that time as a potential threat; I honestly don't know how that 'snuck up' on me.