Porch swing Supports
You need a structural analysis that considers the load on the roof, ceiling load, and the proposed swing load. Way too many variables to address here. If you can’t handle the structural analysis, you can do it the old fashioned way. Put a piece of plywood across the ceiling members and add 50 lbs and measure the deflection. Keep adding weight to 1.5 times the weight you plan on the swing. Don’t exceed 3/8” deflection at the center of the room. 

My gut tells me that your roof won’t support the load without truss support to spread the load.