I solved the problem. it was my mistake in writing TextField.

it says, "This post contains sensitive language. please revise it in order to continue."

I tried: I removed Assets.xcassets file from project and Warning disappeared. ( I have no idea why)

First, I'm sorry for not expressing my question clearly. I'd like to ask if I can implement property observer on "oppositePos" computed property to monitor change of opposite point according to change of original coordinatePoint. However, on my second thought, I realized I couldn't implement it on the property and I didn't need to do it. anyway, I got my answer for the question thanks to your advice. again, thanks for your advice . c00012

I solved this question myself. I should completely implement protocol function in the class not to cause such an error like this: class A: feature {     func getPayment(o: Offer) {         <#code#>     }     func Amortize(o: Offer) {         <#code#>     }     func Export() {         <#code#>     } }