ios - Hot to add a View to a Scrollview programmatically using layout constraints -

i'm triying instert programatically view inside uiscrollview, doesn't appear here code:

mainscrollview.translatesautoresizingmaskintoconstraints = false           self.view.addsubview(mainscrollview)          //add trailing         let trailingconstraint = nslayoutconstraint(item: mainscrollview, attribute: .trailing, relatedby: .equal, toitem: self.view, attribute: .trailing, multiplier: 1, constant: 0)         self.view.addconstraint(trailingconstraint)          //add leading         let leadingconstraint = nslayoutconstraint(item: mainscrollview, attribute: .leading, relatedby: .equal, toitem: self.view, attribute: .leading, multiplier: 1, constant: 0)         self.view.addconstraint(leadingconstraint)          //add top         let topconstraint = nslayoutconstraint(item: mainscrollview, attribute: .top, relatedby: .equal, toitem: self.view, attribute: .top, multiplier: 1, constant: 0)         self.view.addconstraint(topconstraint)          //add bottom         let bottomconstraint = nslayoutconstraint(item: mainscrollview, attribute: .bottom, relatedby: .equal, toitem: self.view, attribute: .bottom, multiplier: 1, constant: 0)         self.view.addconstraint(bottomconstraint)          let contentview = uiview()         contentview.translatesautoresizingmaskintoconstraints = false         contentview.backgroundcolor = .blue         mainscrollview.addsubview(contentview)          //add trailing         let trailingconstraintcontent = nslayoutconstraint(item: contentview, attribute: .trailing, relatedby: .equal, toitem: mainscrollview, attribute: .trailing, multiplier: 1, constant: 0)         mainscrollview.addconstraint(trailingconstraintcontent)          //add leading         let leadingconstraintcontent = nslayoutconstraint(item: contentview, attribute: .leading, relatedby: .equal, toitem: mainscrollview, attribute: .leading, multiplier: 1, constant: 0)         mainscrollview.addconstraint(leadingconstraintcontent)          //add top         let topconstraintcontent = nslayoutconstraint(item: contentview, attribute: .top, relatedby: .equal, toitem: mainscrollview, attribute: .top, multiplier: 1, constant: 0)         mainscrollview.addconstraint(topconstraintcontent)          //add bottom         let bottomconstraintcontent = nslayoutconstraint(item: contentview, attribute: .bottom, relatedby: .equal, toitem: mainscrollview, attribute: .bottom, multiplier: 1, constant: 0)         mainscrollview.addconstraint(bottomconstraintcontent) 

the first scrollview inserted because added background color , can see it, cant see contentview added background color blue.

any help?

update

i have tried following , no success:

mainscrollview.translatesautoresizingmaskintoconstraints = false         let contentview = uiview()         contentview.translatesautoresizingmaskintoconstraints = false          mainscrollview.backgroundcolor = .red         contentview.backgroundcolor = .blue          self.view.addsubview(mainscrollview)         mainscrollview.addsubview(contentview)          let viewsdictionary = ["mainscrollview": mainscrollview, "contentview": contentview]          let mainscrollviewverticalconstraint = nslayoutconstraint.constraints(withvisualformat: "v:|[mainscrollview]|", options: [], metrics: nil, views: viewsdictionary)         let mainscrollviewhorizontalconstraint = nslayoutconstraint.constraints(withvisualformat: "h:|[mainscrollview]|", options: [], metrics: nil, views: viewsdictionary)         self.view.addconstraints(mainscrollviewverticalconstraint)         self.view.addconstraints(mainscrollviewhorizontalconstraint)          let contentviewverticalconstraint = nslayoutconstraint.constraints(withvisualformat: "v:|[contentview]|", options: [], metrics: nil, views: viewsdictionary)         let contentviewhorizontalconstraint = nslayoutconstraint.constraints(withvisualformat: "h:|[contentview]|", options: [], metrics: nil, views: viewsdictionary)         mainscrollview.addconstraints(contentviewverticalconstraint)         mainscrollview.addconstraints(contentviewhorizontalconstraint) 

when adding sub views important thing remember pin them in way make possible auto layout evaluate height, because scroll view determines content size determining sum of heights sub views (in case of vertical). should specify height in vfl . eg.

let contentviewverticalconstraint=nslayoutconstraint.constraints(withvisualformat: "v:|[contentview(300)]", options: [], metrics: nil, views: viewsdictionary)