swift - Trouble adjusting constraints programmatically -

hey have setup this:

the left constraint constant 111. want change constraint 20 , increase length of box. have:

@ibaction func didtapbutton(sender: anyobject) {          view.removeconstraint(leadingconstraint)         leadingconstraint = nslayoutconstraint(item: view.superview!, attribute: .leading, relatedby: .equal, toitem: greenview, attribute: .leading, multiplier: 1.0, constant:20)         view.addconstraint(leadingconstraint)      } 

i had error leadingconstraint = nslayoutconstraint(item: view.superview!, attribute: .leading, relatedby: .equal, toitem: greenview, attribute: .trailing, multiplier: 1.0, constant:20). changed .leading , .leading both attributes. rectangle stretching it's not correct size, it's going off screen:

what doing wrong here? (note: want change these constraints adding new constraint not changing constant value. i'm trying better understanding!). pointers on appreciated! thanks

the way should is:

storyboard:

• create 2 constraints (one first state , 1 end one).
• change priority of each constraint. first 1 must higher second, not 1000. let's 751 first , 749 second.
• create iboutlet each constraint.

code:

• change priority of each constraint (firstconstraint.priority = 749, secondconstraint.priority = 751).
• do view.layoutifneeded().
• all these must done in main thread.

hope works.