Autolayout constraints warning "Will attempt to recover by breaking constraint"

You are constraining your views too much. You must always leave for example one of leftmargin, width and rightmargin flexible, and let the autolayout engine stretch that one.

A single dash without any number means default distance, which is 20 pixels. So, if you remove your trailing constraints on

@"H:|-78-[myNext(==173)]-|"
@"H:|-11.5-[myBtn(==54)]-|"

and change them to

@"H:|-78-[myNext(==173)]"
@"H:|-11.5-[myBtn(==54)]"

the layout engine will then stretch the rightmargin of the button to make it fit the containing view. There are of course other ways of loosening up these constraints, all depending on your design goals.

Personally, I never use the standard API:s for autolayout anymore. I use the library PureLayout, which abstracts autolayout wonderfully!

These three constraints:

"<NSLayoutConstraint:0xb8282f0 H:|-(50)-[UITextField:0xb823fc0]   (Names: '|':UIView:0xb82f360 )>",
"<NSLayoutConstraint:0xb828320 H:[UITextField:0xb823fc0(210)]>",
"<NSLayoutConstraint:0xb82be10 H:[UITextField:0xb823fc0]-(NSSpace(20))-|   (Names: '|':UIView:0xb82f360 )>",

dictate that the superview be 50 + 210 + 20 == 280 points wide.

These three constraints:

"<NSLayoutConstraint:0xb82d3d0 H:|-(78)-[UIButton:0xb82ad50]   (Names: '|':UIView:0xb82f360 )>",
"<NSLayoutConstraint:0xb82d420 H:[UIButton:0xb82ad50(173)]>",
"<NSLayoutConstraint:0xb82d450 H:[UIButton:0xb82ad50]-(NSSpace(20))-|   (Names: '|':UIView:0xb82f360 )>"

dictate that the same superview be 78 + 173 + 20 == 271 points wide.

Obviously, those can't both be true at the same time. You need to decide what you really want to happen here. We can't read your mind and neither can UIKit.

Usually, you don't set a width constraint on a button; you let it use its intrinsic size with appropriate priorities for content hugging and compression resistance. Also, you may not wish to set hard spacing on either side of the button. Either let that spacing vary without constraint so the button can be its intrinsic size, lower the priority of that constraint so it's optional, or make an inequality so you have a minimum spacing but not an exact spacing.