UIView的高度限制打破了子视图的高度限制

Question

I have a really simple setup that I can't get to do what I want.

I'm trying to add an inputAccessoryView .

For the inputAccessoryView I have a UIView ( ViewA ) with a subview ( ViewB ). ViewB is fixed to all edges of ViewA . ViewB has a fixed height, which needs to be the height of both ViewA and ViewB .

ViewA 's isn't set by me, but is 0 which breaks the height of ViewB (set to 42).

ViewA is added as a subview on a UIViewController 's view and fixed to the bottom and both the leading and trailing edges (so width of ViewA and ViewB should be that of the UIViewController's view, which it is).

Here's the code I have:

override init(frame: CGRect) {
    super.init(frame: frame)

    let viewB = UIView()
    addSubview(viewB)

    viewB.translatesAutoresizingMaskIntoConstraints = false
    viewB.topAnchor.constraint(equalTo: topAnchor).isActive = true
    viewB.leadingAnchor.constraint(equalTo: leadingAnchor).isActive = true
    viewB.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
    viewB.trailingAnchor.constraint(equalTo: trailingAnchor).isActive = true
    viewB.heightAnchor.constraint(equalToConstant: 42).isActive = true
}

Which results in the following error:

2017-12-02 15:44:59.150228-0800 Ping[37511:5808904] [LayoutConstraints] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
    (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSAutoresizingMaskLayoutConstraint:0x608000283b60 h=--& v=--& MyApp.ViewA:0x7fe606511360.height == 0   (active)>",
    "<NSLayoutConstraint:0x608000282030 UIView:0x7fe606511550.height == 42   (active)>",
    "<NSLayoutConstraint:0x608000281d60 V:|-(0)-[UIView:0x7fe606511550]   (active, names: '|':MyApp.ViewA:0x7fe606511360 )>",
    "<NSLayoutConstraint:0x608000281ef0 UIView:0x7fe606511550.bottom == MyApp.ViewA:0x7fe606511360.bottom   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x608000282030 UIView:0x7fe606511550.height == 42   (active)>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

ViewA has a pretty simple setup:

extension MessageViewController {

    open override var canBecomeFirstResponder: Bool {
        return true
    }

    open override var inputAccessoryView: UIView? {
        return messageInputView
    }

}

Why does ViewA height that I never set break the constraint that I setup?

Answer 1

I think the solution is to disable the Autoresizing Mask for ViewA :

viewA.translatesAutoresizingMaskIntoConstraints = false

Since you are laying it out with the constraints. Now it tries to set your ViewA's height to 0 which causes the problem.