程序定义的约束Swift iOS

Question

I have a UICollectionView . I also have the following view which I generate in code:

let messageInputContainerView: UIView = {
    let view = UIView()
    view.backgroundColor = UIColor.whiteColor()
    return view
}()

Now, I want to add the messageInputContainerView so that it is attached to the bottom of the screen, and the UICollectionView is directly above it. At the moment I have the following constraints :

view.addSubview(messageInputContainerView)
    view.addConstraintsWithFormat("H:|[v0]|", views: messageInputContainerView)
    view.addConstraintsWithFormat("V:[v0(48)]", views: messageInputContainerView)

    bottomConstraint = NSLayoutConstraint(item: messageInputContainerView, attribute: .Bottom, relatedBy: .Equal, toItem: view, attribute: .Bottom, multiplier: 1, constant: 0)
    view.addConstraint(bottomConstraint!)

The problem is that the collectionView is now touching the bottom of the screen as well, and the two views overlap. I tried solving this with the following constraint :

let newConstraint = NSLayoutConstraint(item: messageInputContainerView, attribute: .Top, relatedBy: .Equal, toItem: self.collectionView! , attribute: .Bottom, multiplier: 1, constant: 0)

view.addConstraint(newConstraint)

however this just through a bunch of errors :

 016-05-30 19:50:26.153 City[1858:79868] 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:0x7fece04d10e0 h=-&- v=-&- UICollectionView:0x7fece08c0a00.midY == UICollectionViewControllerWrapperView:0x7fece04125f0.midY>", "<NSAutoresizingMaskLayoutConstraint:0x7fece04d1150 h=-&- v=-&- UICollectionView:0x7fece08c0a00.height == UICollectionViewControllerWrapperView:0x7fece04125f0.height>", "<NSLayoutConstraint:0x7fece049a460 V:[UIView:0x7fece04992d0(48)]>", "<NSLayoutConstraint:0x7fece04d7620 UIView:0x7fece04992d0.bottom == UICollectionViewControllerWrapperView:0x7fece04125f0.bottom>", "<NSLayoutConstraint:0x7fece04a9990 V:[UICollectionView:0x7fece08c0a00]-(0)-[UIView:0x7fece04992d0]>" ) Will attempt to recover by breaking constraint <NSLayoutConstraint:0x7fece04a9990 V:[UICollectionView:0x7fece08c0a00]-(0)-[UIView:0x7fece04992d0]> 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. 

Answer 1

Please checkout the basic tutorials for autolayout. It will simply give You the needed background for IB and constraints.

The debug log commonly all you need - it tells You the exact info for your problem. Let's think about it in detail

NSLayoutConstraint:0x7fece04a9990 V:[UICollectionView:0x7fece08c0a00]-(0)-[UIView:0x7fece04992d0]

UICollectionView:0x7fece08c0a00 - it's your collection view, it's simple UIView:0x7fece04992d0 - it's your messageInputContainerView - that's clear from the following constraint:

"<NSLayoutConstraint:0x7fece049a460 V:[UIView:0x7fece04992d0(48)]>"

which generated by this line of code:

view.addConstraintsWithFormat("V:[v0(48)]", views: messageInputContainerView)

So, if you can see - uicollection view has NSAutoresizingMaskLayoutConstraint - so, those one were generated by default. You have two ways - set up your collection view constraints via IB - set leading, trainling, top (to whatever value you need) and bottom constraint with the value 48.

The second one - remove autogenerated constraint programmatically and add new one, as following:

[collectionView removeConstraints: [collectionView constraints]]; 
        // top constraint
    view.addConstraint(NSLayoutConstraint(item: self.collectionView, attribute: .Top, relatedBy: .Equal, toItem: view, attribute: .Top, multiplier: 1, constant: 0))
    // leading constraint
    view.addConstraint(NSLayoutConstraint(item: self.collectionView, attribute: .Leading, relatedBy: .Equal, toItem: view, attribute: .Leading, multiplier: 1, constant: 0))
    // trailing constaint
    view.addConstraint(NSLayoutConstraint(item: self.collectionView, attribute: .Trailing, relatedBy: .Equal, toItem: view, attribute: .Trailing, multiplier: 1, constant: 0))
    // your constraint
    let newConstraint = NSLayoutConstraint(item: messageInputContainerView, attribute: .Top, relatedBy: .Equal, toItem: self.collectionView! , attribute: .Bottom, multiplier: 1, constant: 0)

    view.addConstraint(newConstraint)

Hope this helps.