以编程方式定义的约束 Swift iOS

Question

我有一个UICollectionView。我还有以下在代码中生成的视图：

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

现在，我想添加messageInputContainerView，使其附在屏幕底部，UICollectionView 就在它的正上方。目前我有以下限制：

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!)

问题是collectionView现在也接触到了屏幕的底部，并且两个视图重叠了。我尝试使用以下约束来解决这个问题：

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

view.addConstraint(newConstraint)

然而这只是通过一堆错误：

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

请查看基本的tutorials 以获取自动布局。它只会为您提供 IB 和约束所需的背景。

调试日志通常是您所需要的 - 它告诉您问题的确切信息。让我们仔细考虑一下

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

UICollectionView:0x7fece08c0a00 - 这是你的收藏视图，很简单 UIView:0x7fece04992d0 - 这是您的 messageInputContainerView - 从以下约束中可以清楚地看出：

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

由这行代码生成的：

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

所以，如果你可以看到 - uicollection 视图具有 NSAutoresizingMaskLayoutConstraint - 那么，那些是默认生成的。你有两种方法——通过 IB 设置你的集合视图约束——设置前导、训练、顶部（到你需要的任何值）和底部约束，值为 48。

第二个 - 以编程方式删除自动生成的约束并添加新的约束，如下所示：

[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)

希望这会有所帮助。