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iOS9下代码创建约束,不是把重点放在约束上,而是把重点放在约束所涉及到的属性上,这些属性为UIView为的anchor属性,包括为:
widthAnchor
,heightAnchor
topAnchor
,bottomAnchor
leftAnchor
,rightAnchor
,leadingAnchor
,trailingAnchor
centerXAnchor
,centerYAnchor
firstBaselineAnchor
,lastBaselineAnchor
NSLayoutConstraint
实例方法,每种形式有三种,如果忽略constant
,则其值为0,如果忽略multiplier
,则其值为1
:
constraint(equalTo:)
constraint(greaterThanOrEqualTo:)
constraint(lessThanOrEqualTo:)
constraint(equalTo:constant:)
constraint(greaterThanOrEqualTo:constant:)
constraint(lessThanOrEqualTo:constant:)
constraint(equalTo:multiplier:)
constraint(greaterThanOrEqualTo:multiplier:)
constraint(lessThanOrEqualTo:multiplier:)
constraint(equalTo:multiplier:constant:)
constraint(greaterThanOrEqualTo:multiplier:constant:)
constraint(lessThanOrEqualTo:multiplier:constant:)
constraint(equalToConstant:)
constraint(greaterThanOrEqualToConstant:)
constraint(lessThanOrEqualToConstant:)
本文内容来自
VFL的介绍与用法可以参考:
创建约束
The
NSLayoutAnchor
class is a factory class for creatingNSLayoutConstraint
objects using a fluent API.NSLayoutAnchor
是用来创建NSLayoutConstraint
对象的工厂类。相对于原来创建约束的方法更简便。
// 使用NSLayoutConstraint创建约束NSLayoutConstraint(item: subview, attribute: .Leading, relatedBy: .Equal, toItem: view, attribute: .LeadingMargin, multiplier: 1.0, constant: 0.0).active = trueNSLayoutConstraint(item: subview, attribute: .Trailing, relatedBy: .Equal, toItem: view, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true// 使用Layout Anchors来创建约束let margins = view.layoutMarginsGuidesubview.leadingAnchor.constraintEqualToAnchor(margins.leadingAnchor).active = truesubview.trailingAnchor.constraintEqualToAnchor(margins.trailingAnchor).active = true
NSLayoutAnchor
有三个子类:
- NSLayoutXAxisAnchor
- NSLayoutYAxisAnchor
- NSLayoutDimension
例如,如下创建的约束:
// 1bookTextView.translatesAutoresizingMaskIntoConstraints = false// 2bookTextView.leadingAnchor.constraintEqualToAnchor( view.leadingAnchor).active = truebookTextView.trailingAnchor.constraintEqualToAnchor( view.trailingAnchor).active = truebookTextView.bottomAnchor.constraintEqualToAnchor( view.bottomAnchor, constant: -20).active = true// 3bookTextView.heightAnchor.constraintEqualToAnchor( view.heightAnchor,
active
设置为true,表示其马上生效。
View Layout Margins
所有的view都有一个layoutMarginsGuide
属性。相对于View Layout Margins创建的约束,在其view的边缘会留下一些空白的距离。 // 1avatarView.translatesAutoresizingMaskIntoConstraints = false// 2avatarView.topAnchor.constraintEqualToAnchor( view.topAnchor).active = true// 3avatarView.leadingAnchor.constraintEqualToAnchor( view.layoutMarginsGuide.leadingAnchor).active = trueavatarView.trailingAnchor.constraintEqualToAnchor( view.layoutMarginsGuide.trailingAnchor).active = true// 4avatarView.heightAnchor.constraintEqualToConstant(200).active = true
如下,avatarView在左右两侧会留下空白的距离:
View Controller Layout Guides
同view一样,view controller 都有一个top和bottom的layout guide。上图中,avatarView在状态栏的底部,但如果还有其他的透明的bars,例如导航栏和底部的tab bar,那么上面的avatarView,就会被遮挡住。
所以当给view controller的view的subviews,添加约束时,要约束到view controller的top guide的 bottom和bottom guide的top anchor
avatarView.topAnchor.constraintEqualToAnchor( topLayoutGuide.bottomAnchor).active = truebookTextView.bottomAnchor.constraintEqualToAnchor( bottomLayoutGuide.topAnchor, constant: -20).active = true
效果如下:
Readable Content Guide
上图中绿色的bookTextView
,当前左右边缘对齐到屏幕的边缘,这样在iPad上显示时,非常不便于阅读。 使用 readable content guides
,会根据size class来调整大小,这样会在边缘添加空白的距离,来跟适合阅读。
bookTextView.leadingAnchor.constraintEqualToAnchor( view.readableContentGuide.leadingAnchor).active = truebookTextView.trailingAnchor.constraintEqualToAnchor( view.readableContentGuide.trailingAnchor).active = true
在iPad横屏下,显示如下:
Intrinsic Content Size
所有的view都有一个intrinsic content size
, 如果设置了content size,就不用创建width 和 height 约束。
Intrinsic Content Size
由font和text决定 UIView默认的Intrinsic Content Size
是UIViewNoIntrinsicMetric
,表示的是没有大小。 设置AvatarView的UIViewNoIntrinsicMetric
override func intrinsicContentSize() -> CGSize { return CGSize(width: UIViewNoIntrinsicMetric, height: 100)}
其效果就如下:
注意:如果想要在app运行时改变 intrinsic content size
,可以调用invalidateIntrinsicContentSize()
方法来更新
优先级
可能会出现如下的情况,chapterLabel被拉伸了,而我们希望的是avatarView被拉伸,如下:
通过设置ContentHuggingPriority
和ContentCompressionResistancePriority
来解决
setContentHuggingPriority(_:forAxis:)
takes a priority and an axis to determine how much a view wants to stretch. A high priority means that a view wants to stay the same size. A low priority allows the view to stretch.高优先级表示view想要保持其原来的size,低优先级则允许view拉伸
setContentCompressionResistancePriority(_:forAxis:)
also takes a priority and an axis. This method determines how much a view wants to shrink. A high priority means that a view tries not to shrink and a low priority means that the view can squish.高优先级表示view尽量不被压缩,低优先级表示view会被压缩
优先级大小在1到1000之间,1000是最高的,标准的优先值如下:
UILayoutPriorityRequired = 1000UILayoutPriorityDefaultHigh = 750UILayoutPriorityDefaultLow = 250
给chapterLabel设置优先级:
chapterLabel.setContentHuggingPriority( UILayoutPriorityRequired, forAxis: .Vertical)chapterLabel.setContentCompressionResistancePriority( UILayoutPriorityRequired, forAxis: .Vertical)
这样保证chapterLabel
其不会被拉伸和压缩
激活约束数组
上面的例子中,都是一个一个的激活约束,在设置好约束后,一起激活它们,这种方式会高效
如下的约束:
// 1let labelBottom = titleLabel.bottomAnchor.constraint(equalTo: bottomAnchor)let labelCenterX = titleLabel.centerXAnchor.constraint( equalTo: centerXAnchor)// 2let imageViewTop = imageView.topAnchor.constraint(equalTo: topAnchor)let imageViewBottom = imageView.bottomAnchor.constraint( equalTo: titleLabel.topAnchor)let imageViewCenterX = imageView.centerXAnchor.constraint( equalTo: centerXAnchor)// 3let socialMediaTrailing = socialMediaView.trailingAnchor.constraint(equalTo: trailingAnchor)let socialMediaTop = socialMediaView.topAnchor.constraint(equalTo: topAnchor)//一起激活约束NSLayoutConstraint.activate([ imageViewTop, imageViewBottom, imageViewCenterX, labelBottom, labelCenterX, socialMediaTrailing, socialMediaTop])
由于imageView和titleLabel都有intrinsic size,所以需要设置优先级:
imageView.setContentCompressionResistancePriority( UILayoutPriorityDefaultLow, for: .vertical)imageView.setContentCompressionResistancePriority( UILayoutPriorityDefaultLow, for: .horizontal)
Constraint Activation and Deactivation
根据不同的size class来active约束,即根据size class应用不同的约束
在traitCollectionDidChange(_:)
方法中,捕捉trait collection
的变化
override func traitCollectionDidChange(previousTraitCollection: UITraitCollection?) { super.traitCollectionDidChange(previousTraitCollection) // 1 if traitCollection.horizontalSizeClass == .Regular { // 2 激活和取消激活约束 NSLayoutConstraint.deactivateConstraints(compactConstraints) NSLayoutConstraint.activateConstraints(regularConstraints) // 3 socialMediaView.axis = .Horizontal } else { // 4 激活和取消激活约束 NSLayoutConstraint.deactivateConstraints(regularConstraints) NSLayoutConstraint.activateConstraints(compactConstraints) socialMediaView.axis = .Vertical }}
traitCollectionDidChange(_:)
方法会捕获trait collection的改变
效果如下:
还有一个小问题是,由于image的intrinsic content size的影响,水平时image并没有左对齐,但其实imageview是左对齐的
更新Constraint
约束更新周期
上面的图展示了view是如何被绘制的,可覆写更新view或约束的三个方法:
- 所有的约束在
updateConstraints()
中被计算。This is where all priorities, compression resistance, hugging and intrinsic content size all come together in one complex algorithm.可以重写此方法来改变约束。 - view在
layoutSubviews()
做布局。 If you need to access the correct view frame, you can override this. - 在
draw(_:)
方法中,view被绘制
当size class发生变更时,view的布局更新是自动的,但你也可以调用左侧列出的setNeeds...()
方法触发布局
如下:
override func updateConstraints() { super.updateConstraints() // 1 计算比率 var aspectRatio: CGFloat = 1 if let image = image { aspectRatio = image.size.width / image.size.height } // 2 添加比率约束 aspectRatioConstraint?.active = false aspectRatioConstraint = imageView.widthAnchor.constraintEqualToAnchor( imageView.heightAnchor, multiplier: aspectRatio) aspectRatioConstraint?.active = true}
而且还需要在image发生变更时,需要一种方式来调用 updateConstraints()
方法,但此方法不可直接调用,需要调用setNeedsUpdateConstraints()
方法,它会在下一个运行循环中调用updateConstraints()
方法
var image: UIImage? { didSet { imageView.image = image setNeedsUpdateConstraints() }}
手工布局view
如果想获取到view的frame,唯一安全的做法是在 layoutSubviews()
中来获取
override func layoutSubviews() { super.layoutSubviews() if bounds.height < socialMediaView.bounds.height { socialMediaView.alpha = 0 } else { socialMediaView.alpha = 1 } if imageView.bounds.height < 30 { imageView.alpha = 0 } else { imageView.alpha = 1 }}
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