我有一个UIViewController四个UIButtons（2 x 2），我在界面构建器中布置，效果很好。我将拥有我的应用程序的免费且受广告支持的版本，因此我需要根据应用程序是付费版本还是受广告支持的版本重做该场景以加载。基于此，我正在尝试使用 Visual Formatting Language 来布置视图。UIButton尽管我在计算它们时考虑了它们，但我的身高却得到了不正确的值。我无法弄清楚我的错误（或遗漏？）。
这是我的界面生成器的屏幕截图。
我对界面生成器中的按钮没有限制，但我确实IBOutlets连接到MyViewController. MyViewController位于导航控制器中，底部有一个标签栏。
我创建了一个名为的方法layoutButtons，我在viewDidLoad之后调用它super.viewDidLoad()。这里是：
func layoutButtons() {
// Configure layout constraints
// Remove interface builder constraints from storyboard
view.removeConstraints(view.constraints)
// create array to dump constraints into
var allConstraints = [NSLayoutConstraint]()
// determine screen size
let screenSize: CGRect = UIScreen.mainScreen().bounds
let navBarRect = navigationController!.navigationBar.frame
let navBarHeight = navBarRect.height
let tabBarRect = tabBarController!.tabBar.frame
let tabBarHeight: CGFloat = tabBarRect.height
// calculate button width based on screen size
// padding for left + middle + right = 8.0 + 8.0 + 8.0 = 24.0
let buttonWidth = (screenSize.width - 24.0) / 2
/*
My buttons are extending under the top & bottom layout guides despite accounting
for them when I set the buttonHeight.
*/
// padding for top + middle + bottom = 8.0 + 8.0 + 8.0 = 24.0
let buttonHeight = (screenSize.height - topLayoutGuide.length - bottomLayoutGuide.length - 24.0) / 2
// create dictionary of metrics
let metrics = ["buttonWidth": buttonWidth,
"buttonHeight": buttonHeight,
"navBarHeight": navBarHeight,
"tabBarHeight": tabBarHeight,
"bannerAdWidth": bannerAdWidth,
"bannerAdHeight": bannerAdHeight]
// create dictionary of views
var views: [String : AnyObject] = ["firstButton": firstButton,
"secondButton": secondButton,
"thirdButton": thirdButton,
"fourthButton": fourthButton,
"topLayoutGuide": topLayoutGuide,
"bottomLayoutGuide": bottomLayoutGuide]
let topRowHorizontalConstraints = NSLayoutConstraint.constraintsWithVisualFormat(
"H:|-[firstButton(buttonWidth)]-[secondButton(buttonWidth)]-|",
options: [.AlignAllCenterY],
metrics: metrics,
views: views)
allConstraints += topRowHorizontalConstraints
let bottomRowHorizontalConstraints = NSLayoutConstraint.constraintsWithVisualFormat(
"H:|-[thirdButton(buttonWidth)]-[fourthButton(buttonWidth)]-|",
options: [.AlignAllCenterY],
metrics: metrics,
views: views)
allConstraints += bottomRowHorizontalConstraints
let leftColumnVerticalConstraints = NSLayoutConstraint.constraintsWithVisualFormat(
"V:|[topLayoutGuide]-[firstButton(buttonHeight)]-[thirdButton(buttonHeight)]-[bottomLayoutGuide]|",
options: [],
metrics: metrics,
views: views)
allConstraints += leftColumnVerticalConstraints
let rightColumnVerticalConstraints = NSLayoutConstraint.constraintsWithVisualFormat(
"V:|[topLayoutGuide]-[secondButton(buttonHeight)]-[fourthButton(buttonHeight)]-[bottomLayoutGuide]|",
options: [],
metrics: metrics,
views: views)
allConstraints += rightColumnVerticalConstraints
NSLayoutConstraint.activateConstraints(allConstraints)
}
我已经摆弄了我的buttonHeight变量，但是我尝试的每次迭代都会导致按钮在topLayoutGuideand下扩展bottomLayoutGuide。这是它在运行时的样子：
我欢迎任何建议在哪里寻找我的错误。感谢您的阅读。