通过视觉格式语言仅设置一个约束时，无法同时满足约束

Question

我创建了一个新项目来练习iOS AutoLayout功能（ 可视格式语言）

我只做了一个按钮，如下

var button:UIButton!

我做了一个方法，它将按钮添加到self.view，命名为initViews，如下所示

func initViews() -> Void {

        button = UIButton()
        button.setTitle("Button", forState: .Normal)
        button.backgroundColor = UIColor.blueColor()

        self.view.addSubview(button)
        self.createConstraints()
    }

之后，我创建了另一个方法，如下所示用于设置约束。

func createConstraints () -> Void {

    //Views to add constraints to
    let views = Dictionary(dictionaryLiteral: ("button",button))

    let horizontalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("H:|-[button]-|", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    self.view.addConstraints(horizontalConstraints)

//        let verticalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("V:|-[button]-|", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)

    //self.view.addConstraints(verticalConstraints)
}

现在，我在ViewDidLoad中调用initViews，但是在执行时，它在LOG中发出警告。

我确定没有垂直约束我看不到按钮。 但我对警告更自觉。

警告：

2015-07-12 11:10:06.495 iOSAdvanceAutoLayout Project[1248:37226] 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) 
(
    "<NSLayoutConstraint:0x7fc9f062ffb0 UIButton:0x7fc9f061ccd0'Button'.leading == UIView:0x7fc9f0572cb0.leadingMargin>",
    "<NSLayoutConstraint:0x7fc9f0630180 UIView:0x7fc9f0572cb0.trailingMargin == UIButton:0x7fc9f061ccd0'Button'.trailing>",
    "<NSAutoresizingMaskLayoutConstraint:0x7fc9f056a820 h=--& v=--& H:[UIButton:0x7fc9f061ccd0'Button'(0)]>",
    "<NSLayoutConstraint:0x7fc9f056d0e0 'UIView-Encapsulated-Layout-Width' H:[UIView:0x7fc9f0572cb0(375)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7fc9f0630180 UIView:0x7fc9f0572cb0.trailingMargin == UIButton:0x7fc9f061ccd0'Button'.trailing>

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.
2015-07-12 11:10:06.496 iOSAdvanceAutoLayout Project[1248:37226] 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) 
(
    "<NSLayoutConstraint:0x7fc9f062ffb0 UIButton:0x7fc9f061ccd0'Button'.leading == UIView:0x7fc9f0572cb0.leadingMargin>",
    "<NSAutoresizingMaskLayoutConstraint:0x7fc9f056c210 h=--& v=--& UIButton:0x7fc9f061ccd0'Button'.midX ==>",
    "<NSAutoresizingMaskLayoutConstraint:0x7fc9f056a820 h=--& v=--& H:[UIButton:0x7fc9f061ccd0'Button'(0)]>",
    "<NSAutoresizingMaskLayoutConstraint:0x7fc9f056d880 h=-&- v=-&- 'UIView-Encapsulated-Layout-Left' H:|-(0)-[UIView:0x7fc9f0572cb0]   (Names: '|':UIWindow:0x7fc9f061b9d0 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7fc9f062ffb0 UIButton:0x7fc9f061ccd0'Button'.leading == UIView:0x7fc9f0572cb0.leadingMargin>

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.

谁能指导我为什么只设置一个约束就出现此警告？
谢谢

Answer 1

创建按钮后添加以下行

    button.translatesAutoresizingMaskIntoConstraints = false

还添加垂直约束，如下编辑方法。

func createConstraints () -> Void {

        //Views to add constraints to
        let views = Dictionary(dictionaryLiteral: ("button",button))

        let horizontalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("H:|-30.0-[button]-30.0-|",
                                    options: NSLayoutFormatOptions(rawValue: 0),
                                    metrics: nil,
                                    views: views)
        self.view.addConstraints(horizontalConstraints)



        let verticalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("V:|-30.0-[button]-30.0-|",
                                    options: NSLayoutFormatOptions(rawValue: 0),
                                    metrics: nil,
                                    views: views)

        self.view.addConstraints(verticalConstraints)
    }