将约束应用于Facebook登录按钮

Question

我正在尝试向Facebook按钮添加约束以使其位于按钮上，这是我的代码；

...
let loginButton = FBSDKLoginButton()
loginButton.readPermissions = ["public_profile", "email"]
loginButton.center = self.view.center

let myConstraint =
NSLayoutConstraint(item: loginButton,
        attribute: NSLayoutAttribute.Bottom,
        relatedBy: NSLayoutRelation.Equal,
        toItem: self.view,
        attribute: NSLayoutAttribute.Bottom,
        multiplier: 1.0,
        constant: -20)
    self.view.addConstraint(myConstraint)

loginButton.addConstraint(myConstraint)

loginButton.delegate = self
self.view.addSubview(loginButton)

它失败，并出现以下错误；

The view hierarchy is not prepared for the constraint: <NSLayoutConstraint:0x15fd258a0 FBSDKLoginButton:0x15fe562d0'Log in'.bottom == UIView:0x15fd2fa60.bottom - 20>
When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. Break on -[UIView(UIConstraintBasedLayout) _viewHierarchyUnpreparedForConstraint:] to debug.

任何想法？

Answer 1

该错误表明您过早添加了约束。 您应该首先添加loginButton ，然后再添加约束：

loginButton.delegate = self

self.view.addSubview(loginButton)
self.view.addConstraint(myConstraint)
loginButton.addConstraint(myConstraint)

该错误消息提到当添加到视图时，约束的项必须是该视图的后代 。 但是，您loginButton在将实际按钮添加到视图之前添加loginButton的约束。