ios - 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处，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)

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.

有什么想法吗？

Answer 1

错误告诉您添加约束太早。您应该首先添加 loginButton，然后才添加约束:

loginButton.delegate = self

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

错误消息提到添加到 View 时，约束的项目必须是该 View 的后代。但是您在将实际按钮添加到 View 之前添加了 loginButton 的约束。