Ios NSGenericeException以编程方式添加某些约束时

这是我的代码

func showMessageView(){
    let leftConstraint = messageView.leftAnchor.constraintEqualToAnchor(self.view.leftAnchor)
    let bottomConstraint = messageView.bottomAnchor.constraintGreaterThanOrEqualToAnchor(self.view.bottomAnchor)
    let highConstratin = messageView.heightAnchor.constraintEqualToConstant(44.0)
    let widthConstraint = messageView.widthAnchor.constraintGreaterThanOrEqualToAnchor(self.view.widthAnchor)
    NSLayoutConstraint.activateConstraints([leftConstraint, bottomConstraint, highConstratin, widthConstraint])
    self.view.translatesAutoresizingMaskIntoConstraints = false
    self.view.addSubview(messageView)
}

我收到了这个例外：

由于未捕获的异常“NSGenericeException”而终止应用程序，原因：“无法激活项的约束，并且因为它们没有共同的祖先。”。约束是否引用不同视图层次结构中的项？这是违法的。”

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我解决了我的问题，只需将messageView首先添加到视图中，如下所示：

func showMessageView(){
        self.view.addSubview(messageView)
        let leftConstraint = messageView.leftAnchor.constraintEqualToAnchor(self.view.leftAnchor)
        let bottomConstraint = messageView.bottomAnchor.constraintGreaterThanOrEqualToAnchor(self.view.bottomAnchor)
        let highConstratin = messageView.heightAnchor.constraintEqualToConstant(44.0)
        let widthConstraint = messageView.widthAnchor.constraintGreaterThanOrEqualToAnchor(self.view.widthAnchor)
        NSLayoutConstraint.activateConstraints([leftConstraint, bottomConstraint, highConstratin, widthConstraint])
        self.view.translatesAutoresizingMaskIntoConstraints = false
}