无法同时满足约束 iOS

Question

我有一个被限制在视图底部的按钮。 该按钮最初是隐藏的，直到点击表视图中的项目。 桌子上有一个搜索控制器，当点击它时，我希望让按钮移动，使其始终位于键盘上方。

    @objc func keyboardWillShow(notification: NSNotification) {
        if let keyboardHeight = (notification.userInfo?[UIResponder.keyboardFrameBeginUserInfoKey] as? NSValue)?.cgRectValue.height {

            startAssessmentBtn.snp.makeConstraints { make in
                make.bottom.equalTo(-keyboardHeight)
            }

            let userInfo = notification.userInfo! as [AnyHashable: Any]
            let animationDuration = userInfo[UIResponder.keyboardAnimationDurationUserInfoKey] as! NSNumber
            let animationCurve = userInfo[UIResponder.keyboardAnimationCurveUserInfoKey] as! NSNumber

            UIView.setAnimationCurve(UIView.AnimationCurve(rawValue: animationCurve.intValue)!)
            UIView.animate(withDuration: animationDuration.doubleValue) {
                self.tableView.setBottomInset(to: keyboardHeight)
                self.view.layoutIfNeeded()
            }
        }
    }

    @objc func keyboardDidHide(notification: NSNotification) {
        if let keyboardHeight = (notification.userInfo?[UIResponder.keyboardFrameBeginUserInfoKey] as? NSValue)?.cgRectValue {

            let userInfo = notification.userInfo! as [AnyHashable: Any]
            let animationDuration = userInfo[UIResponder.keyboardAnimationDurationUserInfoKey] as! NSNumber
            let animationCurve = userInfo[UIResponder.keyboardAnimationCurveUserInfoKey] as! NSNumber

            startAssessmentBtn.snp.makeConstraints { make in
                make.bottom.equalTo(0)
                //make.bottom.equalToSuperview()//.offset(+keyboardHeight)
            }

            UIView.setAnimationCurve(UIView.AnimationCurve(rawValue: animationCurve.intValue)!)
            UIView.animate(withDuration: animationDuration.doubleValue) {
                self.tableView.setBottomInset(to: 0.0)
                self.startAssessmentBtn.setNeedsLayout()
                self.startAssessmentBtn.layoutIfNeeded()
                self.view.layoutIfNeeded()
            }
        }
    }

使用上面的代码，我可以在第一次打开键盘时让按钮出现在键盘上方。 但是当键盘关闭时，按钮不会返回到屏幕底部。 我收到以下约束错误：

2020-04-01 17:38:42.629598+0100 AuditComply 2.0[4550:3372130] [LayoutConstraints] 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. 
(
    "<SnapKit.LayoutConstraint:0x2830057a0@AssetTreeViewController.swift#126 UIButton:0x121deacd0.bottom == UIView:0x121deb100.bottom - 320.0>",
    "<SnapKit.LayoutConstraint:0x283005620@AssetTreeViewController.swift#149 UIButton:0x121deacd0.bottom == UIView:0x121deb100.bottom>"
)

Will attempt to recover by breaking constraint 
<SnapKit.LayoutConstraint:0x283005620@AssetTreeViewController.swift#149 UIButton:0x121deacd0.bottom == UIView:0x121deb100.bottom>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKitCore/UIView.h> may also be helpful.

所以看来keyboardWillShow 的约束覆盖了keyboardDidHide 的约束。 我曾尝试更改约束的优先级，但这也不起作用。

Answer 1

很明显你在使用 SnapKit。 makeConstraints向现有约束添加约束。

要更新现有约束，您应该使用updateConstraints ，但您应该锚定到同一点，

例如： .equalToSuperview().inset(0)和.equalToSuperview().inset(keyboardHeight)

如果没有锚定到同一点的选项，你应该清除以前的约束并做一些新的。 您可以使用.remakeConstraints同时进行，但您应该添加所有其他约束，如前导、尾随、顶部