如何更新之前在代码中设置的 NSLayoutConstraint 常量？ - 斯威夫特

Question

我正在尝试完全以编程方式制作我的视图。它有一个底部视图，当搜索查询完成相关信息时会显示动画。我已将底部视图的 .bottom nslayoutconstraint 设置为可选的 nslayoutconstraint，首先在 override func viewWillLayoutSubviews() 中将其初始化为屏幕外。

    let bottomView = UIView()
    var bottomViewBottomConstraint: NSLayoutConstraint!

override func viewDidLoad() {
    super.viewDidLoad()
    view.addSubview(bottomView)

}

override func viewWillLayoutSubviews() {

    //Bottom View Constraints
    bottomView.translatesAutoresizingMaskIntoConstraints = false
    let bottomViewLeftConstraint = NSLayoutConstraint(item: bottomView, attribute: .left, relatedBy: .equal, toItem: view, attribute: .left, multiplier: 1, constant: 0)
    let bottomViewRightConstraint = NSLayoutConstraint(item: bottomView, attribute: .right, relatedBy: .equal, toItem: view, attribute: .right, multiplier: 1, constant: 0)
    let bottomViewHeightConstraint = NSLayoutConstraint(item: bottomView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 75)
    bottomViewBottomConstraint = NSLayoutConstraint(item: self.bottomView, attribute: .bottom, relatedBy: .equal, toItem: view, attribute: .bottom, multiplier: 1, constant: 100)

    NSLayoutConstraint.activate([
        bottomViewLeftConstraint, bottomViewRightConstraint, bottomViewBottomConstraint, bottomViewHeightConstraint
        ])
   }

一旦搜索查询完成，视图就会出现。

                self.view.layoutIfNeeded()

            UIView.animate(withDuration: 0.8, delay: 1, usingSpringWithDamping: 1, initialSpringVelocity: 0.1, options: .curveEaseIn, animations: {
                    self.bottomViewBottomConstraint.constant = -5
                    self.view.layoutIfNeeded()
                }, completion: nil)

但是在那之后，当我尝试关闭视图并更改 NsLayoutConstraint 的常量时，视图不会移动。

    self.view.layoutIfNeeded()
    UIView.animate(withDuration: 0.8, animations: {
        self.bottomViewBottomConstraint.constant = 100
        self.view.layoutIfNeeded()
    })

在输出控制台中，我收到了这个错误，我不知道如何修复它。

    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.
    2016-10-11 14:50:01.768353 Green Homes Australia[973:232019]   [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. 
(
    "<NSLayoutConstraint:0x17028d6b0 UIView:0x1049365b0.bottom == UIView:0x100b04e40.bottom - 5   (active)>",
    "<NSLayoutConstraint:0x174681cc0 UIView:0x1049365b0.bottom == UIView:0x100b04e40.bottom + 100   (active)>")

    Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x174681cc0 UIView:0x1049365b0.bottom == UIView:0x100b04e40.bottom + 100   (active)>

Answer 1

问题是你的

override func viewWillLayoutSubviews() {

一次又一次地运行。布局发生了很多！因此，这些约束都会一次又一次地创建和添加，包括 在您完成动画并更改底部约束之后。因此，您最终会遇到两个相互冲突的底部约束。 （您实际上有 很多 个底部约束，但所有其他约束都等于冲突约束之一，因此运行时不会抱怨。）

我喜欢使用的一个简单解决方案是放置一个布尔标志，以便我的初始配置只发生一次：

var didConfig = false
override func viewWillLayoutSubviews() {
    if !didConfig {
        didConfig = true
        // create and configure constraints here
    }
}