无法设置约束以将标签带回屏幕中间。

Question

我试图通过向上滑动显示屏幕底部的视图并通过向下滑动隐藏它。我在屏幕中央有一个标签，我希望将其带到高度的1/4点，因为视图占据屏幕的一半。那部分工作正常，但是当我想隐藏视图时将其恢复到原始位置时，它似乎不起作用并且在高度的1/4处重新调整。

anyobdy知道这里的问题是什么吗？

以下是我的代码：

import UIKit
import FBSDKCoreKit
import FBSDKLoginKit
import FBSDKShareKit





class SecondViewController: UIViewController {

    let label = UILabel()

    var v = MyCustomView()

    override func viewDidLoad() {
        super.viewDidLoad()        

        FBSDKGraphRequest(graphPath: "/me", parameters: ["Fields" : "name"]).start {

            (connection, result, err) in

            if err != nil {
                print("Failed to start request", err)
                return
            }

            var res = result as? [String : Any]
            myVars.username = (res?["name"] as! String)

            print(myVars.username)


        }


        self.v = MyCustomView(frame: CGRect(x: 0, y: view.bounds.height / 2, width: view.bounds.width, height: view.bounds.height / 2))


        label.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(label)



        let lblHeight = label.heightAnchor.constraint(equalToConstant : 30)
        let lblWidth = label.widthAnchor.constraint(equalToConstant: 200)
        let yConstraint = NSLayoutConstraint(item: label, attribute: .top, relatedBy: .equal, toItem: view, attribute: .top, multiplier: 1, constant: view.bounds.height / 2.0 - 15)
        let xConstraint = NSLayoutConstraint(item: label, attribute: .leading, relatedBy: .equal, toItem: view, attribute: .leading, multiplier: 1, constant: view.bounds.width / 2.0 - 100)

        NSLayoutConstraint.activate([lblHeight,lblWidth,xConstraint,yConstraint])


        label.backgroundColor = UIColor.red
        label.textAlignment = NSTextAlignment.center
        label.text = "Shit comes up here!"


        let swipeUp = UISwipeGestureRecognizer(target: self, action: "swipeUp")
        swipeUp.direction = UISwipeGestureRecognizerDirection.up
        self.view.addGestureRecognizer(swipeUp)

        let swipeDown = UISwipeGestureRecognizer(target: self, action: "swipeDown")
        swipeDown.direction = UISwipeGestureRecognizerDirection.down
        self.view.addGestureRecognizer(swipeDown)


    }

    struct myVars{

        static var username = "X"

    }


    func swipeUp(){

        if v.isDescendant(of : self.view) {
            v.isHidden = false
        }

        else {
            self.label.text = myVars.username
            self.view.addSubview(v)
        }


        let yConstraint = NSLayoutConstraint(item: label, attribute: .top, relatedBy: .equal, toItem: view, attribute: .top, multiplier: 1, constant: view.bounds.height / 4.0 - 15)
        NSLayoutConstraint.activate([yConstraint])


    }

    func swipeDown(){


        let yConstraint = NSLayoutConstraint(item : label, attribute : .top, relatedBy: .equal, toItem: view, attribute : .top, multiplier: 1, constant: view.bounds.height / 2.0 - 15)

        NSLayoutConstraint.activate([yConstraint])

        print("SwipedDown!")

        v.isHidden = true

    }




}