使用SnapKit时无法正确动画

Question

我试图为我的应用程序动画一个标志(UILabel)，从中间到顶部。我尝试的是更新约束，但它似乎不起作用。问题是动画，即标志，从原点(0,0)，而不是中间的视图到顶部。必要的代码(控制器及其继承的类)：

import UIKit
import SnapKit

class EntryController: LatroController {

    static let spacingFromTheTop: CGFloat = 150
    var latroLabelCenterYConstraint: Constraint?

    override init() {
        super.init()
        self.animateTitleLabel()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override func initTitleLabel() {
        self.latroLabel = UILabel()
        self.latroLabel?.text = General.latro.rawValue
        self.latroLabel?.textAlignment = .center
        self.latroLabel?.font = UIFont (name: General.latroFont.rawValue, size: EntryController.fontSize)
        self.latroLabel?.textColor = .white
        self.latroLabel?.contentMode = .center
        self.view.addSubview(self.latroLabel!)

        self.latroLabel?.snp.makeConstraints({ (make) in
            make.width.equalTo(EntryController.latroWidth)
            make.height.equalTo(EntryController.latroHeight)
            make.centerX.equalTo(self.view.center.x)
            self.latroLabelCenterYConstraint = make.centerY.equalTo(self.view.center.y).constraint
        })
    }

    func animateTitleLabel() {
        UIView.animate(withDuration: 1.5) {
            self.latroLabel?.snp.updateConstraints { (make) in
                make.centerY.equalTo(200)
            }
            self.view.layoutIfNeeded()
        }
    }
}

import UIKit
import SnapKit

class LatroController: UIViewController {

static let latroWidth: CGFloat = 288
static let latroHeight: CGFloat = 98
static let btnWidth: CGFloat = 288
static let btnHeight: CGFloat = 70
static let txtFieldWidth: CGFloat = 288
static let txtFieldHeight: CGFloat = 50
static let fontSize: CGFloat = 70
static let bottomOffset: CGFloat = 100
static let buttonOffset: CGFloat = 20
static let logoOffset: CGFloat = 50

var latroLabel: UILabel?
var signUpBtn: UIButton?
var logInBtn: UIButton?
var titleLabelYConstraint: NSLayoutConstraint?
var usernameTxtField: UITextField?

init() {
    super.init(nibName: nil, bundle: nil)
    self.view.backgroundColor = UIColor(named: General.orange.rawValue)
    self.initTitleLabel()
}

required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
}

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    self.navigationController?.setNavigationBarHidden(true, animated: false)
}

override func viewDidDisappear(_ animated: Bool) {
    super.viewDidDisappear(animated)
    self.navigationController?.setNavigationBarHidden(false, animated: true)
}

func initTitleLabel() {
    self.latroLabel = UILabel()
    self.latroLabel?.text = General.latro.rawValue
    self.latroLabel?.textAlignment = .center
    self.latroLabel?.font = UIFont (name: General.latroFont.rawValue, size: EntryController.fontSize)
    self.latroLabel?.textColor = .white
    self.latroLabel?.contentMode = .center
    self.view.addSubview(self.latroLabel!)

    self.latroLabel?.snp.makeConstraints({ (make) in
        make.width.equalTo(LatroController.latroWidth)
        make.height.equalTo(LatroController.latroHeight)
        let safeAreaLayoutHeight = self.view.safeAreaLayoutGuide.layoutFrame.height
        print(safeAreaLayoutHeight)
        make.top.equalTo(self.view).offset(150)
        make.centerX.equalTo(self.view.center.x)
    })
}
}

Answer 1

除非视图在界面中并执行了初始布局，否则无法对其进行动画化。因此，您调用self.animateTitleLabel()的速度太快了(在init中)。

把它叫做viewDidAppear之类的东西。当然，您必须使用Bool标志属性来确保在每次运行viewDidAppear时，只在第一次运行时调用它。

(可能有必要在viewDidLayoutSubviews中调用它；您将不得不进行实验。)

Answer 2

好吧，我以为这会比我最初预想的要困难。缺少了以下内容：

self.view.updateLayoutIfNeeded() 

设置约束之后！