旋转设备时约束不起作用

Posted 2023-02-23

【中文标题】旋转设备时约束不起作用

【英文标题】：Constraints not working when device is rotated

【发布时间】：2018-02-21 10:23:01

【问题描述】：

我有这个 UIViewController：

import UIKit

class ViewController: UIViewController 

    var object: DraggableView?

    override func viewDidLoad() 
        super.viewDidLoad() 
        // Create the object
        object = DraggableView(parent: self)
    

    override func viewWillAppear(_ animated: Bool) 
        super.viewWillAppear(animated)
        // Add subview
        object?.setup()
    

我有这个类可以在这个 VC 中添加视图：

import UIKit

class DraggableView 

    var parent: UIViewController!

    let pieceOfViewToShow: CGFloat = 30.0

    init(parent: UIViewController) 
        self.parent = parent
    

    func setup() 
        let view = UIView(frame: parent.view.frame)
        view.backgroundColor = UIColor.red
        parent.view.addSubview(view)
        view.translatesAutoresizingMaskIntoConstraints = false

        view.leadingAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.leadingAnchor).isActive = true
        view.trailingAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.trailingAnchor).isActive = true
        view.heightAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.heightAnchor).isActive = true

        // I need to show only a piece of the view at bottom, so:
        view.topAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.topAnchor, constant: parent.view.frame.height - pieceOfViewToShow).isActive = true
    

问题

一切都是正确的，但是当设备旋转时，它会失去约束，添加的视图也会丢失。

我认为问题在于下一行在设备旋转时无法更新正确的高度[parent.view.frame.height]。

view.topAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.topAnchor, constant: parent.view.frame.height - pieceOfViewToShow).isActive = true

如何在旋转时更新这个常量？ 我正在使用 Swift 3。

【参考方案1】：

您可以尝试在UIView 上使用traitCollectionDidChange 回调来在旋转更改时更新约束，要使其正常工作，您需要将DraggableView 设为UIView 的子类：

import UIKit

class DraggableView: UIView 

    var parent: UIViewController!

    let pieceOfViewToShow: CGFloat = 30.0

    // keep the constraint around to have access to it
    var topConstraint: NSLayoutConstraint?

    init(parent: UIViewController) 
        super.init(frame: parent.view.frame)
        self.parent = parent
    

    required init?(coder aDecoder: NSCoder) 
        fatalError("init(coder:) has not been implemented")
    

    func setup() 
        self.backgroundColor = UIColor.red
        parent.view.addSubview(self)
        self.translatesAutoresizingMaskIntoConstraints = false

        self.leadingAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.leadingAnchor).isActive = true
        self.trailingAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.trailingAnchor).isActive = true
        self.heightAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.heightAnchor).isActive = true

        // keep a reference to the constraint
        topConstraint = self.topAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.topAnchor, constant: parent.view.frame.height - pieceOfViewToShow)
        topConstraint?.isActive = true
    

    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) 
        super.traitCollectionDidChange(previousTraitCollection)

        // update the constraints constant
        topConstraint?.constant = parent.view.frame.height - pieceOfViewToShow
    

【讨论】：

@JaredPérez 抱歉，没有注意到 DraggableView 不是 UIView 的子类......只需使用更新版本

我认为这是一个不错的选择，但是在 DraggableView 类中你不能使用该方法：func traitCollectionDidChange