旋转设备时约束不起作用

Question

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 class 可以在 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失去约束并且添加的视图丢失。

我认为问题出在下一行，当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.

Answer 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，为此您需要将 DraggableView 设为UIView:

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
    }
}

旋转设备时约束不起作用

Constraints not working when device is rotated

constraints

ios

nslayoutconstraint

swift

问题