最好在更改约束之前或之后调用 .layoutIfNeeded()

Posted 2023-02-23

【中文标题】最好在更改约束之前或之后调用 .layoutIfNeeded()

【英文标题】：Is best to call .layoutIfNeeded() before or after constraints are changed

【发布时间】：2020-01-11 16:32:29

【问题描述】：

我有一个按钮，可以根据特定情况更改大小。我不需要完成动画，因为用户永远不会看到更改发生。有时，当从较小的尺寸切换回较大的尺寸时，按钮会卡在较小的尺寸上。我认为layoutIfNeeded() 会解决这个问题。

问题是我应该在下面的setCameraButtonToNormalSize() 函数中什么时候调用layoutIfNeeded()？

lazy var cameraButton: UIButton = 
    let button = UIButton()
    button.translatesAutoresizingMaskIntoConstraints = false
    button.addTarget(self, action: #selector(cameraButtonPressed), for: .touchUpInside)
    return button
()

let normalSize: CGFloat = 50
let smallerSize: CGFloat = 5

var cameraButtonWidthConstraint: NSLayoutConstraint?
var cameraButtonHeightConstraint: NSLayoutConstraint?

func setCameraButtonToNormalSize() 

    // before the constraints are changed

    cameraButtonWidthConstraint?.isActive = false
    cameraButtonHeightConstraint?.isActive = false

    cameraButtonWidthConstraint = cameraButton.widthAnchor.constraint(equalToConstant: normalSize)
    cameraButtonWidthConstraint?.isActive = true
    cameraButtonHeightConstraint = cameraButton.heightAnchor.constraint(equalToConstant: normalSize)
    cameraButtonHeightConstraint?.isActive = true

    // after the constraints are changed


fileprivate func setCameraButtonToSmallerSize() 

    cameraButtonWidthConstraint?.isActive = false
    cameraButtonHeightConstraint?.isActive = false

    cameraButtonWidthConstraint = cameraButton.widthAnchor.constraint(equalToConstant: smallerSize)
    cameraButtonWidthConstraint?.isActive = true
    cameraButtonHeightConstraint = cameraButton.heightAnchor.constraint(equalToConstant: smallerSize)
    cameraButtonHeightConstraint?.isActive = true

【参考方案1】：

你需要在之后调用它

// after the constraints are changed
 self.view.layoutIfNeeded()