ios - UIView animateKeyframes 不适用于 NSLayoutConstraints

Question

我正在尝试通过减少宽度约束来为 UIView 设置动画，但在使用 UIView.animateKeyFrames 时效果不佳。这是我的代码:

    UIView.animateKeyframes(withDuration: 2.0, delay: 0, options: [.calculationModeCubic], animations: {
        UIView.addKeyframe(withRelativeStartTime: 0, relativeDuration: 1.0/2.0, animations: {
            self.titleLabel.alpha = 0.0 // works well
        })

       self.titleSquareHeightConstraint?.constant = 20 // previously 170
        UIView.addKeyframe(withRelativeStartTime: 1.0/2.0, relativeDuration: 1.0/2.0, animations: {
            self.titleSquare.layoutIfNeeded() // instantaneously jump to 20
        })

    }, completion:nil)

titleSquare 的高度会瞬间从 170 跳到 20。 注意:titleSquare 是一个 UIView。此外，我曾经使用 UIView.animate 并且下面的代码工作得很好:

    self.titleSquareHeightConstraint?.constant = 20
    UIView.animate(withDuration: 0.5,
                   delay: 0,
                   usingSpringWithDamping: 1,
                   initialSpringVelocity: 1,
                   options: .curveEaseIn,
                   animations: {

                    self.view.layoutIfNeeded() // Perfectly works

    },completion: nil)

我的 UIView.animateKeyframes 方法有什么问题？似乎我不完全了解它是如何工作的？

谢谢

Answer 1

不是很明显或很直观，但是......

您想告诉 View layoutIfNeeded():

    UIView.animateKeyframes(withDuration: 2.0, delay: 0, options: [.calculationModeCubic], animations: {

        UIView.addKeyframe(withRelativeStartTime: 0, relativeDuration: 1.0/2.0, animations: {
            self.titleLabel.alpha = 0.0 // works well
        })

        self.titleSquareHeightConstraint.constant = 20 // previously 170

        UIView.addKeyframe(withRelativeStartTime: 1.0/2.0, relativeDuration: 1.0/2.0, animations: {

            // tell self.view to layout, not the titleSquare view
            self.view.layoutIfNeeded()

        })

    }, completion:nil)