UILabel 大小更改不会使用自动布局设置动画

Question

我有一个动态宽度 UILabel（颜色为石灰绿色以提高可见性），它对其邻居（绿色和红色视图）有固定的前导和尾随约束。我希望此 table 在其邻居之一移动时调整其宽度（圆形 red/green 视图）。但是，由于我的标签 "jumps" 的宽度在没有动画的情况下变为新值，因此我对这种变化的动画有疑问。对于我的右对齐标签，这意味着文本本身也会跳转。

我正在为 red/green 视图的移动设置动画，如下所示：

- (void)show {
    self.edgeConstraint.constant = 0;
    [UIView animateWithDuration:0.25 animations:^{
        [self.contentView layoutIfNeeded];
    }];
}

- (void)hide {
    self.edgeConstraint.constant = -100;
    [UIView animateWithDuration:0.25 animations:^{
        [self.contentView layoutIfNeeded];
    }];
}

Answer 1

问题出在 UILabel 内容模式上。您会看到类似的问题 here。因为您正在缩短绘制文本的标签，所以标签会执行绘制过程，但它会使用动画块的最后一帧进行更新。创建平滑动画的最简单方法是使用 greaterThanOrEqualTo 前导约束并将 textAlignment 设置为 .natural。

当我这样做时，我得到的结果在顶部，而不是在底部设置 textAlignment .right。

演示源代码：

import UIKit
import PlaygroundSupport

final class ViewController: UIViewController {

    var constraint: NSLayoutConstraint?
    var otherConstraint: NSLayoutConstraint?

    lazy private var sideView: UIView = {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(view)
        view.topAnchor.constraint(equalTo: self.view.topAnchor).isActive = true
        self.constraint = view.leadingAnchor.constraint(equalTo: self.view.trailingAnchor)
        self.constraint?.isActive = true
        view.widthAnchor.constraint(equalToConstant: 100).isActive = true
        view.heightAnchor.constraint(equalToConstant: 44).isActive = true
        view.backgroundColor = .yellow
        return view
    }()

    lazy private var secondSideView: UIView = {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(view)
        view.topAnchor.constraint(equalTo: self.view.topAnchor, constant: 44.0).isActive = true
        self.otherConstraint = view.leadingAnchor.constraint(equalTo: self.view.trailingAnchor)
        self.otherConstraint?.isActive = true
        view.widthAnchor.constraint(equalToConstant: 100).isActive = true
        view.heightAnchor.constraint(equalToConstant: 44).isActive = true
        view.backgroundColor = .yellow
        return view
    }()

    lazy private var label: UILabel = {
        let label = UILabel()
        label.text = "Label"
        label.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(label)
        label.topAnchor.constraint(equalTo: self.view.topAnchor).isActive = true
        label.leadingAnchor.constraint(greaterThanOrEqualTo: self.view.leadingAnchor, constant: 8.0).isActive = true
        label.trailingAnchor.constraint(equalTo: self.sideView.leadingAnchor).isActive = true
        return label
    }()

    lazy private var secondLabel: UILabel = {
        let label = UILabel()
        label.textAlignment = .right
        label.text = "Label"
        label.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(label)
        label.topAnchor.constraint(equalTo: self.view.topAnchor, constant: 44.0).isActive = true
        label.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 8.0).isActive = true
        label.trailingAnchor.constraint(equalTo: self.sideView.leadingAnchor).isActive = true
        return label
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .white
        self.secondSideView.isHidden = false
        self.secondLabel.isHidden = false
        self.sideView.isHidden = false
        self.label.isHidden = false
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        self.perform(#selector(self.showSideView), with: nil, afterDelay: 5.0)
    }

    @objc private func showSideView() {
        self.view.layoutIfNeeded()
        UIView.animate(withDuration: 1.5, delay: 0.0, options: [.autoreverse, .repeat], animations: {
            self.otherConstraint?.constant = -100
            self.constraint?.constant = -100
            self.view.layoutIfNeeded()
        })
    }
}

PlaygroundPage.current.liveView = ViewController()

UILabel 大小更改不会使用自动布局设置动画

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