我有一个从另一个推送的 CollectionViewViewController。此 CollectionViewViewController 的 View 有两个 subview :内部带有标签的简单 UIView 和 UICollectionView。当它第一次出现时，一切正常，渐变显示正确。但是当我弹出这个 View Controller(因此它被释放)并从同一个父 View Controlle 再次推送它时，渐变没有显示。

单元格如下出队:

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: PageScrollGeminiCellIdentifier, for: indexPath) as! PageScrollGeminiCell
    let item = viewModel.items[indexPath.item]
    cell.configure(withTitle: item.name, description: item. description, imageURL: item.imageUrl, gradientColor: item.color, theme: theme)
    mainView.geminiView.animateCell(cell)
    return cell
}

下面是我在单元格的类方法和单元格生命周期中所做的:

private var gradientColor: UIColor?
private var gradientLayer: CAGradientLayer?
[...]
override init(frame: CGRect) {
    super.init(frame: .zero)
    setupView()
}
required init?(coder aDecoder: NSCoder) {
    return nil
}
override func layoutSubviews() {
    if gradientLayer != nil {
        gradientView.layer.sublayers = nil
    }
    if let color = gradientColor {
        let gradientPoints = (CGPoint(x: 0.0, y: 0.15), CGPoint(x: 0.0, y: 1.0))
        self.gradientLayer = gradientView.applyGradient(with: [color, color.withAlphaComponent(0.08)], gradient: .vertical(gradientPoints))
    }
}
// MARK: Private
private func setupView() {
    clipsToBounds = true
    layer.cornerRadius = 15
    backgroundColor = .clear
    addSubview(containerView)
    containerView.addSubview(backgroundImageView)
    containerView.addSubview(gradientView)
    containerView.addSubview(labelsContainerView)
    labelsContainerView.addSubview(titleLabel)
    labelsContainerView.addSubview(descriptionLabel)
    containerView.snp.makeConstraints { maker in
        maker.edges.equalToSuperview()
    }
    backgroundImageView.snp.makeConstraints { maker in
        maker.edges.equalToSuperview()
    }
    gradientView.snp.makeConstraints { maker in
        maker.leading.trailing.top.bottom.equalToSuperview()
    }
    labelsContainerView.snp.makeConstraints { maker in
        maker.leading.top.equalTo(16)
        maker.trailing.equalTo(-16)
    }
    titleLabel.snp.makeConstraints { maker in
        maker.top.leading.trailing.equalToSuperview()
    }
    descriptionLabel.snp.makeConstraints { maker in
        maker.top.equalTo(titleLabel.snp.bottom).offset(8)
        maker.leading.trailing.bottom.equalToSuperview()
    }
}
// MARK: - Internal
func configure(withTitle title: String?, description: String?, imageURL: String?, gradientColor: UIColor?, theme: Themeable) {
    backgroundImageView.setImage(withString: imageURL)
    titleLabel.text = title
    titleLabel.font = theme.font.h2
    descriptionLabel.text = description
    descriptionLabel.font = theme.font.b1
    self.gradientColor = gradientColor
}

我还想向您展示 applyGradient 功能:

func applyGradient(with colours: [UIColor], gradient orientation: GradientOrientation) -> CAGradientLayer {
    layoutSubviews()
    let gradient = CAGradientLayer()
    gradient.frame = self.bounds
    gradient.colors = colours.map { $0.cgColor }
    gradient.startPoint = orientation.startPoint
    gradient.endPoint = orientation.endPoint
    gradient.cornerRadius = layer.cornerRadius
    self.layer.insertSublayer(gradient, at: 0)
    return gradient
}

我发现，当 View Controller 第一次呈现时，layoutSubviews() 方法被调用了两次，但每次下一次每个单元格只被调用一次。

现在我不确定我是否没有向我的单元插入超过 1 个子层，但我确定我正在清除子层阵列，所以我认为这不是问题。

最佳答案

不应该调用 layoutSubviews()，应用程序在需要时自行调用此函数(这就是为什么它在开始时被调用两次) 如 Swift 开发者网站所述: https://developer.apple.com/documentation/uikit/uiview/1622482-layoutsubviews

You should not call this method directly. If you want to force a layout update, call the setNeedsLayout() method instead to do so prior to the next drawing update. If you want to update the layout of your views immediately, call the layoutIfNeeded() method.

尝试避免使用或调用 layoutSubviews()，相反，您可以尝试在 func 中设置 gradientLayer 变量并在需要调用时调用它，尝试下面的 setupView()

如果有帮助，请告诉我。

关于ios - CollectionViewCell 的 subview 的渐变层仅在 View CONtroller 首次接纳时存在，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/55707469/