属性包装器：“不能在属性初始化器中使用实例成员；属性初始化器在‘self’可用之前运行”

Question

我正在尝试创建波纹管属性包装器

@propertyWrapper
public struct ConstraintValue {

    private let view:UIView
    private let contraint:NSLayoutConstraint

    public init(contraint:NSLayoutConstraint,view:UIView) {
        self.contraint = contraint
        self.view = view
    }

    public var wrappedValue: CGFloat {
        set {
            contraint.constant = newValue
            view.layoutIfNeeded()

        }

        get {
            return contraint.constant
        }

    }
}

但是当我尝试使用它时，我得到“不能在属性初始化器中使用实例成员；属性初始化器在 'self' 可用之前运行”

@IBOutlet weak private var imageViewHeightConstraint: NSLayoutConstraint!
 @ConstraintValue(contraint: imageViewHeightConstraint, view: self)
 public var imageHeight:CGFloat

有没有办法解决这个错误？

Answer 1

你不能使用self和imageViewHeightConstraint来初始化这个属性，因为它现在不存在。

最简单的解决方案是在视图的init() 方法中将约束和视图传递给包装器，如下所示：

@propertyWrapper
public struct ConstraintValue {

    var view: UIView!
    var constraint: NSLayoutConstraint!

    public var wrappedValue: CGFloat {
        set {
            constraint.constant = newValue
            view.layoutIfNeeded()

        }

        get {
            return constraint.constant
        }
    }
}

class OurView: UIView {

    var imageViewHeightConstraint: NSLayoutConstraint!

    @ConstraintValue public var imageHeight:CGFloat

    init() {
        super.init(size: .zero)
        _imageHeight.view = self
        _imageHeight.constraint = imageViewHeightConstraint
    }
}

或者，您可以将 KeyPath 用于imageViewHeightConstraint，并将self 传递给视图init() 中的属性包装器。像这样的：

protocol DynamicImageView: UIView {
    var imageViewHeightConstraint: NSLayoutConstraint! { get set }
}

@propertyWrapper
public struct ConstraintValue {

    var view: DynamicImageView?
    private let constraintKeyPath: WritableKeyPath<DynamicImageView, NSLayoutConstraint>

    init(constraintKeyPath: WritableKeyPath<DynamicImageView, NSLayoutConstraint>) {
        self.constraintKeyPath = constraintKeyPath
    }

    public var wrappedValue: CGFloat {
        set {
            view?[keyPath: constraintKeyPath].constant = newValue
            view?.layoutIfNeeded()
        }

        get {
            view?[keyPath: constraintKeyPath].constant ?? 0.0
        }
    }
}

class OurView: UIView, DynamicImageView{

    var imageViewHeightConstraint: NSLayoutConstraint!

    @ConstraintValue(constraintKeyPath: \.imageViewHeightConstraint)
    public var imageHeight:CGFloat

    init() {
        super.init(frame: .zero)
        _imageHeight.view = self
    }
}