如何在 Swift 中存储属性，就像我在 Objective-C 上一样? [英] How to have stored properties in Swift, the same way I had on Objective-C?

问题描述

我正在将一个应用程序从 Objective-C 切换到 Swift，我有几个带有存储属性的类别，例如:

I am switching an application from Objective-C to Swift, which I have a couple of categories with stored properties, for example:

@interface UIView (MyCategory)

- (void)alignToView:(UIView *)view
          alignment:(UIViewRelativeAlignment)alignment;
- (UIView *)clone;

@property (strong) PFObject *xo;
@property (nonatomic) BOOL isAnimating;

@end

由于 Swift 扩展不接受这样的存储属性，我不知道如何维护与 Objc 代码相同的结构.存储属性对我的应用程序非常重要，我相信 Apple 一定已经创建了一些解决方案来在 Swift 中执行此操作.

As Swift extensions don't accept stored properties like these, I don't know how to maintain the same structure as the Objc code. Stored properties are really important for my app and I believe Apple must have created some solution for doing it in Swift.

正如 jou 所说，我实际上正在寻找的是使用关联对象，所以我做了(在另一个上下文中):

As said by jou, what I was looking for was actually using associated objects, so I did (in another context):

import Foundation
import QuartzCore
import ObjectiveC

extension CALayer {
    var shapeLayer: CAShapeLayer? {
        get {
            return objc_getAssociatedObject(self, "shapeLayer") as? CAShapeLayer
        }
        set(newValue) {
            objc_setAssociatedObject(self, "shapeLayer", newValue, UInt(OBJC_ASSOCIATION_RETAIN))
        }
    }

    var initialPath: CGPathRef! {
        get {
            return objc_getAssociatedObject(self, "initialPath") as CGPathRef
        }
        set {
            objc_setAssociatedObject(self, "initialPath", newValue, UInt(OBJC_ASSOCIATION_RETAIN))
        }
    }
}

但我在执行时得到 EXC_BAD_ACCESS:

But I get an EXC_BAD_ACCESS when doing:

class UIBubble : UIView {
    required init(coder aDecoder: NSCoder) {
        ...
        self.layer.shapeLayer = CAShapeLayer()
        ...
    }
}

有什么想法吗?

推荐答案

关联对象 API 使用起来有点麻烦.您可以使用辅助类删除大部分样板.

Associated objects API is a bit cumbersome to use. You can remove most of the boilerplate with a helper class.

public final class ObjectAssociation<T: AnyObject> {

    private let policy: objc_AssociationPolicy

    /// - Parameter policy: An association policy that will be used when linking objects.
    public init(policy: objc_AssociationPolicy = .OBJC_ASSOCIATION_RETAIN_NONATOMIC) {

        self.policy = policy
    }

    /// Accesses associated object.
    /// - Parameter index: An object whose associated object is to be accessed.
    public subscript(index: AnyObject) -> T? {

        get { return objc_getAssociatedObject(index, Unmanaged.passUnretained(self).toOpaque()) as! T? }
        set { objc_setAssociatedObject(index, Unmanaged.passUnretained(self).toOpaque(), newValue, policy) }
    }
}

前提是您可以以更易读的方式添加"一个属性到objective-c 类:

Provided that you can "add" a property to objective-c class in a more readable manner:

extension SomeType {

    private static let association = ObjectAssociation<NSObject>()

    var simulatedProperty: NSObject? {

        get { return SomeType.association[self] }
        set { SomeType.association[self] = newValue }
    }
}

至于解决方案:

extension CALayer {

    private static let initialPathAssociation = ObjectAssociation<CGPath>()
    private static let shapeLayerAssociation = ObjectAssociation<CAShapeLayer>()

    var initialPath: CGPath! {
        get { return CALayer.initialPathAssociation[self] }
        set { CALayer.initialPathAssociation[self] = newValue }
    }

    var shapeLayer: CAShapeLayer? {
        get { return CALayer.shapeLayerAssociation[self] }
        set { CALayer.shapeLayerAssociation[self] = newValue }
    }
}

这篇关于如何在 Swift 中存储属性，就像我在 Objective-C 上一样?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！