如何在Swift中存储属性，就像我在Objective-C上一样？

问题描述

我正在将一个应用程序从Objective-C切换到Swift，我有几个具有存储属性的类别，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扩展不接受这些存储的属性，我不知道如何保持与Objc代码相同的结构。存储的属性对我的应用程序非常重要，我相信Apple必须已经在Swift中创建了一些解决方案。

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，我所寻找的实际上是使用关联对象，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))
        }
    }
}

但是我得到了执行时为EXC_BAD_ACCESS：

But I get an EXC_BAD_ACCESS when doing:

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

任何想法？

推荐答案

关联对象API使用起来有点麻烦。您可以使用帮助程序类删除大部分样板文件。

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) }
    }
}

如果你能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 }
    }
}

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