Swift 3.0：“objc_setAssociatedObject”問題

Question

import UIKit
import ObjectiveC

var SubRowObjectKey: String = "subRow"
extension IndexPath {

    var subRow: Int {
        get {
            let subRowObj = objc_getAssociatedObject(self, &SubRowObjectKey)

            if subRowObj != nil {
                return (subRowObj as! Int)
            }

            return 0
        }
        set {
            let subRowObj = (newValue as NSInteger)
            objc_setAssociatedObject(self, &SubRowObjectKey, subRowObj, .OBJC_ASSOCIATION_RETAIN)

        }
    }

    static func indexPathForSubRow(_ subRow: NSInteger, inRow row: NSInteger, inSection section: NSInteger) -> IndexPath {

        var indexPath = IndexPath(row: row, section: section)
        indexPath.subRow = (subRow as Int)
        print(subRow)
        print(indexPath.subRow)
        return indexPath
    }
}

let indexPath = IndexPath.indexPathForSubRow(5, inRow: 1, inSection: 2)
print(indexPath.subRow)

• 在靜態函數indexPathForSubRow - >'subRow'Count = 5（附加圖像中的行號：30）
• 但是在將subRow分配給indexPath.subRow之后，'indexPath.subRow'count = 0而不是5（附加圖像中的第29和31行）

• 在Xcode版本8.2.1和Swift 3.0中測試

  任何幫助將不勝感激。

Answer 1

IndexPath是一個結構，它不支持關聯的對象。 您可以通過直接嘗試回讀設置對象來輕松地在setter中檢查它：

set {
    let subRowObj = (newValue as NSInteger)
    objc_setAssociatedObject(self, &SubRowObjectKey, subRowObj, .OBJC_ASSOCIATION_RETAIN)
    let subRowObj2 = objc_getAssociatedObject(self, &SubRowObjectKey)
    print(subRowObj2 ?? "nil") // prints "nil"
}

即使setter中的代碼有效 ，整個構造仍然不成立：由於結構在傳輸/分配時被復制（至少通過寫入機制上的復制進行修改），因此相關對象將不包含在復制，所以你遲早會丟失這些信息。

然而，不是擴展IndexPath你可以擴展NSIndexPath然后工作正常 - 但我想這不是你想要的，因為你想影響從表視圖得到的IndexPath ...

Answer 2

根據maddy的回答，這是我的IndexPath擴展，它添加了一個subRow屬性。

extension IndexPath {

    init(subRow: Int, row: Int, section: Int) {
        self.init(indexes: [IndexPath.Element(section), IndexPath.Element(row), IndexPath.Element(subRow)])
    }

    var subRow: Int {
        get { return self[index(at: 2)] }
        set { self[index(at: 2)] = newValue }
    }

    var row: Int {
        get { return self[index(at: 1)] }
        set { self[index(at: 1)] = newValue }
    }

    var section: Int {
        get { return self[index(at: 0)] }
        set { self[index(at: 0)] = newValue }
    }

    private func index(at idx: Int) -> IndexPath.Index {
        return self.startIndex.advanced(by: idx)
    }

}