swift - 为什么使用自定义初始化程序时 `var row: Int` 的属性 `var section: Int` 和 `IndexPath` 崩溃？

Question

有必要将属性subrow: Int 添加到IndexPath。为什么属性 row: Int 和 section: Int 崩溃？

import UIKit 

extension IndexPath {
    init(subrow: Int, row: Int, section: Int) {
        self.init(indexes: [section, row, subrow])
    }
    var subrow: Int {
        get { return self[2] }
        set { return self[2] = newValue }
    }
}

let ip = IndexPath(subrow: 0, row: 1, section: 2)
print(ip.subrow == 0) // OK
print(ip.row == 1) // Crash!
print(ip.section == 2) // Crash!

Answer 1

row 和 section 属性在扩展中定义UIKit 框架中的 IndexPath。他们“方便”访问器，旨在与 TableView 或 Collection View 一起使用。

从API文档可以看出，它们只能与索引路径一起使用正是两个元素:

extension IndexPath {

    /// Initialize for use with `UITableView` or `UICollectionView`.
    public init(row: Int, section: Int)

    /// The section of this index path, when used with `UITableView`.
    ///
    /// - precondition: The index path must have exactly two elements.
    public var section: Int

    /// The row of this index path, when used with `UITableView`.
    ///
    /// - precondition: The index path must have exactly two elements.
    public var row: Int
}

另请参阅 UIKit_FoundationExtensions.swift.gyb 处的源代码.

您可以改用下标方法:

let ip = IndexPath(subrow: 0, row: 1, section: 2)

print(ip[0]) // 2
print(ip[1]) // 1
print(ip[2]) // 0