自定义NSTextLocation的NSTextRange如何正确比较相等性？

当使用自定义NSTextLocation实现的NSTextRange调用isEqual(to:)方法时，即使起始和结束位置通过compare(_:)返回.orderedSame，也会判定为不相等。如何正确比较这类范围的相等性？

问题代码示例 #

class Location: NSObject, NSTextLocation {
    let value: Int

    func compare(_ location: NSTextLocation) -> ComparisonResult {
        guard let location = location as? Location else {
            fatalError("Expected Location")
        }

        if self.value == location.value {
            return .orderedSame
        } else if self.value < location.value {
            return .orderedAscending
        } else {
            return .orderedDescending
        }
    }

    init(_ value: Int) {
        self.value = value
    }
}

let textRange1 = NSTextRange(location: Location(1), end: Location(2))!
let textRange2 = NSTextRange(location: Location(1), end: Location(2))!

print("startLocations are equal: \(textRange1.location.compare(textRange2.location) == .orderedSame)")
print("endLocations are equal: \(textRange1.endLocation.compare(textRange2.endLocation) == .orderedSame)")
print("ranges are equal: \(textRange1.isEqual(to: textRange2))")

问题原因 #

NSTextRange的isEqual(to:)方法内部并非使用compare(_:)判断位置相等，而是直接调用NSObject的isEqual(_:)方法。默认的isEqual(_:)比较的是对象的内存地址，因此即使两个Location实例的value相同，也是不同的对象实例，导致NSTextRange判定为不相等。

解决方案 #

给自定义的Location类重写isEqual(_:)方法，根据value判断相等性；同时重写hash方法（这是NSObject子类遵循相等性约定的最佳实践）：

class Location: NSObject, NSTextLocation {
    let value: Int

    func compare(_ location: NSTextLocation) -> ComparisonResult {
        guard let location = location as? Location else {
            fatalError("Expected Location")
        }

        if self.value == location.value {
            return .orderedSame
        } else if self.value < location.value {
            return .orderedAscending
        } else {
            return .orderedDescending
        }
    }
    
    override func isEqual(_ object: Any?) -> Bool {
        guard let other = object as? Location else {
            return false
        }
        return self.value == other.value
    }
    
    override var hash: Int {
        return value.hashValue
    }

    init(_ value: Int) {
        self.value = value
    }
}

let textRange1 = NSTextRange(location: Location(1), end: Location(2))!
let textRange2 = NSTextRange(location: Location(1), end: Location(2))!

print("startLocations are equal: \(textRange1.location.compare(textRange2.location) == .orderedSame)")
print("endLocations are equal: \(textRange1.endLocation.compare(textRange2.endLocation) == .orderedSame)")
print("ranges are equal: \(textRange1.isEqual(to: textRange2))") // 现在会输出true

结果验证 #

修改后运行代码，textRange1.isEqual(to: textRange2)会返回true，符合预期的相等性判断。

内容的提问来源于stack exchange，提问作者georgemp