环境Xcode 11.0 beta4 swift 5.1
用 subscript
关键字定义,可以传一个或者多个参数和一个返回类型且可以是 read-write
或者 read-only
subscript(index: Int) -> Int {
get {
// Return an appropriate subscript value here.
}
set(newValue) {
// Perform a suitable setting action here.
}
}
// newValue的类型与返回类相同
// 与只读的计算属性相同,可以简化定义如下
subscript(index: Int) -> Int {
// Return an appropriate subscript value here.
}
// 只读下标的示例
struct TimesTable {
let multiplier: Int
subscript(index: Int) -> Int {
return multiplier * index
}
}
let threeTimesTable = TimesTable(multiplier: 3)
print("six times three is \(threeTimesTable[6])")
// Prints "six times three is 18"
下标的确切含义要取决于使用它的上下文,可以给特定的类或结构体的功能实现下标
// Swift自带的Dictionary
var numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
numberOfLegs["bird"] = 2
// 自定义的下标如上面的 threeTimesTable[6]
下标不可以使用 in-out
参数
// 如下示例下标接收多个参数
struct Matrix {
let rows: Int, columns: Int
var grid: [Double]
init(rows: Int, columns: Int) {
self.rows = rows
self.columns = columns
grid = Array(repeating: 0.0, count: rows * columns)
}
// 断言:判断传入的值是否有效
func indexIsValid(row: Int, column: Int) -> Bool {
return row >= 0 && row < rows && column >= 0 && column < columns
}
subscript(row: Int, column: Int) -> Double {
get {
assert(indexIsValid(row: row, column: column), "Index out of range")
return grid[(row * columns) + column]
}
set {
assert(indexIsValid(row: row, column: column), "Index out of range")
grid[(row * columns) + column] = newValue
}
}
}
// 初始化一个2 * 2 矩阵
var matrix = Matrix(rows: 2, columns: 2)
// 赋值
matrix[0, 1] = 1.5
matrix[1, 0] = 3.2
// This triggers an assert, because [2, 2] is outside of the matrix bounds.
let someValue = matrix[2, 2]
与实例下标几乎相同,类型下标要回 static
关键字修饰,如果是类也可以用 class
修饰且可以被子类重写实现
// 定义和调用类型下标
enum Planet: Int {
case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune
static subscript(n: Int) -> Planet {
return Planet(rawValue: n)!
}
}
let mars = Planet[4]
print(mars)
原文:https://www.cnblogs.com/tzsh1007/p/11496286.html