在Objective-C中我们可以使调substringWithRange的方法达到截取字符串的效果
[@"123" substringWithRange:NSMakeRange(1, 2)];
在Swift 4之前，也是有类似的方法"ss".substring(with: <Range<String.Index>>) ,但是在Swift 4之后被弃用了，我们可以通过用extension的方法去给他增加截取字符串的方法
extension String{
func to(_ index:Int) -> String? {
let toIndex = String.Index.init(encodedOffset: index)
guard toIndex < self.endIndex else { return nil }
return String(self[...toIndex])
}
func from(_ index:Int) -> String? {
let fromIndex = String.Index.init(encodedOffset: index)
guard fromIndex < self.endIndex else { return nil }
return String(self[fromIndex..<self.endIndex])
}
func subString(_ from:Int,to:Int) -> String? {
let toIndex = String.Index.init(encodedOffset: from)
let fromIndex = String.Index.init(encodedOffset: to)
guard toIndex < self.endIndex,
fromIndex < self.endIndex,
toIndex <= fromIndex else { return nil }
return String(self[toIndex...fromIndex])
}
}
现在又可以方便的截取字符串了
print("12345678".to(3)) //Optional("1234")
print("12345678".from(3)) //Optional("45678")
print("12345678".subString(3, to: 5)) //Optional("456")
另外我们还可以结合Swift下标的特性，达到通过取字符串下标截取字符串
extension String{
subscript(index:Int) -> String? {
return subString(index, to: index)
}
subscript (bounds: CountableClosedRange<Int>) -> String? {
return subString( bounds.lowerBound, to: bounds.upperBound)
}
subscript (bounds: CountableRange<Int>) -> String? {
return subString( bounds.lowerBound, to: bounds.upperBound)
}
}
print("12345678"[2]) //Optional("3")
print("12345678"[3...5]) //Optional("456")
print("12345678"[100]) //nil
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