Swift中如果想获取字符串中的某个字符，通过Index索引获取，先来简单看一下几个简单的字符串获取字符的方法:

var str="My name is FlyElephant"
//start
var char=str.characters[str.startIndex]
print("\(char)")
//middle
var advance=str.characters[str.startIndex.advancedBy(3)]
print("\(advance)")

//end
var end=str.characters[str.endIndex.predecessor()]
print("\(end)")

var startIndex=str.startIndex.advancedBy(0)
var endIndex=str.startIndex.advancedBy(6)
var rangeContent=str[Range(startIndex...endIndex)]
print("\(rangeContent)")

第一个方法获取的是第一个字符，如果想往后获取，需要通过advancedBy往后获取，注意如果获取最后一个字符，还需要调用一下predecessor方法：

FlyElephant.png

我们来看一下下面的实现方式:

var exChar:Character=str[0]
print("Extension:\(exChar)")

var stContent:String=str[3]
print("Extension:\(stContent)")

var range=Range(11...21)
var strRange=str[range]
print("Extension:\(strRange)")

获取结果:

FlyElephant.png

上面的写法比较简单，我们需要扩展String，代码参考如下:

extension String {
    
    subscript(index:NSInteger)->Character {
        return self[self.startIndex.advancedBy(index)]
    }
    
    subscript(index:NSInteger)->String {
        return String(self[index] as Character)
    }
    
    subscript(range:Range<NSInteger>)->String {
        let start = startIndex.advancedBy(range.startIndex)
        let end = start.advancedBy(range.endIndex-range.startIndex)
        return self[Range(start..<end)]
    }
}

Swift 3.0 扩展方法修改如下:

extension String {
    
    subscript(index: Int) -> Character {
        let position = self.index(self.startIndex, offsetBy: index)
        return self[position]
    }
    
    subscript(index: Int) -> String {
        return String(self[index] as Character)
    }
    
    subscript(range: Range<Int>) -> String {
        let start = self.index(self.startIndex, offsetBy: range.lowerBound)
        let end = self.index(start, offsetBy: range.upperBound - range.lowerBound)
        return self[Range(start..<end)]
    }
    
}

字符串转为Bool也可以扩展:

    var boolValue: Bool {
        return NSString(string: self).boolValue
    }
    
    func toBool() -> Bool? {
        switch self {
        case "True", "true", "yes", "1":
            return true
        case "False", "false", "no", "0":
            return false
        default:
            return nil
        }
    }