Swift-use Set,get to ensure index addition and subtraction within normal range

With the class's computed property set and get, we can protect the addition and subtraction of the index. Here is a sample, the index initial value is 0, the valid range is 0~2. Whether it is index++ or index--, the index is always able to iterate through this range.

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class Test {    var _index = 0    var index:Int {        get{            return _index        }        set{            _index = newValue            if _index < 0 {                _index += 3            }else if _index > 2 {                _index -=3            }        }    }        func onNext(){        index++    }        func onPre(){        index--    }}

Swift-use Set,get to ensure index addition and subtraction within normal range