let inputNumber: Double = 99.99990338164251
let roundedString = String(format: "%.7f", inputNumber) //"99.9999034"
let outputNumber = Double(roundedString) //99.99990339999999
Decimal
类而不是Double
https://developer.apple.com/documentation/foundation/decimal您的问题是浮点数计算的常见问题
关于ObjC类https://developer.apple.com/documentation/foundation/nsdecimalnumber的更多详细信息>