NSNumberFormatter PercentStyle decimal places

With Swift 5, NumberFormatter has an instance property called minimumFractionDigits. minimumFractionDigits has the following declaration:

var minimumFractionDigits: Int { get set }

The minimum number of digits after the decimal separator allowed as input and output by the receiver.

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NumberFormatter also has an instance property called maximumFractionDigits. maximumFractionDigits has the following declaration:

var maximumFractionDigits: Int { get set }

The maximum number of digits after the decimal separator allowed as input and output by the receiver.

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The following Playground code shows how to use minimumFractionDigits and maximumFractionDigits in order to set the number of digits after the decimal separator when using NumberFormatter:

import Foundation

let percentFormatter = NumberFormatter()
percentFormatter.numberStyle = NumberFormatter.Style.percent
percentFormatter.multiplier = 1
percentFormatter.minimumFractionDigits = 1
percentFormatter.maximumFractionDigits = 2

let myDouble1: Double = 8
let myString1 = percentFormatter.string(for: myDouble1)
print(String(describing: myString1)) // Optional("8.0%")

let myDouble2 = 8.5
let myString2 = percentFormatter.string(for: myDouble2)
print(String(describing: myString2)) // Optional("8.5%")

let myDouble3 = 8.5786
let myString3 = percentFormatter.string(for: myDouble3)
print(String(describing: myString3)) // Optional("8.58%")

When in doubt, look in apple documentation for minimum fraction digits and maximum fraction digits which will give you these lines you have to add before formatting your number:

numberFormatter.minimumFractionDigits = 1
numberFormatter.maximumFractionDigits = 2

Also notice, your input has to be 0.085 to get 8.5%. This is caused by the multiplier property, which is for percent style set to 100 by default.

To set the number of fraction digits use:

percentFormatter.minimumFractionDigits = 1
percentFormatter.maximumFractionDigits = 1

Set minimum and maximum to your needs. Should be self-explanatory.