Thomas Step

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1 March 2021

Code For a Number Pad in Swift

by Thomas

I have been developing an iOS application recently and I needed to receive numeric input from the user. I added a TextField and changed the keyboardType to numberPad, but I also stumbled across someone who took it an extra step and added some validation. I wanted to share what that looked like and also show my twist on it. The simple TextField with numeric input looks like this.

TextField("Demo text field", text: $myInput)
  .multilineTextAlignment(.center)
  .textFieldStyle(RoundedBorderTextFieldStyle())
  .keyboardType(.numberPad)

However, if the user has a Bluetooth keyboard synced with their device or if they are on an iPad (iPads do not support numberPad) then it is possible for a user to input other characters as well. To combat bad characters being passed in, we can add some validation logic like this.

import Combine

...

TextField("Demo text field", text: $myInput)
  .multilineTextAlignment(.center)
  .textFieldStyle(RoundedBorderTextFieldStyle())
  .keyboardType(.numberPad)
  .onReceive(Just(myInput)) { newValue in
    let filtered = newValue.filter {
      "0123456789".contains($0)
    }

    if filtered != newValue {
        self.desiredPomodoroRounds = filtered
    }
}

It is worth noting that I needed to import Combine for Just to work. The onReceive function allows us to validate the input text as it comes in. What this snippet does is check that only numbers are input, and if another character is found, that character is dropped. I took this a step further still. I was moving from Slider to TextField so I wanted to put bounds on the numbers that could be input. My solution was to check for those bounds inside of onReceive and force the value back inside of my bounds. Since I am expecting a number but want to make input easy, I felt that this was a decent compromise.

import Combine

...

TextField("Demo text field", text: $myInput)
  .multilineTextAlignment(.center)
  .textFieldStyle(RoundedBorderTextFieldStyle())
  .keyboardType(.numberPad)
  .onReceive(Just(myInput)) { newValue in
    let filtered = newValue.filter {
      "0123456789".contains($0)
    }

    // Arbitrary default value within bounds
    let numberValue = Int(filtered) ?? 4

    // Check for lower bound
    if numberValue < 1 {
        self.myInput = "1"
    }

    // Check for upper bound
    if numberValue > 60 {
        self.myInput = "20"
    }

    if filtered != newValue {
        self.myInput = filtered
    }
}

The bounds and default value would need to be tailored to each application, but this is a decent (albeit a little hacky) way to put numeric bounds on TextField input. The nil coalescing operator on the cast (Int(filtered) ?? 4) also allows for a value to be present if the user just deletes all of the input.

Categories: dev | ios | swift