How to create TextField that only accepts numbers

Although showing a number pad is a good first step, it does not actually prevent bad data from being entered:

1. The user can paste the non-numeric text into the TextField
2. iPad users will still get a full keyboard
3. Anyone with a Bluetooth keyboard attached can type anything

What you really want to do is sanitize the input, like this:

import SwiftUI
import Combine

struct StackOverflowTests: View {
    @State private var numOfPeople = "0"

    var body: some View {
        TextField("Total number of people", text: $numOfPeople)
            .keyboardType(.numberPad)
            .onReceive(Just(numOfPeople)) { newValue in
                let filtered = newValue.filter { "0123456789".contains($0) }
                if filtered != newValue {
                    self.numOfPeople = filtered
                }
            }
    }
}

Whenever numOfPeople changes, the non-numeric values are filtered out, and the filtered value is compared to see if numOfPeople should be updated a second time, overwriting the bad input with the filtered input.

Note that the Just publisher requires that you import Combine.

EDIT:

To explain the Just publisher, consider the following conceptual outline of what occurs when you change the value in the TextField:

1. Because TextField takes a Binding to a String when the contents of the field are changed, it also writes that change back to the @State variable.
2. When a variable marked @State changes, SwiftUI recomputes the body property of the view.
3. During the body computation, a Just publisher is created. Combine has a lot of different publishers to emit values over time, but the Just publisher takes “just” a single value (the new value of numberOfPeople) and emits it when asked.
4. The onReceive method makes a View a subscriber to a publisher, in this case, the Just publisher we just created. Once subscribed, it immediately asks for any available values from the publisher, of which there is only one, the new value of numberOfPeople.
5. When the onReceive subscriber receives a value, it executes the specified closure. Our closure can end in one of two ways. If the text is already numeric only, then it does nothing. If the filtered text is different, it is written to the @State variable, which begins the loop again, but this time the closure will execute without modifying any properties.

Check out Using Combine for more info.