How can I add a bottom line on TextField (SwiftUI)

Divider

A visual element that can be used to separate other content.

You can set color and height

Divider()
 .frame(height: 1)
 .padding(.horizontal, 30)
 .background(Color.red)

struct LoginField: View {

    @State private var email: String = ""
    @State private var password: String = ""

    var body: some View {
        VStack {
            TextField("Email", text: $email)
                .padding(.horizontal, 30).padding(.top, 20)
            Divider()
                .padding(.horizontal, 30)
            TextField("Password", text: $password)
                .padding(.horizontal, 30).padding(.top, 20)
            Divider()
                .padding(.horizontal, 30)
            Spacer()
        }
    }
}

To define a custom style, you can use the code below. The Rectangle is used eventually, but inside the custom style, not in your view code. Is this what you want?

Note: Tested on Beta 3

To use your custom initialiser:

struct ContentView : View {
    @State private var username = ""

    var body: some View {
        VStack {

            TextField("Enter text", text: $username)
                .textFieldStyle(.myOwnStyle)

        }.frame(width:300)
    }
}

Your custom initialiser implementation:

public struct MyOwnTextFieldStyle: TextFieldStyle {
    public func _body(configuration: TextField<Self.Label>) -> some View {

        VStack() {
            configuration
                .border(Color.blue, width: 2)
                .foregroundColor(Color.yellow)
            Rectangle()
                .frame(height: 1.0, alignment: .bottom)
                .relativeWidth(1)
                .foregroundColor(Color.red)

        }
    }
}

extension StaticMember where Base : TextFieldStyle {
    public static var myOwnStyle: MyOwnTextFieldStyle.Member {
        StaticMember<MyOwnTextFieldStyle>(MyOwnTextFieldStyle())
    }
}

Using .overlay

TextField("[email protected]", text: $email)
        .overlay(VStack{Divider().offset(x: 0, y: 15)})

the use of VStack is to ensure that the Divider is always horizontal.

The solution of kontiki does not work with Beta 6, though. So I created a solution where I embed a TextField and a drawn bottom line in a new view. You can just copy the code and use it by writing TextFieldWithBottomLine(placeholder: "My placeholder") Used in a view it looks like:

The first thing I create is a horizontal line:

import SwiftUI

struct HorizontalLineShape: Shape {

    func path(in rect: CGRect) -> Path {

        let fill = CGRect(x: 0, y: 0, width: rect.size.width, height: rect.size.height)
        var path = Path()
        path.addRoundedRect(in: fill, cornerSize: CGSize(width: 2, height: 2))

        return path
    }
}

struct HorizontalLine: View {
    private var color: Color? = nil
    private var height: CGFloat = 1.0

    init(color: Color, height: CGFloat = 1.0) {
        self.color = color
        self.height = height
    }

    var body: some View {
        HorizontalLineShape().fill(self.color!).frame(minWidth: 0, maxWidth: .infinity, minHeight: height, maxHeight: height)
    }
}

struct HorizontalLine_Previews: PreviewProvider {
    static var previews: some View {
        HorizontalLine(color: .black)
    }
}

Next I create a view that contains a TextField and the HorizontalLine below:

import SwiftUI

struct TextFieldWithBottomLine: View {
    @State var text: String = ""
    private var placeholder = ""
    private let lineThickness = CGFloat(2.0)

    init(placeholder: String) {
        self.placeholder = placeholder
    }

    var body: some View {
        VStack {
         TextField(placeholder, text: $text)
            HorizontalLine(color: .black)
        }.padding(.bottom, lineThickness)
    }
}

struct TextFieldWithBottomLine_Previews: PreviewProvider {
    static var previews: some View {
        TextFieldWithBottomLine(placeholder: "My placeholder")
    }
}

To see it in action I created a sample view:

import SwiftUI

struct SampleView: View {
    @State var text: String = ""
    @ObservedObject var model: LoginModel

    init() {
        self.model = LoginModel()
    }

    init(model: LoginModel) {
        self.model = model
    }

    var body: some View {
        TextFieldWithBottomLine(placeholder: "My placeholder").padding(24)
    }

    func initialActions() {

    }

    func reactOnButtonClick() {

    }
}

struct SampleView_Previews: PreviewProvider {
    static var previews: some View {
        SampleView()
    }
}