How do I create a multiline TextField in SwiftUI?

Ok, I started with @sas approach, but needed it really look&feel as multi-line text field with content fit, etc. Here is what I've got. Hope it will be helpful for somebody else... Used Xcode 11.1.

Provided custom MultilineTextField has:
1. content fit
2. autofocus
3. placeholder
4. on commit

import SwiftUI
import UIKit

fileprivate struct UITextViewWrapper: UIViewRepresentable {
    typealias UIViewType = UITextView

    @Binding var text: String
    @Binding var calculatedHeight: CGFloat
    var onDone: (() -> Void)?

    func makeUIView(context: UIViewRepresentableContext<UITextViewWrapper>) -> UITextView {
        let textField = UITextView()
        textField.delegate = context.coordinator

        textField.isEditable = true
        textField.font = UIFont.preferredFont(forTextStyle: .body)
        textField.isSelectable = true
        textField.isUserInteractionEnabled = true
        textField.isScrollEnabled = false
        textField.backgroundColor = UIColor.clear
        if nil != onDone {
            textField.returnKeyType = .done
        }

        textField.setContentCompressionResistancePriority(.defaultLow, for: .horizontal)
        return textField
    }

    func updateUIView(_ uiView: UITextView, context: UIViewRepresentableContext<UITextViewWrapper>) {
        if uiView.text != self.text {
            uiView.text = self.text
        }
        if uiView.window != nil, !uiView.isFirstResponder {
            uiView.becomeFirstResponder()
        }
        UITextViewWrapper.recalculateHeight(view: uiView, result: $calculatedHeight)
    }

    fileprivate static func recalculateHeight(view: UIView, result: Binding<CGFloat>) {
        let newSize = view.sizeThatFits(CGSize(width: view.frame.size.width, height: CGFloat.greatestFiniteMagnitude))
        if result.wrappedValue != newSize.height {
            DispatchQueue.main.async {
                result.wrappedValue = newSize.height // !! must be called asynchronously
            }
        }
    }

    func makeCoordinator() -> Coordinator {
        return Coordinator(text: $text, height: $calculatedHeight, onDone: onDone)
    }

    final class Coordinator: NSObject, UITextViewDelegate {
        var text: Binding<String>
        var calculatedHeight: Binding<CGFloat>
        var onDone: (() -> Void)?

        init(text: Binding<String>, height: Binding<CGFloat>, onDone: (() -> Void)? = nil) {
            self.text = text
            self.calculatedHeight = height
            self.onDone = onDone
        }

        func textViewDidChange(_ uiView: UITextView) {
            text.wrappedValue = uiView.text
            UITextViewWrapper.recalculateHeight(view: uiView, result: calculatedHeight)
        }

        func textView(_ textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool {
            if let onDone = self.onDone, text == "\n" {
                textView.resignFirstResponder()
                onDone()
                return false
            }
            return true
        }
    }

}

struct MultilineTextField: View {

    private var placeholder: String
    private var onCommit: (() -> Void)?

    @Binding private var text: String
    private var internalText: Binding<String> {
        Binding<String>(get: { self.text } ) {
            self.text = $0
            self.showingPlaceholder = $0.isEmpty
        }
    }

    @State private var dynamicHeight: CGFloat = 100
    @State private var showingPlaceholder = false

    init (_ placeholder: String = "", text: Binding<String>, onCommit: (() -> Void)? = nil) {
        self.placeholder = placeholder
        self.onCommit = onCommit
        self._text = text
        self._showingPlaceholder = State<Bool>(initialValue: self.text.isEmpty)
    }

    var body: some View {
        UITextViewWrapper(text: self.internalText, calculatedHeight: $dynamicHeight, onDone: onCommit)
            .frame(minHeight: dynamicHeight, maxHeight: dynamicHeight)
            .background(placeholderView, alignment: .topLeading)
    }

    var placeholderView: some View {
        Group {
            if showingPlaceholder {
                Text(placeholder).foregroundColor(.gray)
                    .padding(.leading, 4)
                    .padding(.top, 8)
            }
        }
    }
}

#if DEBUG
struct MultilineTextField_Previews: PreviewProvider {
    static var test:String = ""//some very very very long description string to be initially wider than screen"
    static var testBinding = Binding<String>(get: { test }, set: {
//        print("New value: \($0)")
        test = $0 } )

    static var previews: some View {
        VStack(alignment: .leading) {
            Text("Description:")
            MultilineTextField("Enter some text here", text: testBinding, onCommit: {
                print("Final text: \(test)")
            })
                .overlay(RoundedRectangle(cornerRadius: 4).stroke(Color.black))
            Text("Something static here...")
            Spacer()
        }
        .padding()
    }
}
#endif

iOS 14 - Native SwiftUI

It is called TextEditor

struct ContentView: View {
    @State var text: String = "Multiline \ntext \nis called \nTextEditor"

    var body: some View {
        TextEditor(text: $text)
    }
}

🎁 Dynamic growing height:

If you want it to grow as you type, embed it in a ZStack with a Text like this:

iOS 13 - Using UITextView

you can use the native UITextView right in the SwiftUI code with this struct:

struct TextView: UIViewRepresentable {
    
    typealias UIViewType = UITextView
    var configuration = { (view: UIViewType) in }
    
    func makeUIView(context: UIViewRepresentableContext<Self>) -> UIViewType {
        UIViewType()
    }
    
    func updateUIView(_ uiView: UIViewType, context: UIViewRepresentableContext<Self>) {
        configuration(uiView)
    }
}

Usage

struct ContentView: View {
    var body: some View {
        TextView() {
            $0.textColor = .red
            // Any other setup you like
        }
    }
}

💡 Advantages:

• Support for iOS 13
• Shared with the legacy code
• Tested for years in UIKit
• Fully customizable
• All other benefits of the original UITextView

Update: While Xcode11 beta 4 now does support TextView, I've found that wrapping a UITextView is still be best way to get editable multiline text to work. For instance, TextView has display glitches where text does not appear properly inside the view.

Original (beta 1) answer:

For now, you could wrap a UITextView to create a composable View:

import SwiftUI
import Combine

final class UserData: BindableObject  {
    let didChange = PassthroughSubject<UserData, Never>()

    var text = "" {
        didSet {
            didChange.send(self)
        }
    }

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

struct MultilineTextView: UIViewRepresentable {
    @Binding var text: String

    func makeUIView(context: Context) -> UITextView {
        let view = UITextView()
        view.isScrollEnabled = true
        view.isEditable = true
        view.isUserInteractionEnabled = true
        return view
    }

    func updateUIView(_ uiView: UITextView, context: Context) {
        uiView.text = text
    }
}

struct ContentView : View {
    @State private var selection = 0
    @EnvironmentObject var userData: UserData

    var body: some View {
        TabbedView(selection: $selection){
            MultilineTextView(text: $userData.text)
                .tabItemLabel(Image("first"))
                .tag(0)
            Text("Second View")
                .font(.title)
                .tabItemLabel(Image("second"))
                .tag(1)
        }
    }
}

#if DEBUG
struct ContentView_Previews : PreviewProvider {
    static var previews: some View {
        ContentView()
            .environmentObject(UserData(
                text: """
                        Some longer text here
                        that spans a few lines
                        and runs on.
                        """
            ))

    }
}
#endif

With a Text() you can achieve this using .lineLimit(nil), and the documentation suggests this should work for TextField() too. However, I can confirm this does not currently work as expected.

I suspect a bug - would recommend filing a report with Feedback Assistant. I have done this and the ID is FB6124711.

EDIT: Update for iOS 14: use the new TextEditor instead.