SwiftUI中TextField选中时如何高亮默认值或显示占位符？

针对你提出的两种需求，分别提供简单实现方式：

方案b：显示占位符，无默认值 #

这种方式更符合用户输入习惯，未输入时显示"Temperature"占位符，输入后展示内容。只需将温度变量改为可选类型，处理计算时的默认值即可：

import SwiftUI

struct ContentView: View {
    @State private var temp: Int? = nil // 改为可选Int，初始为nil
    @State private var tempUnit: String = ""
    @State private var tempConvertUnit: String = ""

    var convertedTemp: Int {
        let inputTemp = temp ?? 0 // 未输入时用0作为计算默认值
        switch tempUnit {
        case "F":
            return tempConvertUnit == "C" ? Int((inputTemp - 32) * 5 / 9) : inputTemp
        case "C":
            return tempConvertUnit == "F" ? Int(inputTemp * 9/5 + 32) : inputTemp
        default:
            return inputTemp
        }
    }
 
    let tempUnits = ["F", "C"]
    @FocusState private var amountIsFocused: Bool

    var body: some View {
        NavigationStack{
            Form{
                Section("Temperature"){
                    TextField("Temperature", value: $temp, format: .number)
                        .keyboardType(.decimalPad)
                        .focused($amountIsFocused)
                 
                    Picker("Unit", selection: $tempUnit){
                        ForEach(tempUnits, id: \.self) { Text($0) }
                    }.pickerStyle(.segmented)
                }
                Section("Converted Temperature"){
                    Text(convertedTemp, format: .number)
                    Picker("Unit", selection: $tempConvertUnit){
                        ForEach(tempUnits, id: \.self) { Text($0) }
                    }.pickerStyle(.segmented)
                }
            }
            .navigationTitle("Converter")
            .toolbar {
                if amountIsFocused {
                    Button("Done") { amountIsFocused = false }
                }
            }
        }
    }
}

方案a：选中TextField时高亮默认值，输入自动覆盖 #

通过监听焦点状态，利用UIKit API实现选中全部文本，输入时直接覆盖默认值：

首先添加UIView扩展用于查找TextField：

extension UIView {
    func findTextField() -> UITextField? {
        if let textField = self as? UITextField {
            return textField
        }
        for subview in subviews {
            if let textField = subview.findTextField() {
                return textField
            }
        }
        return nil
    }
}

然后修改ContentView中的TextField部分，添加焦点变化监听：

TextField("Temperature", value: $temp, format: .number)
    .keyboardType(.decimalPad)
    .focused($amountIsFocused)
    .onChange(of: amountIsFocused) { _, isFocused in
        guard isFocused else { return }
        DispatchQueue.main.async {
            let activeScene = UIApplication.shared.connectedScenes
                .filter { $0.activationState == .foregroundActive }
                .first as? UIWindowScene
            let keyWindow = activeScene?.windows.first(where: { $0.isKeyWindow })
            keyWindow?.rootViewController?.view.findTextField()?.selectAll(nil)
        }
    }

这样当TextField获得焦点时，默认值0会被全部选中，输入新内容会直接覆盖。

内容的提问来源于stack exchange，提问作者Laura Mansfield