如何创建绑定与显示类型不同的自定义SwiftUI Binding？

你需要绑定Date和Int类型数据，但以字符串形式显示，现有代码出现两个核心错误：

1. 逃逸闭包捕获可变self参数
2. 找不到$descriptiveDate/$descriptiveInt作用域

问题代码回顾 #

import SwiftUI

struct BindingButton: View {
    @Binding var title: String
    var onTap: () -> Void
    var body: some View {
        Button {
            onTap()
        } label: {
            Text(title)
        }
    }
}

struct CustomBinding: View {
    @State var date: Date
    @State var int: Int
    
    lazy var descriptiveDate = Binding(
        get: { //Escaping closure captures mutating 'self' parameter
            return self.date.description
        },
        set: { _ in }
    )
    
    lazy var descriptiveInt = Binding(
        get: { //Escaping closure captures mutating 'self' parameter
            return String(self.int)
        },
        set: { _ in }
    )
    
    var body: some View {
        VStack {
            // Cannot find '$descriptiveDate' in scope
            BindingButton(title: $descriptiveDate) {
                date = Date()
            }
            // Cannot find '$descriptiveInt' in scope
            BindingButton(title: $descriptiveInt) {
                int += 2
            }
        }
    }
}

优雅解决方案（无需双重绑定） #

方案1：在View body中直接创建Binding

把Binding的创建逻辑移到body内部，直接访问@State属性，既避免self捕获问题，也不需要使用$投影符号（因为我们直接生成了Binding<String>实例）：

struct CustomBinding: View {
    @State var date: Date = Date()
    @State var int: Int = 0
    
    var body: some View {
        VStack {
            BindingButton(title: Binding(
                get: { date.description },
                set: { _ in }
            )) {
                date = Date()
            }
            
            BindingButton(title: Binding(
                get: { String(int) },
                set: { _ in }
            )) {
                int += 2
            }
        }
    }
}

方案2：封装为计算属性

将自定义Binding封装成计算属性，替代lazy存储属性。计算属性不会修改self，因此不会触发逃逸闭包捕获可变参数的错误，同时直接返回Binding<String>实例，无需投影符号：

struct CustomBinding: View {
    @State var date: Date = Date()
    @State var int: Int = 0
    
    // 可优化：添加自定义日期格式化
    private static let dateFormatter: DateFormatter = {
        let formatter = DateFormatter()
        formatter.dateStyle = .medium
        formatter.timeStyle = .short
        return formatter
    }()
    
    var descriptiveDate: Binding<String> {
        Binding(
            get: { Self.dateFormatter.string(from: date) },
            set: { _ in }
        )
    }
    
    var descriptiveInt: Binding<String> {
        Binding(
            get: { String(int) },
            set: { _ in }
        )
    }
    
    var body: some View {
        VStack {
            BindingButton(title: descriptiveDate) {
                date = Date()
            }
            BindingButton(title: descriptiveInt) {
                int += 2
            }
        }
    }
}

关键说明 #

• $符号仅用于@State/@Binding等属性的投影，直接创建的Binding实例无需使用$
• 计算属性或body内创建Binding时，Swift 5.3+可自动省略self，避免捕获可变self的问题
• 若需更友好的日期显示，推荐使用自定义DateFormatter而非默认的description

内容的提问来源于stack exchange，提问作者kunass2