SwiftUI中如何处理可选选择？解决onChange可选值报错问题

针对你遇到的onChange监听可选值报错的问题，提供以下几种可行方案：

方案一：让SomeType遵循Equatable协议 #

onChange(of:)要求监听的类型遵循Equatable，如果你的自定义类型SomeType还没实现该协议，添加后即可正常使用监听：

// 修改SomeType，添加Equatable协议
struct SomeType: Equatable {
    // 你的自定义属性
}

// 主视图中正常使用onChange
struct MainView: View {
    @State private var showSomeList: Bool = false
    @State private var someListSelection: SomeType?

    var body: some View {
        Button("Show Sheet") {
            showSomeList = true
        }
        .sheet(isPresented: $showSomeList) {
            SomeList(selection: $someListSelection)
        }
        .onChange(of: someListSelection) { newValue in
            guard let selection = newValue else { return }
            // 处理选中的结果
            print("已选择：\(selection)")
        }
    }
}

方案二：将可选值转为Any类型监听 #

如果不想让SomeType遵循Equatable，可以把监听对象转为Any类型，代价是视图更新时可能会触发多余的回调：

struct MainView: View {
    @State private var showSomeList: Bool = false
    @State private var someListSelection: SomeType?

    var body: some View {
        Button("Show Sheet") {
            showSomeList = true
        }
        .sheet(isPresented: $showSomeList) {
            SomeList(selection: $someListSelection)
        }
        .onChange(of: someListSelection as Any) { _ in
            guard let selection = someListSelection else { return }
            // 处理选中的结果
            print("已选择：\(selection)")
        }
    }
}

方案三：利用Sheet的onDismiss回调 #

如果选择完成后会关闭Sheet，可在Sheet关闭时统一处理选中结果，无需实时监听：

struct MainView: View {
    @State private var showSomeList: Bool = false
    @State private var someListSelection: SomeType?

    var body: some View {
        Button("Show Sheet") {
            showSomeList = true
        }
        .sheet(isPresented: $showSomeList, onDismiss: {
            guard let selection = someListSelection else { return }
            // 处理选中的结果
            print("已选择：\(selection)")
        }) {
            SomeList(selection: $someListSelection)
        }
    }
}

方案四：给子视图添加完成回调 #

直接在SomeList中定义闭包，选择完成后主动通知主视图处理，逻辑更直接：

// 修改SomeList，添加完成回调闭包
struct SomeList: View {
    @Binding var selection: SomeType?
    var onSelectionCompleted: (SomeType) -> Void

    var body: some View {
        Button("Selected") {
            let newItem = SomeType()
            selection = newItem
            onSelectionCompleted(newItem)
        }
    }
}

// 主视图中传入处理逻辑
struct MainView: View {
    @State private var showSomeList: Bool = false
    @State private var someListSelection: SomeType?

    var body: some View {
        Button("Show Sheet") {
            showSomeList = true
        }
        .sheet(isPresented: $showSomeList) {
            SomeList(selection: $someListSelection) { selectedItem in
                // 处理选中的结果
                print("已选择：\(selectedItem)")
            }
        }
    }
}

内容的提问来源于stack exchange，提问作者Mark