为什么watchOS中Picker会触发“ScrollView contentOffset binding has been read”警告

你遇到的警告原文如下：

"ScrollView contentOffset binding has been read; this will cause grossly inefficient view performance as the ScrollView's content will be updated whenever its contentOffset changes. Read the contentOffset binding in a view that is not parented between the creator of the binding and the ScrollView to avoid this."

问题原因 #

• watchOS端SwiftUI的Picker默认基于ScrollView实现滚轮选择效果，未明确指定样式时，系统内部会主动读取ScrollView的contentOffset绑定用于适配布局
• 你当前使用的已废弃的NavigationView导航栈实现存在bug，跳转时父视图会意外读取子页面Picker内部的ScrollView偏移量绑定，触发性能警告

修复方案 #

按以下两个步骤修改即可消除警告：

1. 替换根视图废弃的NavigationView为watchOS 9+推荐的NavigationStack，如果需要兼容更低版本，可以给NavigationView加.navigationViewStyle(.stack)显式指定样式
2. 给DistanceSelectView中的Picker明确指定.pickerStyle(.wheel)样式，避免系统默认样式的多余绑定读取

修改后的代码示例 #

PickerTestApp 代码 #

import SwiftUI    
@main
struct PickerTestApp: App {
    var body: some Scene {
        WindowGroup {
            NavigationStack {
                ContentView()
            }
            // 兼容watchOS 8及更低版本可替换为以下代码
            // NavigationView {
            //     ContentView()
            // }
            // .navigationViewStyle(.stack)
        }
    }
}

DistanceSelectView 代码 #

import SwiftUI

struct DistanceSelectView: View {
    @State var age = 1
    var body: some View {
        VStack {
            Picker(selection: $age, label: Text("Select your age.[\(age)]")) {
                ForEach(10 ..< 100, id: \.self) { num in
                    Text("\(num)")
                }
            }
            .pickerStyle(.wheel)
        }
    }
}

内容的提问来源于stack exchange，提问作者Janis Jakaitis