SwiftUI运动App持久化报错：无法访问exerciseTime成员

问题背景 #

开发运动App时，通过TimerStructManager管理记录运动时长的TimerStruct数据，集成到HomeView时遇到两个错误：

• 类型ObservedObject<TimerStructManager>.Wrapper没有来自根类型TimerStructManager的动态成员exerciseTime
• 无法将类型Binding<Subject>转换为预期参数类型Int，报错行：let percent = Double($timer.exerciseTime/1500)

相关代码结构 #

TimerStruct 代码

import Foundation
import SwiftUI

struct TimerStruct: Codable, Identifiable {
    var id = UUID()
    var exerciseTime = 0.0
     var isAlertpresented = false
     var countdownTimer = 300
     var timerRunning = false
     var isPaused = false
     var isActive = false
    var isCompleted = false
}

TimerStructManager 代码

import Foundation
import SwiftUI

class TimerStructManager: ObservableObject {
    @Published var timerStructs: [TimerStruct] = [] {
        didSet {
            save()
        }
    }
    
    let sampleTimerStructs: [TimerStruct] = []
    
    init() {
        load()
    }
    
    func getArchiveURL() -> URL {
        let plistName = "timerStructs.plist"
        let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
        
        return documentsDirectory.appendingPathComponent(plistName)
    }
    
    func save() {
        let archiveURL = getArchiveURL()
        let propertyListEncoder = PropertyListEncoder()
        let encodedTimerStructs = try? propertyListEncoder.encode(timerStructs)
        try? encodedTimerStructs?.write(to: archiveURL, options: .noFileProtection)
    }
    
    func load() {
        let archiveURL = getArchiveURL()
        let propertyListDecoder = PropertyListDecoder()
        
        var finalTimerStructs: [TimerStruct]!
        
        if let retrievedTimerStructData = try? Data(contentsOf: archiveURL),
           let decodedTimerStructs = try? propertyListDecoder.decode([TimerStruct].self, from: retrievedTimerStructData) {
            finalTimerStructs = decodedTimerStructs
        } else {
            finalTimerStructs = sampleTimerStructs
        }
        
        timerStructs = finalTimerStructs
    }
}

HomeView 报错代码片段

let percent = Double($timer.exerciseTime/1500)
Text("\(round(result: $timer.exerciseTime/60)) mins of exercise completed today")

错误原因 #

1. 第一个错误：$timer是TimerStructManager的绑定对象，但TimerStructManager仅持有timerStructs数组属性，没有直接的exerciseTime成员。错误地将管理数组的对象当作单个TimerStruct实例访问属性。
2. 第二个错误：$timer.exerciseTime返回的是Binding<Double>类型，不能直接参与数值计算，也无法传给需要Int类型参数的round函数。

修复方案 #

步骤1：定位目标TimerStruct实例 #

TimerStructManager管理的是[TimerStruct]数组，需先确定要操作的实例（比如当日的运动记录）。示例中先获取数组首个元素，实际可根据日期筛选：

// HomeView中声明ObservedObject
@ObservedObject var manager = TimerStructManager()

// 获取当日记录，示例简化取第一个元素
private var currentTimerStruct: TimerStruct? {
    manager.timerStructs.first
}

步骤2：修正计算与视图代码 #

针对报错代码，改为直接访问数值而非绑定对象，并处理空值情况：

if let exerciseTime = currentTimerStruct?.exerciseTime {
    let percent = exerciseTime / 1500.0
    Text("\(round(exerciseTime / 60)) mins of exercise completed today")
} else {
    Text("0 mins of exercise completed today")
}

额外优化：自动创建当日记录 #

避免空值问题，可在TimerStructManager中添加创建当日记录的方法：

// TimerStructManager中新增方法
func getOrCreateTodayTimerStruct() -> TimerStruct {
    // 实际需根据日期判断是否存在当日记录，此处简化处理
    if let todayTimer = timerStructs.first {
        return todayTimer
    } else {
        let newTimer = TimerStruct()
        timerStructs.append(newTimer)
        return newTimer
    }
}

在HomeView中调用：

private var currentTimerStruct: TimerStruct {
    manager.getOrCreateTodayTimerStruct()
}

绑定的正确用法 #

若需修改exerciseTime，才需要使用绑定，比如向子视图传递：

TimerView(exerciseTime: $manager.timerStructs[0].exerciseTime)

内容的提问来源于stack exchange，提问作者Zudoturiku