这个时间这部分真的太难了。再练习一遍。增强熟练度。

时间选择器样式

时间选择器隐藏文本标签，时间选择器只显示小时和分钟。

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DatePicker(selection: $wakeUp, displayedComponents: .hourAndMinute, label: { Text("Please enter a date") })
                     .labelsHidden()
                     .datePickerStyle(WheelDatePickerStyle())

读取和写入时间的特定部分

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var components = DateComponents()
components.hour = 8
components.minute = 0
let date = Calendar.current.date(from: components)

有可能出错的情况下使用 do catch 结构抓取错误

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do {
                let sleepTime = try model.prediction(wake: time, estimatedSleep: Double(self.sleepTimeLength), coffee: Double(self.coffeeCups))
                
                let bestSleepTime = self.wakeDate - sleepTime.actualSleep
                
                let formatter = DateFormatter()
                formatter.timeStyle = .short
                let dateString = formatter.string(from: bestSleepTime)
                
                self.alertTitle = "你的睡眠时间应该在"
                self.alertMessage = "\(dateString)"
                self.alertShow.toggle()
            }catch{
                self.alertTitle = "出现了未知错误"
                self.alertMessage = "无法计算出推荐睡眠时间"
                self.alertShow.toggle()

第二遍练习代码

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//
//  ContentView.swift
//  Practise2-4for2
//
//  Created by 张洪Hoo on 2020/4/3.
//  Copyright © 2020 张洪Hoo. All rights reserved.
//

import SwiftUI

struct ContentView: View {
    @State private var wakeDate = compDate
    @State private var sleepTimeLength: Double = 8
    @State private var coffeeCups = 1
    
    @State private var alertShow = false
    @State private var alertTitle = ""
    @State private var alertMessage = ""
    
    static var compDate: Date {
        var com = DateComponents()
        
        com.hour = 8
        com.minute = 0
        
        return Calendar.current.date(from: com) ?? Date()
    }
    
    var body: some View {
        NavigationView {
            Form {
                VStack(alignment: .leading) {
                    Text("你什么时候想起床？")
                        .bold()
                    
                    DatePicker(selection: $wakeDate,displayedComponents: .hourAndMinute, label: { Text("你什么时候想起床？") })
                    .labelsHidden()
                    .datePickerStyle(WheelDatePickerStyle())
                }
                
                VStack(alignment: .leading) {
                    Text("期望睡眠时长")
                        .bold()
                    Stepper(value: $sleepTimeLength, in: 4...12, step: 0.25) {
                        Text("\(self.sleepTimeLength, specifier: "%.2f") 小时")
                    }
                }
                
                VStack(alignment: .leading) {
                    Text("每日饮用咖啡杯数")
                        .bold()
                    Stepper(value: $coffeeCups, in: 1...20) {
                        Text("\(self.coffeeCups) 杯")
                    }
                }
            }
        .navigationBarTitle(Text("最佳睡眠时间计算"))
        .navigationBarItems(trailing: Button(action: {
            //do something
            
            let model = SleepCalculator()
            
            let wakeTime = Calendar.current.dateComponents([.hour,.minute], from: self.wakeDate)
            
            let hour = (wakeTime.hour ?? 0) * 60 * 60
            
            let minute = (wakeTime.minute ?? 0) * 60
            
            let time = Double(hour + minute)
            
            do {
                let sleepTime = try model.prediction(wake: time, estimatedSleep: Double(self.sleepTimeLength), coffee: Double(self.coffeeCups))
                
                let bestSleepTime = self.wakeDate - sleepTime.actualSleep
                
                let formatter = DateFormatter()
                formatter.timeStyle = .short
                let dateString = formatter.string(from: bestSleepTime)
                
                self.alertTitle = "你的睡眠时间应该在"
                self.alertMessage = "\(dateString)"
                self.alertShow.toggle()
            }catch{
                self.alertTitle = "出现了未知错误"
                self.alertMessage = "无法计算出推荐睡眠时间"
                self.alertShow.toggle()
            }
        }) {
        Text("计算")
        })
        }
        .alert(isPresented: $alertShow){
            Alert(---
title: Text(alertTitle), message: Text(alertMessage), dismissButton: .default(Text("确定")))
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

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SwiftPlayground 进度

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