Xamarin使用MVVM Helpers时XAML中Label文本属性变更不更新问题

问题原因 #

1. 模型类未实现属性变更通知
   你当前的LevelProgress是普通POCO类，仅外层的use属性在整体替换时会触发变更通知，嵌套属性（如use.currentlevel、use.muscularprogress）修改时不会向绑定发送通知。你手动修改XAML绑定表达式再还原会刷新，是因为XAML热重载触发了重新绑定，直接读取了当前最新值，不属于运行时自动变更通知的正常流程。
2. 后台线程更新绑定源
   你在ViewModel构造函数中用Task.Run开启后台线程执行Loadup方法，在非UI线程更新绑定源属性时，Xamarin.Forms可能不会触发UI刷新。
3. 首次启动空值逻辑缺陷
   首次启动数据库为空时，GetProgress会返回null，直接访问use.currentlevel会触发空引用异常，你当前的判断逻辑无法覆盖空值场景。

解决方案 #

第一步：修改LevelProgress模型类 #

因为你已经引入了MvvmHelpers库，直接让LevelProgress继承ObservableObject，将所有字段改为带变更通知的属性：

using MvvmHelpers;
using SQLite;

namespace INWORK.Models
{
    public class LevelProgress : ObservableObject
    {
        [PrimaryKey, AutoIncrement]
        public int Id { get; set; }

        private int _currentlevel;
        public int currentlevel
        {
            get => _currentlevel;
            set => SetProperty(ref _currentlevel, value);
        }

        private bool _pushups;
        public bool pushups
        {
            get => _pushups;
            set => SetProperty(ref _pushups, value);
        }

        private bool _squats;
        public bool squats
        {
            get => _squats;
            set => SetProperty(ref _squats, value);
        }

        private bool _pullups;
        public bool pullups
        {
            get => _pullups;
            set => SetProperty(ref _pullups, value);
        }

        private bool _splitsquats;
        public bool splitsquats
        {
            get => _splitsquats;
            set => SetProperty(ref _splitsquats, value);
        }

        private bool _stepups;
        public bool stepups
        {
            get => _stepups;
            set => SetProperty(ref _stepups, value);
        }

        private bool _tricepdips;
        public bool tricepdips
        {
            get => _tricepdips;
            set => SetProperty(ref _tricepdips, value);
        }

        private bool _legraises;
        public bool legraises
        {
            get => _legraises;
            set => SetProperty(ref _legraises, value);
        }

        private bool _running;
        public bool running
        {
            get => _running;
            set => SetProperty(ref _running, value);
        }

        private bool _intervals;
        public bool intervals
        {
            get => _intervals;
            set => SetProperty(ref _intervals, value);
        }

        private double _muscularprogress;
        public double muscularprogress
        {
            get => _muscularprogress;
            set => SetProperty(ref _muscularprogress, value);
        }

        private double _cardioprogress;
        public double cardioprogress
        {
            get => _cardioprogress;
            set => SetProperty(ref _cardioprogress, value);
        }
    }
}

第二步：修正AboutViewModel的加载逻辑 #

移除后台线程调用，确保在UI线程更新绑定源，同时补全空值判断：

public class AboutViewModel : ViewModelBase
{
    public ICommand GoInfoCommand { get; set; }
    public AboutViewModel()
    {
        Title = "About";
        OpenWebCommand = new Command(async () => await Browser.OpenAsync("https://aka.ms/xamarin-quickstart"));
        // 不要用Task.Run开后台线程，直接异步加载
        _ = Loadup();
    }

    private LevelProgress _use;
    public LevelProgress use
    {
        get => _use;
        set
        {
            _use = value;
            OnPropertyChanged();
        }
    }

    public async Task Loadup()
    {
        var temps = await DataStorage.GetProgress();
        // 补全空值判断，覆盖首次启动场景
        if (temps == null || temps.currentlevel == 0)
        {
            await DataStorage.FirstCreation();
            temps = await DataStorage.GetProgress();
        }
        // 切到UI线程赋值，确保触发UI刷新
        await Device.InvokeOnMainThreadAsync(() => 
        {
            use = temps;
        });
    }

    public ICommand OpenWebCommand { get; }
}

修改完成后运行即可看到绑定值会随数据库加载自动刷新。

内容的提问来源于stack exchange，提问作者Emils Legzdins