Prism Forms移除OnNavigatingTo后，如何在View构造前执行代码？

针对你遇到的Prism移除OnNavigatingTo后，Promotion属性未初始化导致CanEditPromotion方法崩溃的问题，这里有几个可行的解决方案：

方案1：使用Prism的ObservableCommand自动处理CanExecute评估 #

Prism提供的ObservableCommand（替代DelegateCommand）能自动监听依赖属性的变化，当Promotion赋值完成后，它会自动重新执行CanEditPromotion方法，无需手动触发，非常适配这类场景。

修改后的ViewModel代码如下：

public ObservableCommand DeletePromotionCommand { get; set; }
public ObservableCommand EditPromotionCommand { get; set; }

public PromotionDetailViewModel(INavigationService navigationService, IPageDialogService pageDialogService) : base(navigationService, pageDialogService)
{
    Title = "Promoção";
    // 用ObservableCommand替代DelegateCommand，自动跟踪属性变化
    DeletePromotionCommand = new ObservableCommand(DeletePromotion, CanDeletePromotion);
    EditPromotionCommand = new ObservableCommand(EditPromotion, CanEditPromotion);
}

private bool CanEditPromotion()
{
    // 先处理Promotion为null的边界情况
    if (Promotion == null) return false;
    
    var userString = Preferences.Get("user", string.Empty);
    if (userString == string.Empty) return false;
    var userId = (Guid)JObject.Parse(userString)["id"];
    return userId == Promotion.CreatedBy;
}

// 确保Promotion是Observable属性，触发PropertyChanged通知
private Promotion _promotion;
public Promotion Promotion
{
    get => _promotion;
    set => SetProperty(ref _promotion, value);
}

// 在IInitialize的Initialize方法中完成参数接收与属性赋值
public override async void Initialize(INavigationParameters parameters)
{
    base.Initialize(parameters);
    try
    {
        IsBusy = true;
        Promotion = parameters["promotion"] as Promotion;
        var marketService = new Service<Market>();
        Market = await marketService.GetAsync(Promotion.MarketId);
    }
    catch (Exception)
    {
        // 这里可以添加具体的异常处理逻辑，比如弹窗提示
    }
    finally
    {
        IsBusy = false;
    }
}

方案2：延迟初始化DelegateCommand #

如果不想更换命令类型，可以把DelegateCommand的初始化延迟到Promotion赋值完成之后，比如放在Initialize方法里：

public DelegateCommand DeletePromotionCommand { get; set; }
public DelegateCommand EditPromotionCommand { get; set; }

public PromotionDetailViewModel(INavigationService navigationService, IPageDialogService pageDialogService) : base(navigationService, pageDialogService)
{
    Title = "Promoção";
    // 构造函数中暂不初始化命令
}

public override async void Initialize(INavigationParameters parameters)
{
    base.Initialize(parameters);
    try
    {
        IsBusy = true;
        Promotion = parameters["promotion"] as Promotion;
        var marketService = new Service<Market>();
        Market = await marketService.GetAsync(Promotion.MarketId);
        
        // 此时Promotion已赋值，再初始化命令
        DeletePromotionCommand = new DelegateCommand(DeletePromotion, CanDeletePromotion);
        EditPromotionCommand = new DelegateCommand(EditPromotion, CanEditPromotion);
    }
    catch (Exception)
    {
        // 异常处理逻辑
    }
    finally
    {
        IsBusy = false;
    }
}

方案3：手动触发CanExecute重新评估 #

保持DelegateCommand在构造函数中初始化，但修改CanEditPromotion处理Promotion为null的情况，然后在Promotion赋值后手动调用命令的RaiseCanExecuteChanged方法：

public DelegateCommand DeletePromotionCommand { get; set; }
public DelegateCommand EditPromotionCommand { get; set; }

public PromotionDetailViewModel(INavigationService navigationService, IPageDialogService pageDialogService) : base(navigationService, pageDialogService)
{
    Title = "Promoção";
    DeletePromotionCommand = new DelegateCommand(DeletePromotion, CanDeletePromotion);
    EditPromotionCommand = new DelegateCommand(EditPromotion, CanEditPromotion);
}

private bool CanEditPromotion()
{
    if (Promotion == null) return false; // 处理null边界
    
    var userString = Preferences.Get("user", string.Empty);
    if (userString == string.Empty) return false;
    var userId = (Guid)JObject.Parse(userString)["id"];
    return userId == Promotion.CreatedBy;
}

private Promotion _promotion;
public Promotion Promotion
{
    get => _promotion;
    set
    {
        if (SetProperty(ref _promotion, value))
        {
            // 赋值完成后手动触发命令的CanExecute重新评估
            EditPromotionCommand?.RaiseCanExecuteChanged();
            DeletePromotionCommand?.RaiseCanExecuteChanged();
        }
    }
}

public override async void Initialize(INavigationParameters parameters)
{
    base.Initialize(parameters);
    try
    {
        IsBusy = true;
        Promotion = parameters["promotion"] as Promotion;
        var marketService = new Service<Market>();
        Market = await marketService.GetAsync(Promotion.MarketId);
    }
    catch (Exception)
    {
        // 异常处理逻辑
    }
    finally
    {
        IsBusy = false;
    }
}

推荐优先使用方案1，因为ObservableCommand是Prism专门为这类依赖属性变化的场景设计的，能减少手动代码，更贴合MVVM的设计规范。

内容的提问来源于stack exchange，提问作者Sérgio Damasceno