在 Timer 内启动 Viewmodel 不起作用答案

【问题标题】：Launching a Viewmodel inside a Timer elapsed not working在 Timer 内启动 Viewmodel 不起作用
【发布时间】：2019-05-29 14:38:26
【问题描述】：

我正在尝试在计时器事件中启动另一个视图模型。当我跳过这个计时器事件时，似乎启动器使用得很好。但是，如果启动是在活动期间完成的，则不会。谁能看看问题出在哪里？

using System.Timers;
using Caliburn.Micro.Xamarin.Forms;
using MyProject.Resources;
using System.Windows.Input;
using Xamarin.Forms;
using System.Diagnostics;

namespace MyProject.ViewModels
{
    public class PairingDeviceViewModel : BaseScreen {
    public INavigationService NavigationService { get; set; }

    private Timer timer;
    private const float MAX = 20;
    private string percentage;
    private float rate;

    public float Rate { get => rate; set => Set(ref rate, value); }
    public string Percentage { get => percentage; set => Set(ref percentage, value); }


    protected override void OnInitialize()
    {
        base.OnInitialize();

        timer = new Timer
        {
            Interval = MAX,
        };
    }

    protected override void OnActivate()
    {
        base.OnActivate();
        timer.Elapsed += Timer_Elapsed;
        timer.Start();
    }

    protected override void OnDeactivate(bool close)
    {
        base.OnDeactivate(close);
        timer.Elapsed -= Timer_Elapsed;
    }

    private void Timer_Elapsed(object sender, ElapsedEventArgs e)
    {
        Debug.WriteLine($"Timer_Elapsed");
        Debug.WriteLine($"Rate {Rate}");
        Rate++;
        Percentage = string.Format(StringResources.PercentageNumber, Rate);
        NotifyOfPropertyChange(() => Percentage);
        if (Rate == MAX)
        {
            timer.Stop();
            Debug.WriteLine($"navigation begins");
            NavigationService.NavigateToViewModelAsync<ConfigurationListViewModel>();
        }
        Debug.WriteLine($"Timer_Elapsed END");
    }

    public ICommand SkipBluetoothScanning => new Command(() => {
        timer.Stop();
        NavigationService.NavigateToViewModelAsync<ConfigurationListViewModel>();
    });
}

所以在我的Timer_elapsed 方法中，一旦Rate 等于MAX，我就会调用NavigationService.NavigateToViewModelAsync<>..。这个问题是这个条件Rate==MAX 被多次调用。

请查看日志文件。

Timer_Elapsed
Timer_Elapsed
Timer_Elapsed
Timer_Elapsed
Timer_Elapsed
Timer_Elapsed
Timer_Elapsed
Timer_Elapsed
Timer_Elapsed
Timer_Elapsed
Timer_Elapsed
Timer_Elapsed
Rate 0
Rate 0
Rate 0
Rate 0
Rate 0
Rate 0
Rate 0
Rate 0
Rate 0
Rate 0
Rate 0
Timer_Elapsed END
Timer_Elapsed
Rate 11
Timer_Elapsed END
Timer_Elapsed
Rate 12
Timer_Elapsed END
Timer_Elapsed
Rate 13
Timer_Elapsed END
Timer_Elapsed
Rate 14
Timer_Elapsed END
Timer_Elapsed
Rate 15
Timer_Elapsed END
Timer_Elapsed
Rate 16
Timer_Elapsed END
Timer_Elapsed
Timer_Elapsed END
Rate 17
Timer_Elapsed
Rate 18
Timer_Elapsed END
Timer_Elapsed
Rate 19
navigation begins
navigation begins
navigation begins
navigation begins
navigation begins
navigation begins
navigation begins
navigation begins
navigation begins
navigation begins
navigation begins
navigation begins
Timer_Elapsed END
[InputMethodManager] HSIFW - flag : 0 Pid : 6116
[InputMethodManager] HSIFW - flag : 0 Pid : 6116
Timer_Elapsed END
Timer_Elapsed END
Timer_Elapsed END
Timer_Elapsed END
Timer_Elapsed END
Timer_Elapsed END
[InputMethodManager] HSIFW - flag : 0 Pid : 6116
[InputMethodManager] HSIFW - flag : 0 Pid : 6116
Timer_Elapsed END
Timer_Elapsed END
[InputMethodManager] HSIFW - flag : 0 Pid : 6116
Timer_Elapsed END
Timer_Elapsed END
[InputMethodManager] HSIFW - flag : 0 Pid : 6116
Timer_Elapsed END

DispatcherTimer 是针对此功能或其他功能的更好解决方案吗？

非常感谢。

【问题讨论】：

首先，将浮点数与“==”进行比较是有风险的。如果要增加 1，最好使用整数。还有，timer的默认间隔是100ms，这真的是你需要的粒度吗？
我可以增加 timer.interval。对我来说应该没问题。由于 Timer.Interval 是一个浮点数，这也是我使用 Rate 作为浮点数的原因。我遇到的问题是条件 `if(Rate == Max) 被多次调用。
stackoverflow.com/questions/1530069/…
谢谢杰森。这是不调用 NavigateToViewModelAsync 的唯一可能原因吗？
没有。您可能需要使用 BeginInvokeOnMainThread()

标签： c# visual-studio xamarin xamarin.forms caliburn.micro

【解决方案1】：

您可以像这样锁定线程以避免重复执行：

private void Timer_Elapsed(object sender, ElapsedEventArgs e)
  {
     lock (this)
        {
         if (Math.Abs(Rate - MAX) < double.Epsilon){
             //something you want to do
               ...
            }
        }
  }

【讨论】：

谢谢，我试试看。