未在 Blazor 服务器应用程序的 Hangfire 作业中注入服务

Question

免责声明：我对 C#、ASP.NET Core 和依赖注入领域非常陌生。我从默认模板创建了一个简单的 Blazor Server 应用程序，该模板构建了一个模拟天气服务，并在表格中显示了从中获取的数据。现在我希望表格每五秒自动更新一次，为此我使用了Hangfire.AspNetCore 和Hangfire.MemoryStorage 包。所以我稍微修改了FetchData.razor 组件，看起来像这样：

@page "/fetchdata"

@using WeatherTest.Data
@using Hangfire

@inject WeatherForecastService ForecastService

<h1>Weather forecast</h1>

<p>This component demonstrates fetching data from a service.</p>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <table class="table">
        <thead>
            <tr>
                <th>Date</th>
                <th>Temp. (C)</th>
                <th>Temp. (F)</th>
                <th>Summary</th>
            </tr>
        </thead>
        <tbody>
            @foreach (var forecast in forecasts)
            {
                <tr>
                    <td>@forecast.Date.ToShortDateString()</td>
                    <td>@forecast.TemperatureC</td>
                    <td>@forecast.TemperatureF</td>
                    <td>@forecast.Summary</td>
                </tr>
            }
        </tbody>
    </table>
}

@code {
    private WeatherForecast[] forecasts;

    public async Task UpdateForecasts()
    {
      forecasts = await ForecastService.GetForecastAsync(DateTime.Now);
      BackgroundJob.Schedule(() => UpdateForecasts(), TimeSpan.FromSeconds(5));
    }

    protected override async Task OnInitializedAsync()
    {
      await UpdateForecasts();
    }
}

这是气象服务：

using System;
using System.Linq;
using System.Threading.Tasks;

namespace WeatherTest.Data
{
    public class WeatherForecastService
    {
        private static readonly string[] Summaries = new[]
        {
            "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
        };

        public Task<WeatherForecast[]> GetForecastAsync(DateTime startDate)
        {
            var rng = new Random();
            return Task.FromResult(Enumerable.Range(1, 5).Select(index => new WeatherForecast
            {
                Date = startDate.AddDays(index),
                TemperatureC = rng.Next(-20, 55),
                Summary = Summaries[rng.Next(Summaries.Length)]
            }).ToArray());
        }
    }
}

以及服务配置：

public void ConfigureServices(IServiceCollection services)
{
    services.AddRazorPages();
    services.AddServerSideBlazor();
    services.AddSingleton<WeatherForecastService>();
    services.AddHangfire(c => c.UseMemoryStorage());
    services.AddHangfireServer();
}

问题是UpdateForecasts()在OnInitializedAsync()调用时成功，但在Hangfire调用时失败，在forecasts = await ForecastService.GetForecastAsync(DateTime.Now);抛出以下异常：

System.NullReferenceException: '对象引用未设置为 对象的实例。'

WeatherTest.Pages.FetchData.ForecastService.get 返回 null。

在我看来，由于 Hangfire 工作程序在另一个线程中运行，因此 WeatherForecastService 不会被注入。我对吗？是否可以从多个线程中使用单例，或者每个线程都应该有自己的服务实例？最后，我应该如何解决这个问题？

Answer 1

这是使用标准Timer 的FetchData 组件。

几点：

1. 我添加了一个Task.Delay 来模拟慢速连接并显示页面正在刷新。
2. StateHasChanged 像 await this.InvokeAsync(StateHasChanged) 这样包装，以确保它在 UI 上下文线程上运行。
3. 实现 IDisposable，因为我们有 Timer 事件处理程序要断开连接。

@page "/fetchdata"
@implements IDisposable
<PageTitle>Weather forecast</PageTitle>

@using StackOverflow.Server.Data
@inject WeatherForecastService ForecastService

<h1>Weather forecast</h1>

<p>This component demonstrates fetching data from a service.</p>

<div class="p-2">
    <button class="btn @this.btnCss" @onclick="ToggleTimer">@this.btnText</button>
</div>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <table class="table">
        <thead>
            <tr>
                <th>Date</th>
                <th>Temp. (C)</th>
                <th>Temp. (F)</th>
                <th>Summary</th>
            </tr>
        </thead>
        <tbody>
            @foreach (var forecast in forecasts)
            {
                <tr>
                    <td>@forecast.Date.ToShortDateString()</td>
                    <td>@forecast.TemperatureC</td>
                    <td>@forecast.TemperatureF</td>
                    <td>@forecast.Summary</td>
                </tr>
            }
        </tbody>
    </table>
}

@code {
    private System.Timers.Timer aTimer = new System.Timers.Timer(2000);

    private WeatherForecast[]? forecasts;

    private string btnCss => aTimer.Enabled ? "btn-danger" : "btn-success";

        private string btnText => aTimer.Enabled ? "Stop" : "Start";

    protected override async Task OnInitializedAsync()
    {
        aTimer.Elapsed += TimerElapsed;
        aTimer.AutoReset = true;
        aTimer.Enabled = true;
        forecasts = await ForecastService.GetForecastAsync(DateTime.Now);
    }

    public async void TimerElapsed(Object? sender, System.Timers.ElapsedEventArgs e)
    {
        forecasts = null;
        await this.InvokeAsync(StateHasChanged);
        await Task.Delay(500);
        forecasts = await ForecastService.GetForecastAsync(DateTime.Now);
        await this.InvokeAsync(StateHasChanged);
    }

    public void ToggleTimer()
    {
        aTimer.Enabled = !aTimer.Enabled;
        this.InvokeAsync(StateHasChanged);
    }

    public void Dispose()
       => aTimer!.Elapsed -= TimerElapsed;
}