Blazor Webassembly - 为什么在 TimerOnElapsed 上调用函数？

Question

我正在使用 Blazor Webassembly 模板，特别是天气预报页面。

我只是添加了一个计时器并有一个增加变量的函数，该函数显示在页面上。我没有得到的是， addText 函数从未被专门调用，但变量随着计时器的经过而不断增加。所以我不明白这里发生了什么。

有人可以启发我吗？ html 区域内的所有函数都在 StateHasChanged() 上调用吗？如果我将 html 中的行更改为 '@((MarkupString)sometext.ToString())'，它不会增加。代码如下：

<h1>Weather forecast</h1>

<p>This component demonstrates fetching data from the server.</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>
    <br />

    @((MarkupString)addText())
}

@code {
    private WeatherForecast[] forecasts;

    private int sometext = 0;
    Timer timer;

    protected override async Task OnInitializedAsync()
    {
        timer = new Timer();
        timer.Interval = 1000;
        timer.Elapsed += TimerOnElapsed;

        timer.Start();

        forecasts = await Http.GetFromJsonAsync<WeatherForecast[]>("WeatherForecast");
    }

    private void TimerOnElapsed(object sender, ElapsedEventArgs e)
    {
        StateHasChanged();
    }

    private string addText()
    {
        return sometext++.ToString();
    }
}

Answer 1

StateHaseChanged 正在强制渲染。渲染正在调用函数...标记区域被编译为渲染时调用的委托。您的函数只是一个子程序调用。您正在计算渲染调用。

<h1>Weather forecast</h1>

<p>This component demonstrates fetching data from the server.</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>
    <br />

    @sometext 
}

@code {
    private WeatherForecast[] forecasts;

    private int sometext = 0;
    Timer timer;

    protected override async Task OnInitializedAsync()
    {
        timer = new Timer();
        timer.Interval = 1000;
        timer.Elapsed += TimerOnElapsed;

        timer.Start();

        forecasts = await Http.GetFromJsonAsync<WeatherForecast[]>("WeatherForecast");
    }

    private void TimerOnElapsed(object sender, ElapsedEventArgs e)
    {
        sometext++;
        StateHasChanged(); // <- probably not needed not sure
    }   
}