为什么 Blazor 渲染组件两次

Question

我有一个简单的 Blazor 组件。

<div @onclick="HandleClick">Click me</div>

@code {

    public async Task HandleClick()
    {
        await Task.Run(()=> System.Threading.Thread.Sleep(1000));
    }

    protected override void OnAfterRender(bool firstRender)
    {
        Console.WriteLine("Rendered");
    }
}

当我单击 div 时，“Rendered”被打印到控制台，并在 1 秒后再次打印，这意味着 blazor 已渲染组件两次。我知道 Blazor 会触发组件的自动重新渲染，作为向其分派事件的一部分。

但是为什么它在任务完成后重新渲染？如何避免二次渲染？

我在 OnAfterRender 生命周期钩子中有一些 JS 互操作，现在运行两次。我可以添加某种计数器，但这会污染我的代码，我想避免这种情况。我HandleClick是一个简单的 public void 方法，然后一切正常，但这并不总是可能的

Answer 1

您可以像这样使用firstRender变量：

if(firstRender)
{
   // Call JSInterop to initialize your js functions, etc.
   // This code will execute only once in the component life cycle.
   // The variable firstRender is true only after the first rendering of the 
   // component; that is, after the component has been created and initialized.
   // Now, when you click the div element firstRender is false, but still the 
   // component is rendered twice, before the awaited method (Task.Run) is called,
   // and after the awaited method completes. The first render occurs because UI 
   // event automatically invoke the StateHasChanged method. The second render 
   // occurs also automatically after an awaited method in an async method 
   // completes. This is how Blazor works, and it shouldn't bother you. 
}