維護 ASP.NET MVC 專案遇上巢狀 Layout 引用 Partial View 的情境,無法斷定執行先後順序,想必是自己觀念不清,做了以下實驗驗證,順手分享之。

假設有 ASP.NET MVC 巢狀 Layout 並混用 Partial View 結構如下:

_Layout.cshtml

@{
    System.Diagnostics.Debug.WriteLine("_Layout.cshtml");
}
<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>@ViewBag.Title</title>
</head>
<body>

@Html.Partial("_PartView1")

@RenderBody()

</body>
</html>

_PartView1.cshtml (_PartView2.cshtml 及 _PartView3.cshtml 做法相同,只有數字不同)

@{
    System.Diagnostics.Debug.WriteLine("PartialView1");
}
<div>Partial View 1</div>

_NestedLayout.cshtml

@{
    Layout = "~/Views/Shared/_Layout.cshtml";
    System.Diagnostics.Debug.WriteLine("_NestedLayout.cshtml");
}
@Html.Partial("_PartView2")
@RenderBody()

Index.cshtml

@{
    Layout = "~/Views/Shared/_NestedLayout.cshtml";
    System.Diagnostics.Debug.WriteLine("Index.cshtml");
}

@Html.Partial("_PartView3")

<h2>Index</h2>

HomeController.cs

    public class HomeController : Controller
    {
        // GET: Home
        public ActionResult Index()
        {
            System.Diagnostics.Debug.WriteLine("Index Action");
            return View();
        }
    }

執行結果不難預期:

問題來了,HomeController.cs、Index.cshtml、_PartView1.cshtml、_PartView2.cshtml、_PartView3.cshtml、_NestedLayout.cshtml、_Layout.cshtml 都埋了 System.Diagnostics.Debug.WriteLine(),將以什麼順序執行?

給大家 20 秒自我測驗。

答案揭曉:

HomeController Index Action –> Index.cshtml –> Partial View 3 –> _NestedLayout –> Partial View 2 -> _Layout –> Partial View 1

這順序不難理解,基本上就是從 HomeController.cs 開始,從 Index.cshtml、_NestedLayout.cshtml 到 _Layout.cshtml,由內而外的順序將 Razor View 轉為 HTML,生成 HTML 過程才載入 Partial View。

(原想找到官方文件證實,搜索未獲,十方大德如有知悉懇請不吝補充)


Comments

# by 布萊恩

https://docs.microsoft.com/zh-tw/aspnet/web-pages/overview/ui-layouts-and-themes/3-creating-a-consistent-look 可供參考

# by Jeffrey

to 布萊恩, 感謝補充。

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