[译]【NetCore学习笔记之Anchor TagHelper】ASP.NET Core MVC Anchor Tag Helper

Anchor Tag Helper，主要是用于构建一个href属性的链接，包含一些对controller、action或者mvc route的处理，它是Mvc6中新增的功能，在NetCore中也是很方便的使用的。

先看看源码定义：

[HtmlTargetElement("a", Attributes = "asp-action")]
[HtmlTargetElement("a", Attributes = "asp-controller")]
[HtmlTargetElement("a", Attributes = "asp-area")]
[HtmlTargetElement("a", Attributes = "asp-fragment")]
[HtmlTargetElement("a", Attributes = "asp-host")]
[HtmlTargetElement("a", Attributes = "asp-protocol")]
[HtmlTargetElement("a", Attributes = "asp-route")]
[HtmlTargetElement("a", Attributes = "asp-all-route-data")]
[HtmlTargetElement("a", Attributes = "asp-route-*")]
public class AnchorTagHelper : TagHelper
{
/// <summary>
/// Creates a new <see cref="T:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper" />.
/// </summary>
/// <param name="generator">The <see cref="T:Microsoft.AspNetCore.Mvc.ViewFeatures.IHtmlGenerator" />.</param>
public AnchorTagHelper(IHtmlGenerator generator);
/// <inheritdoc />
/// <remarks>Does nothing if user provides an <c>href</c> attribute.</remarks>
/// <exception cref="T:System.InvalidOperationException">
/// Thrown if <c>href</c> attribute is provided and <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Action" />, <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Controller" />,
/// <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Fragment" />, <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Host" />, <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Protocol" />, or <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Route" /> are
/// non-<c>null</c> or if the user provided <c>asp-route-*</c> attributes. Also thrown if <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Route" />
/// and one or both of <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Action" /> and <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Controller" /> are non-<c>null</c>.
/// </exception>
public override void Process(TagHelperContext context, TagHelperOutput output);
/// <inheritdoc />
public override int Order { get; }
protected IHtmlGenerator Generator { get; }
/// <summary>The name of the action method.</summary>
/// <remarks>Must be <c>null</c> if <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Route" /> is non-<c>null</c>.</remarks>
[HtmlAttributeName("asp-action")]
public string Action { get; set; }
/// <summary>The name of the controller.</summary>
/// <remarks>Must be <c>null</c> if <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Route" /> is non-<c>null</c>.</remarks>
[HtmlAttributeName("asp-controller")]
public string Controller { get; set; }
/// <summary>The name of the area.</summary>
/// <remarks>Must be <c>null</c> if <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Route" /> is non-<c>null</c>.</remarks>
[HtmlAttributeName("asp-area")]
public string Area { get; set; }
/// <summary>The protocol for the URL, such as "http" or "https".</summary>
[HtmlAttributeName("asp-protocol")]
public string Protocol { get; set; }
/// <summary>The host name.</summary>
[HtmlAttributeName("asp-host")]
public string Host { get; set; }
/// <summary>The URL fragment name.</summary>
[HtmlAttributeName("asp-fragment")]
public string Fragment { get; set; }
/// <summary>Name of the route.</summary>
/// <remarks>
/// Must be <c>null</c> if <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Action" /> or <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Controller" /> is non-<c>null</c>.
/// </remarks>
[HtmlAttributeName("asp-route")]
public string Route { get; set; }
/// <summary>Additional parameters for the route.</summary>
[HtmlAttributeName("asp-all-route-data", DictionaryAttributePrefix = "asp-route-")]
public IDictionary<string, string> RouteValues { get; set; }
/// <summary>
/// Gets or sets the <see cref="T:Microsoft.AspNetCore.Mvc.Rendering.ViewContext" /> for the current request.
/// </summary>
[HtmlAttributeNotBound]
[ViewContext]
public ViewContext ViewContext { get; set; }
}

其实在早期的mvc中，是有类似的扩展方法的，比如@Html.ActionLink、@Url.Action 等帮助方法。使用也很简单，比如：

@Html.ActionLink("Register", "Register", "Account")

<a href="@Url.Action("Register", "Account")">Register</a>

那么，对应的生成的html代码如下：

<a href="/Account/Register">Register</a>
<a href="/Account/Register">Register</a>

如果，我们使用anchor tag helper构建同样的一个html的话，就需要用到anchor的扩展属性asp-controller,asp-action，比如：

<a asp-controller="Account" asp-action="Register">Register</a>

Adding Route Parameters

很多时候，比如你想从列表页跳转到详情页去查看或者修改，那么就需要传递一些参数用于区分是哪条数据，这个时候怎么办呢？如果用Anchor Tag Helper处理的话，就需要用到她的另外一个属性asp-route- 前缀开头的。

比如，我想编辑对应记录，我需要传递一个id参数，怎么办呢，那就可以这么用：

<a asp-action="Edit" asp-controller="Todo" asp-route-id="@item.Id">@item.Title</a>

对应的Html代码如下:

<a href="/Todo/Edit/6">Add an appsetting in config and access it in app.</a>

这里，我只是用到了id,所以就构建了一个asp-route-id属性，如果你用到了别的，依然可以自己构建，前提是必须是以“asp-route-” 开头的。

Named Routes

另外一种使用方式，就是利用MapRoute中已经命名好的路由规则处理。比如：

routes.MapRoute(
  name: "login",
template: "login",
defaults: new { controller = "Account", action = "Login" });

此时，你可以这么玩：

<a asp-route="login">Login</a>

Protocol, Host and Fragment

anchor tag helper 也可以支持其他一些属性（你可以从源码中定义的属性中可以看到），比如Protocol协议、host、fragment的分片名称。

如果你要用老版本的MVC中的ActionLink方法实现的话，可以这么玩：

@Html.ActionLink("Register", "Register", "Account",  "https", "core.todolist.com", "fragment", null, null)

对应的Html代码如下：

<a href="https://core.todolist.com/Account/Register#fragment">Register</a>

如果你要使用anchor tag helper的话，就要用到另外三个属性：asp-protocol、asp-host、asp-fragment。

<a asp-controller="Account"
asp-action="Register"
asp-protocol="https"
asp-host="core.todolist.com"
asp-fragment="fragment">Register</a>

<!--or with the protocol only -->
<a asp-controller="Account"
asp-action="Register"
asp-protocol="https">Register</a>

很明显，你会觉得这种方式相对可读性更加直观，因为你可以一目了然的看到一些属性的意思和对应的值。另外一个好处就是，可以在只预览html时，不会报错。

that's all~

参考资料：
http://www.davepaquette.com/archive/2015/06/01/mvc-6-anchor-tag-helper.aspx