ASP.Net Core-TagHelpers

当我们新建了一个.Net Core类型的Project时，我们会看到页面上有类似于这样的代码：

当我们运行项目，查看源代码会发现，浏览器中的就是Html代：

那么，为什么我们在页面写的代码会转化为html标签呢？接下来我们就来探索这个秘密。

当我们在vs中把鼠标放到这样的元素上时发现这样的元素都是一个Microsoft.AspNetCore.Mvc.TagHelpers命名空间下的对象，我们通过Reflector查看a标签的对象AnchorTagHelper

[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
{
    // Fields
    private IDictionary<string, string> _routeValues;
    private const string ActionAttributeName = "asp-action";
    private const string AreaAttributeName = "asp-area";
    private const string ControllerAttributeName = "asp-controller";
    private const string FragmentAttributeName = "asp-fragment";
    private const string HostAttributeName = "asp-host";
    private const string Href = "href";
    private const string ProtocolAttributeName = "asp-protocol";
    private const string RouteAttributeName = "asp-route";
    private const string RouteValuesDictionaryName = "asp-all-route-data";
    private const string RouteValuesPrefix = "asp-route-";

    // Methods
    public AnchorTagHelper(IHtmlGenerator generator);
    public override void Process(TagHelperContext context, TagHelperOutput output);

    // Properties
    [HtmlAttributeName("asp-action")]
    public string Action { get; set; }
    [HtmlAttributeName("asp-area")]
    public string Area { get; set; }
    [HtmlAttributeName("asp-controller")]
    public string Controller { get; set; }
    [HtmlAttributeName("asp-fragment")]
    public string Fragment { get; set; }
    protected IHtmlGenerator Generator { [CompilerGenerated] get; }
    [HtmlAttributeName("asp-host")]
    public string Host { get; set; }
    public override int Order { get; }
    [HtmlAttributeName("asp-protocol")]
    public string Protocol { get; set; }
    [HtmlAttributeName("asp-route")]
    public string Route { get; set; }
    [HtmlAttributeName("asp-all-route-data", DictionaryAttributePrefix="asp-route-")]
    public IDictionary<string, string> RouteValues { get; set; }
    [HtmlAttributeNotBound, ViewContext]
    public ViewContext ViewContext { get; set; }
}

首先，这个类继承自抽象类TagHelper

public abstract class TagHelper : ITagHelper
{
    // Methods
    protected TagHelper();
    public virtual void Init(TagHelperContext context);
    public virtual void Process(TagHelperContext context, TagHelperOutput output);
    public virtual Task ProcessAsync(TagHelperContext context, TagHelperOutput output);

    // Properties
    public virtual int Order { [CompilerGenerated] get; }
}

TagHelper又继承自ITagHelper接口

public interface ITagHelper
{
    // Methods
    void Init(TagHelperContext context);
    Task ProcessAsync(TagHelperContext context, TagHelperOutput output);

    // Properties
    int Order { get; }
}

这个接口只有两个方法，Init和ProcessAsync，我们也许会猜到，转化的过程就是通过这两个方法来转化的。没错，你猜对了。

我们回到AnchorTagHelper类中，查看Process方法：

public override void Process(TagHelperContext context, TagHelperOutput output)
{
    if (context == null)
    {
        throw new ArgumentNullException("context");
    }
    if (output == null)
    {
        throw new ArgumentNullException("output");
    }
    if (output.Attributes.ContainsName("href"))
    {
        if ((((this.Action != null) || (this.Controller != null)) || ((this.Area != null) || (this.Route != null))) || (((this.Protocol != null) || (this.Host != null)) || ((this.Fragment != null) || (this.RouteValues.Count != ))))
        {
            throw new InvalidOperationException(Resources.FormatAnchorTagHelper_CannotOverrideHref("<a>", "asp-action", "asp-controller", "asp-area", "asp-route", "asp-protocol", "asp-host", "asp-fragment", "asp-route-", "href"));
        }
    }
    else
    {
        TagBuilder builder;
        IDictionary<string, object> routeValues = null;
        if ((this._routeValues != null) && (this._routeValues.Count > ))
        {
            routeValues = new Dictionary<string, object>(this._routeValues.Count, StringComparer.OrdinalIgnoreCase);
            foreach (KeyValuePair<string, string> pair in this._routeValues)
            {
                routeValues.Add(pair.Key, pair.Value);
            }
        }
        if (this.Area != null)
        {
            if (routeValues == null)
            {
                routeValues = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
            }
            routeValues["area"] = this.Area;
        }
        if (this.Route == null)
        {
            builder = this.Generator.GenerateActionLink(this.ViewContext, string.Empty, this.Action, this.Controller, this.Protocol, this.Host, this.Fragment, routeValues, null);
        }
        else
        {
            if ((this.Action != null) || (this.Controller != null))
            {
                throw new InvalidOperationException(Resources.FormatAnchorTagHelper_CannotDetermineHrefRouteActionOrControllerSpecified("<a>", "asp-route", "asp-action", "asp-controller", "href"));
            }
            builder = this.Generator.GenerateRouteLink(this.ViewContext, string.Empty, this.Route, this.Protocol, this.Host, this.Fragment, routeValues, null);
        }
        if (builder != null)
        {
            output.MergeAttributes(builder);
        }
    }
}

，发现这个方法主要就是根据我们填写的属性、ViewContext（View上下文）、routeValue（路由信息）来生成Html标签，主要方法就是在GenerateRouteLink生成一个TagBuilder类型的对象，再通过output.MergeAttributes(builder);生成TagHelperOutput类型的对象。

那么，我们在页面上写的属性是怎么与AnchorTagHelper中的属性关联的呢？

我们可以看这个类中的属性，每个属性前面都有一个[HtmlAttributeName("asp-action")] ，标记这个属性和我们页面上写的asp-**属性关联的。

综上，如果我们不清楚具体的某个TagHelper有哪些属性，我们可以通过反编译查看，当然没必要。我们只要通过VS直接通过智能感知系统就可以点出来这些属性。

我们还可以自定义自己的Tag，只要继承自ITagHelper,实现方法就可以

public class WebsiteInformationTagHelper : TagHelper
{
    public WebsiteContext Info { get; set; }

    public override void Process(TagHelperContext context, TagHelperOutput output)
    {
        output.TagName = "section";
        output.PostContent.SetContent(string.Format(
            "<p><strong>Version:</strong> {0}</p>" + Environment.NewLine +
            "<p><strong>Copyright Year:</strong> {1}</p>" + Environment.NewLine +
            "<p><strong>Approved:</strong> {2}</p>" + Environment.NewLine +
            "<p><strong>Number of tags to show:</strong> {3}</p>" + Environment.NewLine,
            Info.Version.ToString(),
            Info.CopyrightYear.ToString(),
            Info.Approved.ToString(),
            Info.TagsToShow.ToString()));
        output.SelfClosing = false;
    }
}

<website-information info="new WebsiteContext {
                                Version = new Version(1, 1),
                                CopyrightYear = 1990,
                                Approved = true,
                                TagsToShow = 30 }"/>