带有.NET MVC 3 Razor 编辑器的 Html5占位符用于扩展？

You may take a look at the following article for writing a custom DataAnnotationsModelMetadataProvider.

And here's another, more ASP.NET MVC 3ish way to proceed involving the newly introduced IMetadataAware interface.

Start by creating a custom attribute implementing this interface:

public class PlaceHolderAttribute : Attribute, IMetadataAware
{
private readonly string _placeholder;
public PlaceHolderAttribute(string placeholder)
{
_placeholder = placeholder;
}


public void OnMetadataCreated(ModelMetadata metadata)
{
metadata.AdditionalValues["placeholder"] = _placeholder;
}
}

And then decorate your model with it:

public class MyViewModel
{
[PlaceHolder("Enter title here")]
public string Title { get; set; }
}

Next define a controller:

public class HomeController : Controller
{
public ActionResult Index()
{
return View(new MyViewModel());
}
}

A corresponding view:

@model MyViewModel
@using (Html.BeginForm())
{
@Html.EditorFor(x => x.Title)
<input type="submit" value="OK" />
}

And finally the editor template (~/Views/Shared/EditorTemplates/string.cshtml):

@{
var placeholder = string.Empty;
if (ViewData.ModelMetadata.AdditionalValues.ContainsKey("placeholder"))
{
placeholder = ViewData.ModelMetadata.AdditionalValues["placeholder"] as string;
}
}
<span>
@Html.Label(ViewData.ModelMetadata.PropertyName)
@Html.TextBox("", ViewData.TemplateInfo.FormattedModelValue, new { placeholder = placeholder })
</span>

As smnbss comments in Darin Dimitrov's answer, Prompt exists for exactly this purpose, so there is no need to create a custom attribute. From the the documentation:

Gets or sets a value that will be used to set the watermark for prompts in the UI.

To use it, just decorate your view model's property like so:

[Display(Prompt = "numbers only")]
public int Age { get; set; }

This text is then conveniently placed in ModelMetadata.Watermark. Out of the box, the default template in MVC 3 ignores the Watermark property, but making it work is really simple. All you need to do is tweaking the default string template, to tell MVC how to render it. Just edit String.cshtml, like Darin does, except that rather than getting the watermark from ModelMetadata.AdditionalValues, you get it straight from ModelMetadata.Watermark:

~/Views/Shared/EditorTemplates/String.cshtml:

@Html.TextBox("", ViewData.TemplateInfo.FormattedModelValue, new { @class = "text-box single-line", placeholder = ViewData.ModelMetadata.Watermark })

And that is it.

As you can see, the key to make everything work is the placeholder = ViewData.ModelMetadata.Watermark bit.

If you also want to enable watermarking for multi-line textboxes (textareas), you do the same for MultilineText.cshtml:

~/Views/Shared/EditorTemplates/MultilineText.cshtml:

@Html.TextArea("", ViewData.TemplateInfo.FormattedModelValue.ToString(), 0, 0, new { @class = "text-box multi-line", placeholder = ViewData.ModelMetadata.Watermark })

I actually prefer to use the display name for the placeholder text majority of the time. Here is an example of using the DisplayName:

  @Html.TextBoxFor(x => x.FirstName, true, null, new { @class = "form-control", placeholder = Html.DisplayNameFor(x => x.FirstName) })

I've wrote such a simple class:

public static class WatermarkExtension
{
public static MvcHtmlString WatermarkFor<TModel, TValue>(this HtmlHelper<TModel> html, Expression<Func<TModel, TValue>> expression)
{
var watermark = ModelMetadata.FromLambdaExpression(expression, html.ViewData).Watermark;
var htmlEncoded = HttpUtility.HtmlEncode(watermark);
return new MvcHtmlString(htmlEncoded);
}
}

The usage as such:

@Html.TextBoxFor(model => model.AddressSuffix, new {placeholder = Html.WatermarkFor(model => model.AddressSuffix)})

And property in a viewmodel:

[Display(ResourceType = typeof (Resources), Name = "AddressSuffixLabel", Prompt = "AddressSuffixPlaceholder")]
public string AddressSuffix
{
get { return _album.AddressSuffix; }
set { _album.AddressSuffix = value; }
}

Notice Prompt parameter. In this case I use strings from resources for localization but you can use just strings, just avoid ResourceType parameter.

I use this way with Resource file (don't need Prompt anymore !)

@Html.TextBoxFor(m => m.Name, new
{
@class = "form-control",
placeholder = @Html.DisplayName(@Resource.PleaseTypeName),
autofocus = "autofocus",
required = "required"
})

Here is a solution I made using the above ideas that can be used for TextBoxFor and PasswordFor:

public static class HtmlHelperEx
{
public static MvcHtmlString TextBoxWithPlaceholderFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper,
Expression<Func<TModel, TProperty>> expression, object htmlAttributes)
{
var metadata = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);
return htmlHelper.TextBoxFor(expression, htmlAttributes.AddAttribute("placeholder", metadata.Watermark));


}


public static MvcHtmlString PasswordWithPlaceholderFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper,
Expression<Func<TModel, TProperty>> expression, object htmlAttributes)
{
var metadata = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);
return htmlHelper.PasswordFor(expression, htmlAttributes.AddAttribute("placeholder", metadata.Watermark));


}
}


public static class HtmlAttributesHelper
{
public static IDictionary<string, object> AddAttribute(this object htmlAttributes, string name, object value)
{
var dictionary = htmlAttributes == null ? new Dictionary<string, object>() : htmlAttributes.ToDictionary();
if (!String.IsNullOrWhiteSpace(name) && value != null && !String.IsNullOrWhiteSpace(value.ToString()))
dictionary.Add(name, value);
return dictionary;
}


public static IDictionary<string, object> ToDictionary(this object obj)
{
return TypeDescriptor.GetProperties(obj)
.Cast<PropertyDescriptor>()
.ToDictionary(property => property.Name, property => property.GetValue(obj));
}
}

I think create a custom EditorTemplate is not good solution, beause you need to care about many possible tepmlates for different cases: strings, numsers, comboboxes and so on. Other solution is custom extention to HtmlHelper.

Model:

public class MyViewModel
{
[PlaceHolder("Enter title here")]
public string Title { get; set; }
}

Html helper extension:

   public static MvcHtmlString BsEditorFor<TModel, TValue>(this HtmlHelper<TModel> htmlHelper,
Expression<Func<TModel, TValue>> expression, string htmlClass = "")
{
var modelMetadata = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);
var metadata = modelMetadata;


var viewData = new
{
HtmlAttributes = new
{
@class = htmlClass,
placeholder = metadata.Watermark,
}
};
return htmlHelper.EditorFor(expression, viewData);

}

A corresponding view:

@Html.BsEditorFor(x => x.Title)