I'm trying to create a custom HTML helper and I would like to know how I can access the Model object without passing it as a parameter.
Thanks
If you are using strongly typed views which you should:
public static MvcHtmlString MyHelper<TModel>(this HtmlHelper<TModel> htmlHelper)
{
TModel model = htmlHelper.ViewData.Model;
return MvcHtmlString.Empty;
}
If you are not using strongly typed views which you shouldn't:
public static MvcHtmlString MyHelper(this HtmlHelper htmlHelper)
{
object model = htmlHelper.ViewData.Model;
return MvcHtmlString.Empty;
}
HTML helpers are a bad way to generate HTML programmatically. Web Forms is much better with code in a page class file and HTML markup in a separate file. Yes HTML helpers put some code in separate class files but you are calling code from your HTML page. Whats to stop you from writing code directly in your view page. MVC is supportive of lots of bad practices which you don't have to do but for some reason in Web Forms developers have to do bad practices because it is allowed. If you learn Web Forms well, you will develop maintainable and scalable web applications using modern object oriented patterns instead of procedural logic like HTML helpers.
Related
I'm trying to create custom Html Helpers in my sample MVC Project.
Here is what i've done so far:
public static MvcHtmlString ImageFor<TModel,TValue>(this HtmlHelper<TModel> htmlHelper,
Expression<Func<TModel,TValue>> expression,
string alternateText,
object htmlAttributes)
{
ModelMetadata metadata = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);
string name = ExpressionHelper.GetExpressionText(expression);
TagBuilder tagBuilder = new TagBuilder("img");
tagBuilder.MergeAttribute("src", metadata.Model.ToString());
tagBuilder.MergeAttribute("alt", alternateText);
tagBuilder.MergeAttribute("name", name);
return MvcHtmlString.Create(tagBuilder.ToString(TagRenderMode.SelfClosing));
}
In my controller I'm passing the image path to a property(imgUrl).
When i try to create a tag for it it doesnot give me the image on browser, instead it is always displaying the alternate text.
Here is the rendered markup for it.
<img name="imageUrl" alt="alernate" src="C:\Users\Administrator\Desktop\down.png"/>
While i've done the loosely typed helper for #Html.Image, it works perfect !.
What has gone wrong with my code.
I've searched alot for creating Strongly Typed Helpers, but none of the tutorials explained me what is going on when we access the strongly typed helpers.
I've also seen posts in SO, when someone post code regarding helpers, its just a piece of code without any explanation.
If anyone has a tutorial for creating strongly typed custom helpers, which gives a breif of please share the link.
Your src attribute points to your local drive. That is not going to work. Your browser displays the alternate text because it can't find the image. You should use the URL of the image instead. For example src="/images/down.png".
Is there a performance difference between using an HtmlHelper or a Partial for a given task?
For example, I'm writing an HtmlHelper "control" to create a link in an editor with the following signature:
public static HtmlString RecordNameLink(
this HtmlHelper htmlHelper,
string linkText,
string editActionName,
object editRouteValues,
string deleteActionName = null,
object deleteRouteValues = null)
In this case the edit button will always be displayed, and the delete button will only be displayed if it is included. Alternately, I could do this:
#Html.Partial("Controls/RecordNameLink", Model)
(Or pass a partial-specific model)
But is there a reason to choose one over the other, specifically does one have better performance than the other? (though I'm open to learning more about the differences in general)
Thanks.
In MVC 3, for this sort of thing its going to be faster for you to render with an html helper than a Partial.
Do a test where you render a partial 100+ times in a loop, vs have the partial contain the loop (partial render per row of a table, vs partial render of all rows of a table). You might be quite surprised at the result.
Your HTML Helper will skip the viewengine having to hunt for the partial, the call to the Virtual Path Provider to load it, etc.
I have a partial view that I want to be generic. According to this question, partial views cannot be generic. So I instead made an HtmlHelper extension that handles the pieces for which I want type-safety, then hands off the rest to a real partial view.
Usually my helper is called on page load, which works fine, but sometimes I want to add a row or something through AJAX. When this happens, the controller cannot use my "partial view" since it does not have access to the HtmlHelper.
Apart from having a partial view with a model of type object, is there anything I can do?
I'm using Razor, if that is important.
A simplified version of what I'm doing:
public static MvcHtmlString DoStuff<T>(this HtmlHelper html, IEnumerable<T> data,
Func<T, ViewModelType> StronglyTypedFn, string PartialName)
{
// the pre- and post-processing for the partial view is complex enough I'd like
// to encapsulate it. But I want the encapsulation to include the safety
// benefits that generics give.
var mappedData = data.Select(StronglyTypedFn);
string htmlData = "";
foreach(var model in mappedData){
htmlData += html.Partial(PartialName, model);
}
htmlData += "some boilerplate footer html";
return htmlData;
}
I realize that in this example I have so few lines of code outside the partial view that it seems pointless to have a helper, but in my real example it is more complex.
Now, in an ajax call I want to return Html.DoStuff(). But I can't, because this requires access to the HtmlHelper, and the helper isn't available inside a controller.
You could just have a simple action method that calls the partial for one model instance
public PartialViewResult Single(string partialName) {
return PartialView(partialName);
}
You could use a View with a Dynamic type instead of object.
But... It seems as if there's some misunderstanding here because the Controller shouldn't try to render the view at all. Could you post the Controller code?
The better option is, IMO, returning a JsonResult for your ajax request and adding the row/rows on client side using JS.
Ok, this is for me a very tough challenge. We're taking our existing ASP.NET website and converting (redesigning the PL only) to MVC. Our site is very complex. But the hard part is to convert the existing custom controls to MVC equivilant. The custom controls (I am not talking about user controls) are just of course a class currently that inherits System.Web.UI.Control and uses that object throughout. For example, we have some properties at the top of this existing custom class like so:
Dictionary<int, Control> configControls;
DropDownList kControl;
CheckBox confirmBox;
These all are variables of type Web controls in classic ASP.NET.
So I figured maybe what I could do (without building entire new custom controls from scratch) is to use the HtmlHelper object. So I tried this:
(include first the using statement that includes System.Web.MVC.Html at the top of my new custom class in our new web project)
private HtmlHelper helper;
Dictionary configControls;
helper.DropDownList
but this is not working. I guess I can't use this object just like this ?? I figured I can use HtmlHelper in the Dictionary and then make variable types off of helper. but those are just extension methods, not objects
I don't know of an equivalent to something like the generic "Control" we had available to us to inherit from such as in classic ASP.NET. Surely it won't be the same in MVC obviusly (stateless and a completely diff way of doing things) but what can I use in MVC with the same concept sort of?
So I figured maybe what I could do (without building entire new custom controls from scratch) is to use the HtmlHelper object. So I tried this:
(include first the using statement that includes System.Web.MVC.Html at the top of my new custom class in our new web project)
private HtmlHelper helper;
Dictionary configControls;
helper.DropDownList
but this is not working. I don't even know if this approach will work in my custom control. And when I try to use my helper variable, I get no extension methods unless it's inside an existing extension method where the signature has an HtmlHelper param passed in. So when I create that private variable just in my custom class outside, I get nothing in intellisense to choose from when doing "helper.". So do I need to define that object like this: ?
private HtmlHelper htmlHelper = new HtmlHelper();
but it's asking for a ViewContext and an IViewDataContainer as params. If I'm building out a custom method that knows nothing yet about its view (it shouldn't need to) because I'm simply creating strings of HMTL in this custom class to be passed to the Extension method to ultimately spit out fields then maybe I can't use HtmlHelper this way in a custom class like this.
So can I use that object in a way instead of "Control"? Maybe I can even in my dictionary variable use type object in place of control ? I don't know and then cast object to type HtmlHelper when I need to use or reference that value from the dictionary? But for now, I figured I can use HtmlHelper object in the Dictionary and then make variable types off of helper. but those are just extension methods, not objects.
I hope I am making any sense here when you read this.
I just blogged about this last night, some of this might be helpful for you.
WebForms And MVC In Harmony — Almost…
Basically it discusses some options for emulating "WebControls" using MVC.
Additionally, you can still use WebControls like you could before (granted they may not work if they need things like the ViewState). The problem I've discovered with that is you have a disconnect from the inline render code and the WebControls themselves.
I did write this method last night which let you use WebControls with inline code.
using System.Reflection;
using System.IO;
using System.Web.UI;
using System.Web;
using System.Web.Mvc;
public static class MyExtensionMethods {
//example method - renders a webcontrol to the page
public static void RenderControl(this HtmlHelper helper, Control control) {
//perform databinding if needed
MethodInfo bind = control.GetType().GetMethod("DataBind");
if (bind is System.Reflection.MethodInfo) {
bind.Invoke(control, null);
}
//render the HTML for this control
StringWriter writer = new StringWriter();
HtmlTextWriter html = new HtmlTextWriter(writer);
control.RenderControl(html);
//write the output
HttpContext.Current.Response.Write(writer.ToString());
//and cleanup the writers
html.Dispose();
writer.Dispose();
}
}
//then used like...
<% int[] numbers = { 1, 2, 3, 4, 5 }; %>
<% this.Html.RenderControl(new DataGrid() { DataSource = numbers }); %>
Just an interesting concept you might be interested in.
Short of hacking webforms controls into your MVC application, servercontrols with many methods do not map to MVC.
They are replaced by partials and controllers(or subcontrollers if you like that sort of thing).
If all you want to do is render some HTML based on a few parameters, then a Helper is what you are after. Static Class, static methods. If however, you need to keep state, and do a bunch of stateful stuff, then a partial, JS, and controller(or subcontroller) are really what you are after.
Server Controls that manage their own state really are a thing of the past in MVC.
Remember that MVC is an attempt to use the web the way it was meant to work, particularly if you bring REST into the picture. Webforms is a fudge to make the web work like windows forms.
I would create needed business logic, shared partial view (probably, with quite a lot of well structured javascript lines attached) and seperated controller.
Then i would use this bunch of code through partial request technique.
Not sure how much this will be of help but, do have a look at this series of blog post
Custom controls everywhere
Also have a look at the Catharsis project
Web-Application Framework - Catharsis - Part I - New Solution
The codeplex URL for the same is
Catharsis
This project has some good examples of control creating for asp.net mvc.
I am trying to write my own LightWeight MVC for .Net 2.0 using NHaml as the view engine.
In ASP.Net 3.5 MVC the View file we used to specify the link by the code snippet.
Html.ActionLink("Add Product","Add");
In MVC binary there is no function to match this call.
I Only found:
(In class System.Web.Mvc.Html.LinkExtensions )
public static string ActionLink(this System.Web.Mvc.HtmlHelper htmlHelper,
string linkText, string actionName)
There are more similar static classes like FormExtensions, InputExtensions etc.
How does ASP.Net MVC handle it? Does it generates dynamic code for Html.ActionLink?
The ActionLink method is an extension method (hence the this before the type of the first parameter). This means you can use this method as an instance method on all HtmlHelper instances, even though it is not defined on HtmlHelper.
Html is a property on the View of type HtmlHelper. This means you can use the ActionLink extensionmethod on it.
The ActionLink method itself does nothing more than generate a link string (with regards to its arguments) and return that string.
Have you checked out the code on Codeplex? The MVC Framwork is open source, so you can dig around as much as you need to.