Override sc:Image alt text

How can I override Sitecore <sc:Image> control so that it reads the alt text from another field in the same item?

I want to be able to give a tag,

<sc:Image contextAltText="fieldName" ...>

and it should first look at this field and if empty, it should render the Media Item Alt text as it normally does.

I don't need detailed implementation instructions. I only need to know which class and functions I have to overwrite.

Solution

You can create you own class inheriting from the Sitecore.Web.UI.WebControls.Image and override it like this:

namespace My.Assembly.Namespace
{
    public class MyImage : Sitecore.Web.UI.WebControls.Image
    {
        public virtual string ContentAltText { get; set; }

        protected override void PopulateParameters(Sitecore.Collections.SafeDictionary<string> parameters)
        {
            base.PopulateParameters(parameters);
            if (!String.IsNullOrEmpty(Item[ContentAltText] ))
            {
                parameters.Add("alt", Item[ContentAltText]);
            }
        }
    }
}

Maybe instead of calling parameters.Add you will need to replace the original alt value. I haven't tested the code so as you asked, it should only point you in the right direction.

EDIT: code below will not work with the ContentAltText being field name instead of different alt text.

There is also another way, but I don't know if this will override default alt value or this only works for new attributes:

<sc:Image runat="server" Field="Field Name" Parameters="alt=OverridenAltValue" />

EDIT 2:

To register your controls, find section below in web.config and add your namespace with your custom prefix:

<system.web>
  <pages>
    <controls>
      <add tagPrefix="sc" namespace="Sitecore.Web.UI.WebControls" assembly="Sitecore.Kernel"/>
      <add tagPrefix="msc" namespace="My.Assembly.Namespace" assembly="My.Assembly"/>
      ...
    </controls>
  </pages>
</system.web>

You may need to restart Visual Studio to make sure it recognizes new namespace.

Then use:

<msc:ImageWithAlt runat="server" ... />