Image handling in EPiServer

IImageRequirement.cs

namespace MyProject.Infrastructure.EPiServer.Attributes
{
    public interface IImageRequirement
    {
        string GetRequirement();
    }
}

Image.cshtml

@using MyProject.Infrastructure.EPiServer.Extensions
@using MyProject.Infrastructure.EPiServer.Models.Media
@using MyProject.Web.Components.Image
@using EPiServer
@using EPiServer.Editor
@using EPiServer.ServiceLocation
@model EPiServer.Core.ContentReference

@if (Model != null)
{
    var imageFile = ServiceLocator.Current.GetInstance<IContentLoader>().Get<ImageFile>(Model);
    var imageDimensions = imageFile.GetDimensions();
    var halfWidth = imageDimensions.Width/2;
    var halfHeight = imageDimensions.Height/2;

    <img class="@ViewData["class"]" src="@Url.ContentUrl(Model)" 
         srcset="@Url.ContentUrl(Model) 2x, @Url.ContentUrl(Model)?width=@halfWidth&amp;height=@halfHeight 1x" 
         alt="@imageFile.Description" title="@imageFile.Credits"/>
}
else if (PageEditing.PageIsInEditMode)
{
    <text>Currently no picture - size requirement: @ViewData[ImageHelper.ImageConstraints]</text>
}

ImageFile.cs

using System.ComponentModel.DataAnnotations;
using EPiServer.Core;
using EPiServer.DataAbstraction;
using EPiServer.DataAnnotations;
using EPiServer.Framework.DataAnnotations;

namespace MyProject.Infrastructure.EPiServer.Models.Media
{
    [ContentType(DisplayName = "Image", GUID = "BE8C5F3B-8AA4-4870-AFF3-5F1208015722")]
    [MediaDescriptor(ExtensionString = "jpg,jpeg,jpe,ico,gif,bmp,png")]
    public class ImageFile : ImageData
    {
        [CultureSpecific]
        [Editable(true)]
        [Display(
        Name = "Alternate text",
        Description = "Description of the image",
        GroupName = SystemTabNames.Content,
        Order = 10)]
        public virtual string Description { get; set; }

        [CultureSpecific]
        [Editable(true)]
        [Display(
        Name = "Credits",
        GroupName = SystemTabNames.Content,
        Order = 20)]
        public virtual string Credits { get; set; }
    }
}

ImageHelper.cs

using System;
using System.Linq;
using System.Reflection;
using MyProject.Infrastructure.EPiServer.Attributes;

namespace MyProject.Web.Components.Image
{
    public static class ImageHelper
    {

        public static string GetImageRequirement(this PropertyInfo property)
        {
            return property
                .GetCustomAttributes(false)
                .OfType<IImageRequirement>()
                .First()
                .GetRequirement();
        }

        public const string ImageConstraints = "constraints";
    }
}

Index.cshtml

@model ImageBlockViewModel

<div class="image">
    @Html.PropertyFor(m => m.CurrentBlock.Image, new
    {
        constraints = Model.GetImageConstraints
    } )
</div>

RequiredAltTextAttribute.cs

using System;
using System.ComponentModel.DataAnnotations;
using MyProject.Infrastructure.EPiServer.Models.Media;
using EPiServer;
using EPiServer.Core;
using EPiServer.ServiceLocation;

namespace MyProject.Infrastructure.EPiServer.Attributes
{
    [AttributeUsage(AttributeTargets.Property)]
    public class RequiredAltTextAttribute : ValidationAttribute
    {

        public override bool IsValid(object value)
        {
            var contentReference = value as ContentReference;
            if (contentReference == null)
            {
                return true;
            }

            var imageFile = ServiceLocator.Current.GetInstance<IContentLoader>().Get<ImageFile>(contentReference);
            if (imageFile == null)
            {
                ErrorMessage = "The content selected for property '{0}' cannot be found!";
                return false;
            }

            if (string.IsNullOrEmpty(imageFile.Description))
            {
                ErrorMessage =
                    $"The item referenced by '{{0}}' has no description. Please add a description to '{imageFile.Name}' before use.";
                return false;
            }

            return true;
        }
    }
}

RequiredImageSizeAttribute.cs

using System;
using System.ComponentModel.DataAnnotations;
using MyProject.Infrastructure.EPiServer.Extensions;
using MyProject.Infrastructure.EPiServer.Models.Media;
using EPiServer;
using EPiServer.Core;
using EPiServer.ServiceLocation;

namespace MyProject.Infrastructure.EPiServer.Attributes
{

    // Adapted from https://github.com/markeverard/Chief2moro.ImageDataExtensions
    [AttributeUsage(AttributeTargets.Property)]
    public class RequiredImageSizeAttribute : ValidationAttribute, IImageRequirement
    {

        public int Width { get; set; }
        public int Height { get; set; }

        public override bool IsValid(object value)
        {
            if (value == null)
            {
                return true;
            }

            if (Height < 1 && Width < 1)
            {
                return true;
            }

            var contentReference = value as ContentReference;
            var imageFile = ServiceLocator.Current.GetInstance<IContentLoader>().Get<ImageFile>(contentReference);
            if (imageFile == null)
            {
                ErrorMessage = "The content selected for property '{0}' cannot be found!";
                return false;
            }

            var dimensions = imageFile.GetDimensions();

            if (IsNotRequiredHeight(dimensions) && IsNotRequiredWidth(dimensions))
            {
                ErrorMessage = $"The image used in reference '{{0}}' is {dimensions}, but must have a size of {Width}x{Height} pixels";
                return false;
            }

            if (IsNotRequiredHeight(dimensions))
            {
                ErrorMessage = $"The image used in property {{0}} is {dimensions}, but must have a height of {Height} pixels";
                return false;
            }

            if (IsNotRequiredWidth(dimensions))
            {
                ErrorMessage = $"The image used in property {{0}} is {dimensions}, but must have a width of {Width} pixels";
                return false;
            }


            return true;
        }

        private bool IsNotRequiredWidth(Dimensions dimensions)
        {
            return IsNonZero(Width) && Width != dimensions.Width;
        }

        private bool IsNotRequiredHeight(Dimensions dimensions)
        {
            return IsNonZero(Height) && Height != dimensions.Height;
        }

        private bool IsNonZero(int size)
        {
            return size > 0;
        }

        public string GetRequirement()
        {
            return $"{Width}x{Height}";
        }
    }
}