Allow for filtering of document type allowed children and allowed at root when creating new content.

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

This addresses part of the discussion in #16329 relating to the removal of SendingXNotifications in 14+, to provide an alternative for the SendingAllowedChildrenNotification.

Description

This PR implements an alternative to the SendingAllowedChildrenNotification that isn't available in Umbraco 14+ but is documented as a feature.

A filter is defined by the interface IContentTypeFilter. Methods are asynchronous. My guess is that most implementations won't need to use asynchronous methods, but given they might, it seemed better to define them this way.

Originally I registered a single instance of this with a "no-op" implementation, but then realised it would be better as a collection. That way there could be more than one filter (perhaps one added by a package, and another explicitly by the implementor).

The registered filters are called in the ContentTypeServiceBase methods used for retrieving the "allowed children" and the "allowed at root" content types. So we get whatever we get from the Umbraco schema, and then run the results through the appropriate method on the filters, before returning the data to the front-end for rendering.

An implementor could register their own filter that implements the features that this notification was used for - e.g. restricting the ability to create a node of a particular type if it already exists.

To Test:

• First, just make sure you can create content, media and members at the root and as child items as expected.
• Register filters and verify that the expected items are removed. You can use a composer like the below.

This sample shows a couple of filters:

• An illustrative one that allows only those with an alias beginning with "t".
• A more real-world one that ensures you can only create the home page once at the root level.

using Umbraco.Cms.Core.Composing;
using Umbraco.Cms.Core.Models;
using Umbraco.Cms.Core.Services;
using Umbraco.Cms.Core.Services.Filters;

public class TestingComposer : IComposer
{
    public void Compose(IUmbracoBuilder builder)
    {
        builder.ContentTypeFilters()
            .Append<MyContentTypeFilterService>()
            .Append<MyContentTypeFilterService2>();
    }
}

internal class MyContentTypeFilterService : IContentTypeFilter
{
    public Task<IEnumerable<TItem>> FilterAllowedAtRootAsync<TItem>(IEnumerable<TItem> contentTypes)
        where TItem : IContentTypeComposition
        => Task.FromResult(contentTypes);

    public Task<IEnumerable<ContentTypeSort>> FilterAllowedChildrenAsync(IEnumerable<ContentTypeSort> contentTypes, Guid parentKey)
        => Task.FromResult(contentTypes
            .Where(x => x.Alias.StartsWith("t")));
}

internal class MyContentTypeFilterService2 : IContentTypeFilter
{
    private readonly IContentService _contentService;

    public MyContentTypeFilterService2(IContentService contentService) => _contentService = contentService;

    public Task<IEnumerable<TItem>> FilterAllowedAtRootAsync<TItem>(IEnumerable<TItem> contentTypes)
        where TItem : IContentTypeComposition
    {
        var docTypeAliasesToExclude = new List<string>();

        const string HomePageDocTypeAlias = "homePage1";
        var docTypeAliasesAtRoot = _contentService.GetRootContent().Select(x => x.ContentType.Alias).Distinct().ToList();
        if (docTypeAliasesAtRoot.Contains(HomePageDocTypeAlias))
        {
            docTypeAliasesToExclude.Add(HomePageDocTypeAlias);
        }

        return Task.FromResult(contentTypes
            .Where(x => docTypeAliasesToExclude.Contains(x.Alias) is false));
    }

    public Task<IEnumerable<ContentTypeSort>> FilterAllowedChildrenAsync(IEnumerable<ContentTypeSort> contentTypes, Guid parentKey)
        => Task.FromResult(contentTypes);
}

[Zeegaan]

Looks good, tests good 🚀

[AndyButland]

Docs PR is here: umbraco/UmbracoDocs#6808