Improve components, proper cache refresher ordering

src/Umbraco.Core/Components/RequiredComponentAttribute.cs

@@ -0,0 +1,41 @@
+using System;
+
+namespace Umbraco.Core.Components
+{
+    /// <summary>
+    /// Indicates that a component is required by another component.
+    /// </summary>
+    /// <remarks>
+    /// fixme
+    /// <para>This attribute is *not* inherited. This means that a component class inheriting from 
+    /// another component class does *not* inherit its requirements. However, the bootloader checks
+    /// the *interfaces* of every component for their requirements, so requirements declared on
+    /// interfaces are inherited by every component class implementing the interface.</para>
+    /// <para>When targetting a class, indicates a dependency on the component which must be enabled,
+    /// unless the requirement has explicitely been declared as weak (and then, only if the component
+    /// is enabled).</para>
+    /// <para>When targetting an interface, indicates a dependency on enabled components implementing 
+    /// the interface. It could be no component at all, unless the requirement has explicitely been 
+    /// declared as strong (and at least one component must be enabled).</para>
+    /// </remarks>
+
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface, AllowMultiple = true, Inherited = false)]
+    public class RequiredComponentAttribute : Attribute
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="RequiredComponentAttribute"/> class.
+        /// </summary>
+        /// <param name="requiringType">The type of the required component.</param>
+        public RequiredComponentAttribute(Type requiringType)
+        {
+            if (typeof(IUmbracoComponent).IsAssignableFrom(requiringType) == false)
+                throw new ArgumentException($"Type {requiringType.FullName} is invalid here because it does not implement {typeof(IUmbracoComponent).FullName}.");
+            RequiringType = requiringType;
+        }
+
+        /// <summary>
+        /// Gets the required type.
+        /// </summary>
+        public Type RequiringType { get; }
+    }
+}