Add parameter value provider support

Adds support for parameter value providers which makes it
possible to set custom values for parameters.

src/Spectre.Console.Tests/Data/Validators/EvenNumberValidatorAttribute.cs

@@ -11,19 +11,19 @@ namespace Spectre.Console.Tests.Data
         {
         }
 
-        public override ValidationResult Validate(ICommandParameterInfo info, object value)
+        public override ValidationResult Validate(CommandParameterContext context)
         {
-            if (value is int integer)
+            if (context.Value is int integer)
             {
                 if (integer % 2 == 0)
                 {
                     return ValidationResult.Success();
                 }
 
-                return ValidationResult.Error($"Number is not even ({info?.PropertyName}).");
+                return ValidationResult.Error($"Number is not even ({context.Parameter.PropertyName}).");
             }
 
-            throw new InvalidOperationException($"Parameter is not a number ({info?.PropertyName}).");
+            throw new InvalidOperationException($"Parameter is not a number ({context.Parameter.PropertyName}).");
         }
     }
 }

src/Spectre.Console.Tests/Data/Validators/PositiveNumberValidatorAttribute.cs

@@ -11,19 +11,19 @@ namespace Spectre.Console.Tests.Data
         {
         }
 
-        public override ValidationResult Validate(ICommandParameterInfo info, object value)
+        public override ValidationResult Validate(CommandParameterContext context)
         {
-            if (value is int integer)
+            if (context.Value is int integer)
             {
                 if (integer > 0)
                 {
                     return ValidationResult.Success();
                 }
 
-                return ValidationResult.Error($"Number is not greater than 0 ({info?.PropertyName}).");
+                return ValidationResult.Error($"Number is not greater than 0 ({context.Parameter.PropertyName}).");
             }
 
-            throw new InvalidOperationException($"Parameter is not a number ({info?.PropertyName}).");
+            throw new InvalidOperationException($"Parameter is not a number ({context.Parameter.PropertyName}).");
         }
     }
 }

src/Spectre.Console.Tests/Unit/Cli/CommandAppTests.ValueProviders.cs

@@ -0,0 +1,89 @@
+using System.Linq;
+using Shouldly;
+using Spectre.Console.Cli;
+using Spectre.Console.Testing;
+using Spectre.Console.Tests.Data;
+using Xunit;
+
+namespace Spectre.Console.Tests.Unit.Cli
+{
+    public sealed partial class CommandAppTests
+    {
+        public sealed class ValueProviders
+        {
+            public sealed class ValueProviderSettings : CommandSettings
+            {
+                [CommandOption("-f|--foo <VALUE>")]
+                [IntegerValueProvider(32)]
+                public int Foo { get; set; }
+            }
+
+            public sealed class IntegerValueProvider : ParameterValueProviderAttribute
+            {
+                private readonly int _value;
+
+                public IntegerValueProvider(int value)
+                {
+                    _value = value;
+                }
+
+                public override bool TryGetValue(CommandParameterContext context, out object result)
+                {
+                    if (context.Parameter.ParameterType == typeof(int))
+                    {
+                        if (context.Value == null)
+                        {
+                            result = _value;
+                            return true;
+                        }
+                    }
+
+                    result = null;
+                    return false;
+                }
+            }
+
+            [Fact]
+            public void Should_Use_Provided_Value_If_No_Value_Was_Specified()
+            {
+                // Given
+                var app = new CommandAppTester();
+                app.SetDefaultCommand<GenericCommand<ValueProviderSettings>>();
+                app.Configure(config =>
+                {
+                    config.PropagateExceptions();
+                });
+
+                // When
+                var result = app.Run();
+
+                // Then
+                result.Settings.ShouldBeOfType<ValueProviderSettings>().And(settings =>
+                {
+                    settings.Foo.ShouldBe(32);
+                });
+            }
+
+            [Fact]
+            public void Should_Not_Override_Value_If_Value_Was_Specified()
+            {
+                // Given
+                var app = new CommandAppTester();
+                app.SetDefaultCommand<GenericCommand<ValueProviderSettings>>();
+                app.Configure(config =>
+                {
+                    config.PropagateExceptions();
+                });
+
+                // When
+                var result = app.Run("--foo", "12");
+
+                // Then
+                result.Settings.ShouldBeOfType<ValueProviderSettings>().And(settings =>
+                {
+                    settings.Foo.ShouldBe(12);
+                });
+            }
+        }
+    }
+}

src/Spectre.Console/Cli/Annotations/ParameterValidationAttribute.cs

@@ -27,9 +27,8 @@ namespace Spectre.Console.Cli
         /// <summary>
         /// Validates the parameter value.
         /// </summary>
-        /// <param name="info">The parameter info.</param>
+        /// <param name="context">The parameter context.</param>
-        /// <param name="value">The parameter value.</param>
         /// <returns>The validation result.</returns>
-        public abstract ValidationResult Validate(ICommandParameterInfo info, object? value);
+        public abstract ValidationResult Validate(CommandParameterContext context);
     }
 }

src/Spectre.Console/Cli/Annotations/ParameterValueProviderAttribute.cs

@@ -0,0 +1,20 @@
+using System;
+
+namespace Spectre.Console.Cli
+{
+    /// <summary>
+    /// An base class attribute used for parameter completion.
+    /// </summary>
+    /// <seealso cref="Attribute" />
+    [AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
+    public abstract class ParameterValueProviderAttribute : Attribute
+    {
+        /// <summary>
+        /// Gets a value for the parameter.
+        /// </summary>
+        /// <param name="context">The parameter context.</param>
+        /// <param name="result">The resulting value.</param>
+        /// <returns><c>true</c> if a value was provided; otherwise, <c>false</c>.</returns>
+        public abstract bool TryGetValue(CommandParameterContext context, out object? result);
+    }
+}

src/Spectre.Console/Cli/CommandParameterContext.cs

@@ -0,0 +1,38 @@
+using System;
+
+namespace Spectre.Console.Cli
+{
+    /// <summary>
+    /// Represents a context for <see cref="ICommandParameterInfo"/> related operations.
+    /// </summary>
+    public sealed class CommandParameterContext
+    {
+        /// <summary>
+        /// Gets the parameter.
+        /// </summary>
+        public ICommandParameterInfo Parameter { get; }
+
+        /// <summary>
+        /// Gets the type resolver.
+        /// </summary>
+        public ITypeResolver Resolver { get; }
+
+        /// <summary>
+        /// Gets tje parameter value.
+        /// </summary>
+        public object? Value { get; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="CommandParameterContext"/> class.
+        /// </summary>
+        /// <param name="parameter">The parameter.</param>
+        /// <param name="resolver">The type resolver.</param>
+        /// <param name="value">The parameter value.</param>
+        public CommandParameterContext(ICommandParameterInfo parameter, ITypeResolver resolver, object? value)
+        {
+            Parameter = parameter ?? throw new ArgumentNullException(nameof(parameter));
+            Resolver = resolver ?? throw new ArgumentNullException(nameof(resolver));
+            Value = value;
+        }
+    }
+}

src/Spectre.Console/Cli/ICommandParameterInfo.cs

@@ -1,3 +1,5 @@
+using System;
+
 namespace Spectre.Console.Cli
 {
     /// <summary>
@@ -9,12 +11,17 @@ namespace Spectre.Console.Cli
         /// Gets the property name.
         /// </summary>
         /// <value>The property name.</value>
-        public abstract string PropertyName { get; }
+        public string PropertyName { get; }
+
+        /// <summary>
+        /// Gets the parameter type.
+        /// </summary>
+        public Type ParameterType { get; }
 
         /// <summary>
         /// Gets the description.
         /// </summary>
         /// <value>The description.</value>
-        public abstract string? Description { get; }
+        public string? Description { get; }
     }
 }

src/Spectre.Console/Cli/Internal/Binding/CommandValueResolver.cs

@@ -18,6 +18,18 @@ namespace Spectre.Console.Cli
                 // Process unmapped parameters.
                 foreach (var parameter in tree.Unmapped)
                 {
+                    // Got a value provider?
+                    if (parameter.ValueProvider != null)
+                    {
+                        var context = new CommandParameterContext(parameter, resolver, null);
+                        if (parameter.ValueProvider.TryGetValue(context, out var result))
+                        {
+                            lookup.SetValue(parameter, result);
+                            CommandValidator.ValidateParameter(parameter, lookup, resolver);
+                            continue;
+                        }
+                    }
+
                     if (parameter.IsFlagValue())
                     {
                         // Set the flag value to an empty, not set instance.
@@ -42,7 +54,7 @@ namespace Spectre.Console.Cli
                             }
 
                             binder.Bind(parameter, resolver, value);
-                            CommandValidator.ValidateParameter(parameter, lookup);
+                            CommandValidator.ValidateParameter(parameter, lookup, resolver);
                         }
                         else if (Nullable.GetUnderlyingType(parameter.ParameterType) != null ||
                                  !parameter.ParameterType.IsValueType)
@@ -88,7 +100,17 @@ namespace Spectre.Console.Cli
                         }
                     }
 
-                    CommandValidator.ValidateParameter(mapped.Parameter, lookup);
+                    // Got a value provider?
+                    if (mapped.Parameter.ValueProvider != null)
+                    {
+                        var context = new CommandParameterContext(mapped.Parameter, resolver, mapped.Value);
+                        if (mapped.Parameter.ValueProvider.TryGetValue(context, out var result))
+                        {
+                            lookup.SetValue(mapped.Parameter, result);
+                        }
+                    }
+
+                    CommandValidator.ValidateParameter(mapped.Parameter, lookup, resolver);
                 }
 
                 tree = tree.Next;

src/Spectre.Console/Cli/Internal/CommandValidator.cs

@@ -23,12 +23,13 @@ namespace Spectre.Console.Cli
             }
         }
 
-        public static void ValidateParameter(CommandParameter parameter, CommandValueLookup settings)
+        public static void ValidateParameter(CommandParameter parameter, CommandValueLookup settings, ITypeResolver resolver)
         {
             var assignedValue = settings.GetValue(parameter);
             foreach (var validator in parameter.Validators)
             {
-                var validationResult = validator.Validate(parameter, assignedValue);
+                var context = new CommandParameterContext(parameter, resolver, assignedValue);
+                var validationResult = validator.Validate(context);
                 if (!validationResult.Successful)
                 {
                     // If there is a error message specified in the parameter validator attribute,

src/Spectre.Console/Cli/Internal/Modelling/CommandArgument.cs

@@ -13,9 +13,10 @@ namespace Spectre.Console.Cli
         public CommandArgument(
             Type parameterType, ParameterKind parameterKind, PropertyInfo property, string? description,
             TypeConverterAttribute? converter, DefaultValueAttribute? defaultValue,
-            CommandArgumentAttribute argument, IEnumerable<ParameterValidationAttribute> validators)
+            CommandArgumentAttribute argument, ParameterValueProviderAttribute? valueProvider,
+            IEnumerable<ParameterValidationAttribute> validators)
                 : base(parameterType, parameterKind, property, description, converter, defaultValue,
-                      null, validators, argument.IsRequired)
+                      null, valueProvider, validators, argument.IsRequired)
         {
             Value = argument.ValueName;
             Position = argument.Position;

src/Spectre.Console/Cli/Internal/Modelling/CommandModelBuilder.cs

@@ -182,6 +182,7 @@ namespace Spectre.Console.Cli
             var description = property.GetCustomAttribute<DescriptionAttribute>();
             var converter = property.GetCustomAttribute<TypeConverterAttribute>();
             var deconstructor = property.GetCustomAttribute<PairDeconstructorAttribute>();
+            var valueProvider = property.GetCustomAttribute<ParameterValueProviderAttribute>();
             var validators = property.GetCustomAttributes<ParameterValidationAttribute>(true);
             var defaultValue = property.GetCustomAttribute<DefaultValueAttribute>();
 
@@ -194,7 +195,8 @@ namespace Spectre.Console.Cli
 
             return new CommandOption(property.PropertyType, kind,
                 property, description?.Description, converter, deconstructor,
-                attribute, validators, defaultValue, attribute.ValueIsOptional);
+                attribute, valueProvider, validators, defaultValue,
+                attribute.ValueIsOptional);
         }
 
         private static CommandArgument BuildArgumentParameter(PropertyInfo property, CommandArgumentAttribute attribute)
@@ -202,6 +204,7 @@ namespace Spectre.Console.Cli
             var description = property.GetCustomAttribute<DescriptionAttribute>();
             var converter = property.GetCustomAttribute<TypeConverterAttribute>();
             var defaultValue = property.GetCustomAttribute<DefaultValueAttribute>();
+            var valueProvider = property.GetCustomAttribute<ParameterValueProviderAttribute>();
             var validators = property.GetCustomAttributes<ParameterValidationAttribute>(true);
 
             var kind = GetParameterKind(property.PropertyType);
@@ -209,7 +212,8 @@ namespace Spectre.Console.Cli
             return new CommandArgument(
                 property.PropertyType, kind, property,
                 description?.Description, converter,
-                defaultValue, attribute, validators);
+                defaultValue, attribute, valueProvider,
+                validators);
         }
 
         private static ParameterKind GetOptionKind(

src/Spectre.Console/Cli/Internal/Modelling/CommandOption.cs

@@ -16,10 +16,11 @@ namespace Spectre.Console.Cli
         public CommandOption(
             Type parameterType, ParameterKind parameterKind, PropertyInfo property, string? description,
             TypeConverterAttribute? converter, PairDeconstructorAttribute? deconstructor,
-            CommandOptionAttribute optionAttribute, IEnumerable<ParameterValidationAttribute> validators,
+            CommandOptionAttribute optionAttribute, ParameterValueProviderAttribute? valueProvider,
+            IEnumerable<ParameterValidationAttribute> validators,
             DefaultValueAttribute? defaultValue, bool valueIsOptional)
                 : base(parameterType, parameterKind, property, description, converter,
-                      defaultValue, deconstructor, validators, false)
+                      defaultValue, deconstructor, valueProvider, validators, false)
         {
             LongNames = optionAttribute.LongNames;
             ShortNames = optionAttribute.ShortNames;

src/Spectre.Console/Cli/Internal/Modelling/CommandParameter.cs

@@ -17,6 +17,7 @@ namespace Spectre.Console.Cli
         public TypeConverterAttribute? Converter { get; }
         public PairDeconstructorAttribute? PairDeconstructor { get; }
         public List<ParameterValidationAttribute> Validators { get; }
+        public ParameterValueProviderAttribute? ValueProvider { get; }
         public bool Required { get; set; }
         public string PropertyName => Property.Name;
 
@@ -28,6 +29,7 @@ namespace Spectre.Console.Cli
             string? description, TypeConverterAttribute? converter,
             DefaultValueAttribute? defaultValue,
             PairDeconstructorAttribute? deconstuctor,
+            ParameterValueProviderAttribute? valueProvider,
             IEnumerable<ParameterValidationAttribute> validators, bool required)
         {
             Id = Guid.NewGuid();
@@ -38,6 +40,7 @@ namespace Spectre.Console.Cli
             Converter = converter;
             DefaultValue = defaultValue;
             PairDeconstructor = deconstuctor;
+            ValueProvider = valueProvider;
             Validators = new List<ParameterValidationAttribute>(validators ?? Array.Empty<ParameterValidationAttribute>());
             Required = required;
         }

src/Spectre.Console/Spectre.Console.csproj

@@ -28,10 +28,6 @@
     </PackageReference>
   </ItemGroup>
 
-  <ItemGroup>
-    <Folder Include="Cli\" />
-  </ItemGroup>
-
   <PropertyGroup>
     <AnnotatedReferenceAssemblyVersion>3.0.0</AnnotatedReferenceAssemblyVersion>
     <GenerateNullableAttributes>False</GenerateNullableAttributes>