Add ExpressionSource operator

src/Bonsai.Scripting.Expressions.Design/ExpressionScriptEditor.cs

@@ -4,6 +4,7 @@
 using System.Windows.Forms.Design;
 using System.Windows.Forms;
 using Bonsai.Design;
+using System.Reactive;
 
 namespace Bonsai.Scripting.Expressions.Design
 {
@@ -53,7 +54,7 @@ public override object EditValue(ITypeDescriptorContext context, IServiceProvide
             var editorService = (IWindowsFormsEditorService)provider.GetService(typeof(IWindowsFormsEditorService));
             if (editorService != null)
             {
-                var itType = GetDataSource(context, provider)?.ObservableType;
+                var itType = context.Instance is ExpressionSource ? typeof(Unit) : GetDataSource(context, provider)?.ObservableType;
                 using var editorDialog = new ExpressionScriptEditorDialog(itType);
                 editorDialog.Script = (string)value;
                 return editorService.ShowDialog(editorDialog) == DialogResult.OK

src/Bonsai.Scripting.Expressions/ExpressionSource.cs

@@ -0,0 +1,78 @@
+using Bonsai.Expressions;
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Linq.Expressions;
+using System.Reactive;
+using System.Reactive.Linq;
+
+namespace Bonsai.Scripting.Expressions
+{
+    /// <summary>
+    /// Represents an operator that uses an expression script to generate an
+    /// observable sequence with a single element.
+    /// </summary>
+    [DefaultProperty(nameof(Expression))]
+    [WorkflowElementCategory(ElementCategory.Source)]
+    [TypeDescriptionProvider(typeof(ExpressionSourceTypeDescriptionProvider))]
+    [Description("An expression script used to generate a sequence with a single element.")]
+    public class ExpressionSource : ZeroArgumentExpressionBuilder, IScriptingElement
+    {
+        /// <summary>
+        /// Gets or sets the name of the expression source.
+        /// </summary>
+        [Externalizable(false)]
+        [Category(nameof(CategoryAttribute.Design))]
+        [Description("The name of the expression source.")]
+        public string Name { get; set; }
+
+        /// <summary>
+        /// Gets or sets a description for the expression source.
+        /// </summary>
+        [Externalizable(false)]
+        [Category(nameof(CategoryAttribute.Design))]
+        [Description("A description for the expression source.")]
+        [Editor(DesignTypes.MultilineStringEditor, DesignTypes.UITypeEditor)]
+        public string Description { get; set; }
+
+        /// <summary>
+        /// Gets or sets the expression that generates the singleton element.
+        /// </summary>
+        /// <remarks>
+        /// The <c>it</c> parameter stores the singleton <see cref="Unit"/> object
+        /// representing no data.
+        /// </remarks>
+        [Editor("Bonsai.Scripting.Expressions.Design.ExpressionScriptEditor, Bonsai.Scripting.Expressions.Design", DesignTypes.UITypeEditor)]
+        [Description("The expression that generates the singleton element.")]
+        public string Expression { get; set; } = "it";
+
+        /// <inheritdoc/>
+        public override Expression Build(IEnumerable<Expression> arguments)
+        {
+            var config = ParsingConfigHelper.CreateParsingConfig(typeof(Unit));
+            var generator = DynamicExpressionHelper.ParseLambda(config, typeof(Unit), null, Expression);
+            return System.Linq.Expressions.Expression.Call(typeof(ExpressionSource), nameof(Process), new[] { generator.ReturnType }, generator);
+        }
+
+        static IObservable<TResult> Process<TResult>(Func<Unit, TResult> generator)
+        {
+            return Observable.Return(Unit.Default).Select(generator);
+        }
+
+        class ExpressionSourceTypeDescriptionProvider : TypeDescriptionProvider
+        {
+            static readonly TypeDescriptionProvider parentProvider = TypeDescriptor.GetProvider(typeof(ExpressionSource));
+
+            public ExpressionSourceTypeDescriptionProvider()
+                : base(parentProvider)
+            {
+            }
+
+            public override ICustomTypeDescriptor GetExtendedTypeDescriptor(object instance)
+            {
+                return new ScriptingElementTypeDescriptor(instance,
+                    "An expression script used to generate a sequence with a single element.");
+            }
+        }
+    }
+}