Add combo box and list box for dynamic collections

src/Bonsai.Gui/ComboBoxDataSourceBuilder.cs

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+using System.ComponentModel;
+
+namespace Bonsai.Gui
+{
+    /// <summary>
+    /// Represents an operator that interfaces with a combo box control bound to each data
+    /// source in the sequence and generates notifications whenever the selection changes.
+    /// </summary>
+    [TypeVisualizer(typeof(ComboBoxDataSourceVisualizer))]
+    [Description("Interfaces with a combo box control bound to each data source in the sequence and generates notifications whenever the selection changes.")]
+    public class ComboBoxDataSourceBuilder : ListControlDataSourceBuilderBase
+    {
+    }
+}

src/Bonsai.Gui/ComboBoxDataSourceVisualizer.cs

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+using System;
+using System.Drawing;
+using System.Windows.Forms;
+
+namespace Bonsai.Gui
+{
+    /// <summary>
+    /// Provides a type visualizer representing a combo box control bound
+    /// to an arbitrary data source.
+    /// </summary>
+    public class ComboBoxDataSourceVisualizer : ControlVisualizerBase<ComboBox, ComboBoxDataSourceBuilder>
+    {
+        /// <inheritdoc/>
+        protected override ComboBox CreateControl(IServiceProvider provider, ComboBoxDataSourceBuilder builder)
+        {
+            var comboBox = new ComboBox();
+            comboBox.Dock = DockStyle.Fill;
+            comboBox.Size = new Size(300, 150);
+            comboBox.SubscribeTo(builder._DisplayMember, value => comboBox.DisplayMember = value);
+            comboBox.SubscribeTo(builder._DataSource, value => comboBox.DataSource = value);
+            comboBox.SelectedIndexChanged += (sender, e) =>
+            {
+                var index = comboBox.SelectedIndex;
+                var selectedValue = index < 0 ? null : comboBox.Items[index];
+                builder._SelectedItem.OnNext(selectedValue);
+            };
+            return comboBox;
+        }
+    }
+}

src/Bonsai.Gui/DataSourceControlBuilderBase.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Linq.Expressions;
+
+namespace Bonsai.Gui
+{
+    /// <summary>
+    /// Provides an abstract base class for UI controls which can be bound to each
+    /// data source from an observable sequence.
+    /// </summary>
+    public abstract class DataSourceControlBuilderBase : ControlBuilderBase
+    {
+        static readonly Range<int> argumentRange = Range.Create(lowerBound: 1, upperBound: 1);
+
+        /// <summary>
+        /// Gets the range of input arguments that this expression builder accepts.
+        /// </summary>
+        public override Range<int> ArgumentRange => argumentRange;
+
+        /// <summary>
+        /// Builds the expression tree for configuring and calling the UI control.
+        /// </summary>
+        /// <inheritdoc/>
+        public override Expression Build(IEnumerable<Expression> arguments)
+        {
+            var source = arguments.First();
+            var sourceType = source.Type.GetGenericArguments()[0];
+            var valueType = ExpressionHelper.GetGenericTypeBindings(typeof(IList<>), sourceType);
+            return Expression.Call(Expression.Constant(this), nameof(Generate), valueType, source);
+        }
+
+        /// <summary>
+        /// Generates an observable sequence of values containing the currently
+        /// selected item from the data source whenever the selection changes.
+        /// </summary>
+        /// <typeparam name="TValue">
+        /// The type of the values in the data source.
+        /// </typeparam>
+        /// <param name="source">
+        /// A sequence of collections representing the data sources to bind to
+        /// the UI control. Only one collection is bound at any one time.
+        /// </param>
+        /// <returns>
+        /// A sequence of values representing the currently selected item in
+        /// the UI control.
+        /// </returns>
+        protected abstract IObservable<TValue> Generate<TValue>(IObservable<IList<TValue>> source);
+    }
+}

src/Bonsai.Gui/ListBoxDataSourceBuilder.cs

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+using System.ComponentModel;
+
+namespace Bonsai.Gui
+{
+    /// <summary>
+    /// Represents an operator that interfaces with a list box control bound to each data
+    /// source in the sequence and generates notifications whenever the selection changes.
+    /// </summary>
+    [TypeVisualizer(typeof(ListBoxDataSourceVisualizer))]
+    [Description("Interfaces with a list box control bound to each data source in the sequence and generates notifications whenever the selection changes.")]
+    public class ListBoxDataSourceBuilder : ListControlDataSourceBuilderBase
+    {
+    }
+}

src/Bonsai.Gui/ListBoxDataSourceVisualizer.cs

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+using System;
+using System.Drawing;
+using System.Windows.Forms;
+
+namespace Bonsai.Gui
+{
+    /// <summary>
+    /// Provides a type visualizer representing a list box control bound
+    /// to an arbitrary data source.
+    /// </summary>
+    public class ListBoxDataSourceVisualizer : ControlVisualizerBase<ListBox, ListBoxDataSourceBuilder>
+    {
+        /// <inheritdoc/>
+        protected override ListBox CreateControl(IServiceProvider provider, ListBoxDataSourceBuilder builder)
+        {
+            var listBox = new ListBox();
+            listBox.Dock = DockStyle.Fill;
+            listBox.Size = new Size(300, 150);
+            listBox.SubscribeTo(builder._DisplayMember, value => listBox.DisplayMember = value);
+            listBox.SubscribeTo(builder._DataSource, value => listBox.DataSource = value);
+            listBox.SelectedIndexChanged += (sender, e) =>
+            {
+                var index = listBox.SelectedIndex;
+                var selectedValue = index < 0 ? null : listBox.Items[index];
+                builder._SelectedItem.OnNext(selectedValue);
+            };
+            return listBox;
+        }
+    }
+}

src/Bonsai.Gui/ListControlDataSourceBuilderBase.cs

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+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Drawing.Design;
+using System.Linq;
+using System.Reactive;
+using System.Reactive.Disposables;
+using System.Reactive.Linq;
+using System.Reactive.Subjects;
+
+namespace Bonsai.Gui
+{
+    /// <summary>
+    /// Provides an abstract base class for interfacing with combo box and list box controls
+    /// bound to each data source in an observable sequence.
+    /// </summary>
+    public class ListControlDataSourceBuilderBase : DataSourceControlBuilderBase
+    {
+        internal readonly BehaviorSubject<string> _DisplayMember = new(string.Empty);
+        internal readonly BehaviorSubject<object> _DataSource = new(null);
+        internal readonly BehaviorSubject<object> _SelectedItem = new(null);
+
+        /// <summary>
+        /// Gets or sets the property to display for this list control.
+        /// </summary>
+        [Editor(typeof(DataMemberSelectorEditor), typeof(UITypeEditor))]
+        [Description("The property to display for this list control.")]
+        public string DisplayMember
+        {
+            get => _DisplayMember.Value;
+            set => _DisplayMember.OnNext(value);
+        }
+
+        /// <summary>
+        /// Generates an observable sequence of values containing the currently
+        /// selected item in the list control whenever the selection changes.
+        /// </summary>
+        /// <typeparam name="TValue">
+        /// The type of the values in the data source.
+        /// </typeparam>
+        /// <param name="source">
+        /// A sequence of collections representing the data sources to bind to
+        /// the list control. Only one collection is bound at any one time.
+        /// </param>
+        /// <returns>
+        /// A sequence of values representing the currently selected item in
+        /// the list control.
+        /// </returns>
+        protected override IObservable<TValue> Generate<TValue>(IObservable<IList<TValue>> source)
+        {
+            return Observable.Create<TValue>(observer =>
+            {
+                var sourceObserver = Observer.Create<IList<TValue>>(collection =>
+                {
+                    _SelectedItem.OnNext(null);
+                    _DataSource.OnNext(collection);
+                }, observer.OnError);
+                var selectedItem = _SelectedItem.Where(value => value is TValue).Cast<TValue>();
+                return new CompositeDisposable(
+                    selectedItem.SubscribeSafe(observer),
+                    source.SubscribeSafe(sourceObserver),
+                    Disposable.Create(() =>
+                    {
+                        _DataSource.OnNext(null);
+                        _SelectedItem.OnNext(null);
+                    }));
+            });
+        }
+    }
+}