Avalonia/src/Markup/Avalonia.Markup/Data/MultiBinding.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Reactive.Linq;
using Avalonia.Data.Converters;
using Avalonia.Metadata;

namespace Avalonia.Data
{
    /// <summary>
    /// A XAML binding that calculates an aggregate value from multiple child <see cref="Bindings"/>.
    /// </summary>
    public class MultiBinding : IBinding
    {
        /// <summary>
        /// Gets the collection of child bindings.
        /// </summary>
        [Content, AssignBinding]
        public IList<IBinding> Bindings { get; set; } = new List<IBinding>();

        /// <summary>
        /// Gets or sets the <see cref="IMultiValueConverter"/> to use.
        /// </summary>
        public IMultiValueConverter Converter { get; set; }

        /// <summary>
        /// Gets or sets a parameter to pass to <see cref="Converter"/>.
        /// </summary>
        public object ConverterParameter { get; set; }

        /// <summary>
        /// Gets or sets the value to use when the binding is unable to produce a value.
        /// </summary>
        public object FallbackValue { get; set; }

        /// <summary>
        /// Gets or sets the value to use when the binding result is null.
        /// </summary>
        public object TargetNullValue { get; set; }

        /// <summary>
        /// Gets or sets the binding mode.
        /// </summary>
        public BindingMode Mode { get; set; } = BindingMode.OneWay;

        /// <summary>
        /// Gets or sets the binding priority.
        /// </summary>
        public BindingPriority Priority { get; set; }
        
        /// <summary>
        /// Gets or sets the relative source for the binding.
        /// </summary>
        public RelativeSource RelativeSource { get; set; }

        /// <summary>
        /// Gets or sets the string format.
        /// </summary>
        public string StringFormat { get; set; }

        public MultiBinding()
        {
            FallbackValue = AvaloniaProperty.UnsetValue;
            TargetNullValue = AvaloniaProperty.UnsetValue;
        }

        /// <inheritdoc/>
        public InstancedBinding Initiate(
            IAvaloniaObject target,
            AvaloniaProperty targetProperty,
            object anchor = null,
            bool enableDataValidation = false)
        {
            var targetType = targetProperty?.PropertyType ?? typeof(object);
            var converter = Converter;
            // We only respect `StringFormat` if the type of the property we're assigning to will
            // accept a string. Note that this is slightly different to WPF in that WPF only applies
            // `StringFormat` for target type `string` (not `object`).
            if (!string.IsNullOrWhiteSpace(StringFormat) && 
                (targetType == typeof(string) || targetType == typeof(object)))
            {
                converter = new StringFormatMultiValueConverter(StringFormat, converter);
            }
            
            var children = Bindings.Select(x => x.Initiate(target, null));

            var input = children.Select(x => x.Observable)
                                .CombineLatest()
                                .Select(x => ConvertValue(x, targetType, converter))
                                .Where(x => x != BindingOperations.DoNothing);

            var mode = Mode == BindingMode.Default ?
                targetProperty?.GetMetadata(target.GetType()).DefaultBindingMode : Mode;

            switch (mode)
            {
                case BindingMode.OneTime:
                    return InstancedBinding.OneTime(input, Priority);
                case BindingMode.OneWay:
                    return InstancedBinding.OneWay(input, Priority);
                default:
                    throw new NotSupportedException(
                        "MultiBinding currently only supports OneTime and OneWay BindingMode.");
            }
        }

        private object ConvertValue(IList<object> values, Type targetType, IMultiValueConverter converter)
        {
            for (var i = 0; i < values.Count; ++i)
            {
                if (values[i] is BindingNotification notification)
                {
                    values[i] = notification.Value;
                }
            }

            var culture = CultureInfo.CurrentCulture;
            var converted = converter.Convert(values, targetType, ConverterParameter, culture);

            if (converted == null)
            {
                converted = TargetNullValue;
            }

            if (converted == AvaloniaProperty.UnsetValue)
            {
                converted = FallbackValue;
            }

            return converted;
        }
    }
}