Avalonia/src/Perspex.Base/PriorityBindingEntry.cs

// Copyright (c) The Perspex Project. All rights reserved.
// Licensed under the MIT license. See licence.md file in the project root for full license information.

using System;
using Perspex.Data;

namespace Perspex
{
    /// <summary>
    /// A registered binding in a <see cref="PriorityValue"/>.
    /// </summary>
    internal class PriorityBindingEntry : IDisposable
    {
        /// <summary>
        /// The binding subscription.
        /// </summary>
        private IDisposable _subscription;

        /// <summary>
        /// Initializes a new instance of the <see cref="PriorityBindingEntry"/> class.
        /// </summary>
        /// <param name="index">
        /// The binding index. Later bindings should have higher indexes.
        /// </param>
        public PriorityBindingEntry(int index)
        {
            Index = index;
        }

        /// <summary>
        /// Gets the observable associated with the entry.
        /// </summary>
        public IObservable<object> Observable { get; private set; }

        /// <summary>
        /// Gets a description of the binding.
        /// </summary>
        public string Description
        {
            get;
            private set;
        }

        /// <summary>
        /// Gets the binding entry index. Later bindings will have higher indexes.
        /// </summary>
        public int Index
        {
            get;
        }

        /// <summary>
        /// The current value of the binding.
        /// </summary>
        public object Value
        {
            get;
            private set;
        }

        /// <summary>
        /// Starts listening to the binding.
        /// </summary>
        /// <param name="binding">The binding.</param>
        /// <param name="changed">Called when the binding changes.</param>
        /// <param name="completed">Called when the binding completes.</param>
        /// <param name="error">Called when a binding error occurs.</param>
        public void Start(
            IObservable<object> binding,
            Action<PriorityBindingEntry> changed,
            Action<PriorityBindingEntry> completed,
            Action<PriorityBindingEntry, BindingError> error)
        {
            Contract.Requires<ArgumentNullException>(binding != null);
            Contract.Requires<ArgumentNullException>(changed != null);
            Contract.Requires<ArgumentNullException>(completed != null);

            if (_subscription != null)
            {
                throw new Exception("PriorityValue.Entry.Start() called more than once.");
            }

            Observable = binding;
            Value = PerspexProperty.UnsetValue;

            if (binding is IDescription)
            {
                Description = ((IDescription)binding).Description;
            }

            _subscription = binding.Subscribe(
                value =>
                {
                    var bindingError = value as BindingError;

                    if (bindingError == null)
                    {
                        Value = value;
                        changed(this);
                    }
                    else if (error != null)
                    {
                        error(this, bindingError);
                    }
                },
                () => completed(this));
        }

        /// <summary>
        /// Ends the binding subscription.
        /// </summary>
        public void Dispose()
        {
            _subscription?.Dispose();
        }
    }
}