Avalonia/src/Avalonia.Base/PropertyStore/BindingEntry.cs

using System;
using Avalonia.Data;
using Avalonia.Threading;

namespace Avalonia.PropertyStore
{
    /// <summary>
    /// Represents an untyped interface to <see cref="BindingEntry{T}"/>.
    /// </summary>
    internal interface IBindingEntry : IBatchUpdate, IPriorityValueEntry, IDisposable
    {
        void Start(bool ignoreBatchUpdate);
    }

    /// <summary>
    /// Stores a binding in a <see cref="ValueStore"/> or <see cref="PriorityValue{T}"/>.
    /// </summary>
    /// <typeparam name="T">The property type.</typeparam>
    internal class BindingEntry<T> : IBindingEntry, IPriorityValueEntry<T>, IObserver<BindingValue<T>>
    {
        private readonly AvaloniaObject _owner;
        private ValueOwner<T> _sink;
        private IDisposable? _subscription;
        private bool _isSubscribed;
        private bool _batchUpdate;
        private Optional<T> _value;

        public BindingEntry(
            AvaloniaObject owner,
            StyledPropertyBase<T> property,
            IObservable<BindingValue<T>> source,
            BindingPriority priority,
            ValueOwner<T> sink)
        {
            _owner = owner;
            Property = property;
            Source = source;
            Priority = priority;
            _sink = sink;
        }

        public StyledPropertyBase<T> Property { get; }
        public BindingPriority Priority { get; private set; }
        public IObservable<BindingValue<T>> Source { get; }
        Optional<object?> IValue.GetValue() => _value.ToObject();

        public void BeginBatchUpdate() => _batchUpdate = true;

        public void EndBatchUpdate()
        {
            _batchUpdate = false;

            if (_sink.IsValueStore)
                Start();
        }

        public Optional<T> GetValue(BindingPriority maxPriority)
        {
            return Priority >= maxPriority ? _value : Optional<T>.Empty;
        }

        public void Dispose()
        {
            _subscription?.Dispose();
            _subscription = null;
            OnCompleted();
        }

        public void OnCompleted()
        {
            var oldValue = _value;
            _value = default;
            Priority = BindingPriority.Unset;
            _isSubscribed = false;
            _sink.Completed(Property, this, oldValue);
        }

        public void OnError(Exception error)
        {
            throw new NotImplementedException("BindingEntry.OnError is not implemented", error);
        }

        public void OnNext(BindingValue<T> value)
        {
            if (Dispatcher.UIThread.CheckAccess())
            {
                UpdateValue(value); 
            }
            else
            {
                // To avoid allocating closure in the outer scope we need to capture variables
                // locally. This allows us to skip most of the allocations when on UI thread.
                var instance = this;
                var newValue = value;

                Dispatcher.UIThread.Post(() => instance.UpdateValue(newValue));
            }
        }

        public void Start() => Start(false);

        public void Start(bool ignoreBatchUpdate)
        {
            // We can't use _subscription to check whether we're subscribed because it won't be set
            // until Subscribe has finished, which will be too late to prevent reentrancy. In addition
            // don't re-subscribe to completed/disposed bindings (indicated by Unset priority).
            if (!_isSubscribed &&
                Priority != BindingPriority.Unset &&
                (!_batchUpdate || ignoreBatchUpdate))
            {
                _isSubscribed = true;
                _subscription = Source.Subscribe(this);
            }
        }

        public void Reparent(PriorityValue<T> parent) => _sink = new(parent);

        public void RaiseValueChanged(
            AvaloniaObject owner,
            AvaloniaProperty property,
            Optional<object?> oldValue,
            Optional<object?> newValue)
        {
            owner.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
                owner,
                (AvaloniaProperty<T>)property,
                oldValue.Cast<T>(),
                newValue.Cast<T>(),
                Priority));
        }

        private void UpdateValue(BindingValue<T> value)
        {
            if (value.HasValue && Property.ValidateValue?.Invoke(value.Value) == false)
            {
                value = Property.GetDefaultValue(_owner.GetType());
            }

            if (value.Type == BindingValueType.DoNothing)
            {
                return;
            }

            var old = _value;

            if (value.Type != BindingValueType.DataValidationError)
            {
                _value = value.ToOptional();
            }

            _sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(_owner, Property, old, value, Priority));
        }
    }
}