Avalonia/samples/MiniMvvm/PropertyChangedExtensions.cs

using System;
using System.ComponentModel;
using System.Linq.Expressions;
using System.Reflection;
using Avalonia.Reactive;

namespace MiniMvvm
{
    public static class PropertyChangedExtensions
    {
        class PropertyObservable<T> : IObservable<T>
        {
            private readonly INotifyPropertyChanged _target;
            private readonly PropertyInfo _info;

            public PropertyObservable(INotifyPropertyChanged target, PropertyInfo info)
            {
                _target = target;
                _info = info;
            }

            class Subscription : IDisposable
            {
                private readonly INotifyPropertyChanged _target;
                private readonly PropertyInfo _info;
                private readonly IObserver<T> _observer;

                public Subscription(INotifyPropertyChanged target, PropertyInfo info, IObserver<T> observer)
                {
                    _target = target;
                    _info = info;
                    _observer = observer;
                    _target.PropertyChanged += OnPropertyChanged;
                    _observer.OnNext((T)_info.GetValue(_target));
                }

                private void OnPropertyChanged(object sender, PropertyChangedEventArgs e)
                {
                    if (e.PropertyName == _info.Name)
                        _observer.OnNext((T)_info.GetValue(_target));
                }

                public void Dispose()
                {
                    _target.PropertyChanged -= OnPropertyChanged;
                    _observer.OnCompleted();
                }
            }

            public IDisposable Subscribe(IObserver<T> observer)
            {
                return new Subscription(_target, _info, observer);
            }
        }

        public static IObservable<TRes> WhenAnyValue<TModel, TRes>(this TModel model,
            Expression<Func<TModel, TRes>> expr) where TModel : INotifyPropertyChanged
        {
            var l = (LambdaExpression)expr;
            var ma = (MemberExpression)l.Body;
            var prop = (PropertyInfo)ma.Member;
            return new PropertyObservable<TRes>(model, prop);
        }
        
        public static IObservable<TRes> WhenAnyValue<TModel, T1, TRes>(this TModel model,
            Expression<Func<TModel, T1>> v1,
            Func<T1,  TRes> cb
        ) where TModel : INotifyPropertyChanged
        {
            return model.WhenAnyValue(v1).Select(cb);
        }
        
        public static IObservable<TRes> WhenAnyValue<TModel, T1, T2, TRes>(this TModel model,
            Expression<Func<TModel, T1>> v1,
            Expression<Func<TModel, T2>> v2,
            Func<T1, T2, TRes> cb
        ) where TModel : INotifyPropertyChanged =>
            Observable.CombineLatest(
                model.WhenAnyValue(v1),
                model.WhenAnyValue(v2),
                cb);

        public static IObservable<ValueTuple<T1, T2>> WhenAnyValue<TModel, T1, T2>(this TModel model,
            Expression<Func<TModel, T1>> v1,
            Expression<Func<TModel, T2>> v2
        ) where TModel : INotifyPropertyChanged =>
            model.WhenAnyValue(v1, v2, (a1, a2) => (a1, a2));

        public static IObservable<TRes> WhenAnyValue<TModel, T1, T2, T3, TRes>(this TModel model,
            Expression<Func<TModel, T1>> v1,
            Expression<Func<TModel, T2>> v2,
            Expression<Func<TModel, T3>> v3,
            Func<T1, T2, T3, TRes> cb
        ) where TModel : INotifyPropertyChanged =>
            model.WhenAnyValue(v1)
                .CombineLatest(
                    model.WhenAnyValue(v2),
                    (l, r) => (l, r))
                .CombineLatest(
                    model.WhenAnyValue(v3),
                    (t, r) => cb(t.l, t.r, r));

        public static IObservable<ValueTuple<T1, T2, T3>> WhenAnyValue<TModel, T1, T2, T3>(this TModel model,
            Expression<Func<TModel, T1>> v1,
            Expression<Func<TModel, T2>> v2,
            Expression<Func<TModel, T3>> v3
        ) where TModel : INotifyPropertyChanged =>
            model.WhenAnyValue(v1, v2, v3, (a1, a2, a3) => (a1, a2, a3));
    }
}