// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved. See License.txt in the project root for license information.

using System.ComponentModel;
using System.Reactive;
using System.Reactive.Disposables;

namespace System
{
    /// <summary>
    /// Provides a set of static methods for subscribing delegates to observables.
    /// </summary>
    public static class ObservableExtensions
    {
        /// <summary>
        /// Subscribes to the observable sequence without specifying any handlers.
        /// This method can be used to evaluate the observable sequence for its side-effects only.
        /// </summary>
        /// <typeparam name="T">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Observable sequence to subscribe to.</param>
        /// <returns>IDisposable object used to unsubscribe from the observable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        public static IDisposable Subscribe<T>(this IObservable<T> source)
        {
            if (source == null)
                throw new ArgumentNullException("source");

            //
            // [OK] Use of unsafe Subscribe: non-pretentious constructor for an observer; this overload is not to be used internally.
            //
            return source.Subscribe/*Unsafe*/(new AnonymousObserver<T>(Stubs<T>.Ignore, Stubs.Throw, Stubs.Nop));
        }

        /// <summary>
        /// Subscribes an element handler to an observable sequence.
        /// </summary>
        /// <typeparam name="T">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Observable sequence to subscribe to.</param>
        /// <param name="onNext">Action to invoke for each element in the observable sequence.</param>
        /// <returns>IDisposable object used to unsubscribe from the observable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="onNext"/> is null.</exception>
        public static IDisposable Subscribe<T>(this IObservable<T> source, Action<T> onNext)
        {
            if (source == null)
                throw new ArgumentNullException("source");
            if (onNext == null)
                throw new ArgumentNullException("onNext");

            //
            // [OK] Use of unsafe Subscribe: non-pretentious constructor for an observer; this overload is not to be used internally.
            //
            return source.Subscribe/*Unsafe*/(new AnonymousObserver<T>(onNext, Stubs.Throw, Stubs.Nop));
        }

        /// <summary>
        /// Subscribes an element handler and an exception handler to an observable sequence.
        /// </summary>
        /// <typeparam name="T">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Observable sequence to subscribe to.</param>
        /// <param name="onNext">Action to invoke for each element in the observable sequence.</param>
        /// <param name="onError">Action to invoke upon exceptional termination of the observable sequence.</param>
        /// <returns>IDisposable object used to unsubscribe from the observable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="onNext"/> or <paramref name="onError"/> is null.</exception>
        public static IDisposable Subscribe<T>(this IObservable<T> source, Action<T> onNext, Action<Exception> onError)
        {
            if (source == null)
                throw new ArgumentNullException("source");
            if (onNext == null)
                throw new ArgumentNullException("onNext");
            if (onError == null)
                throw new ArgumentNullException("onError");

            //
            // [OK] Use of unsafe Subscribe: non-pretentious constructor for an observer; this overload is not to be used internally.
            //
            return source.Subscribe/*Unsafe*/(new AnonymousObserver<T>(onNext, onError, Stubs.Nop));
        }

        /// <summary>
        /// Subscribes an element handler and a completion handler to an observable sequence.
        /// </summary>
        /// <typeparam name="T">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Observable sequence to subscribe to.</param>
        /// <param name="onNext">Action to invoke for each element in the observable sequence.</param>
        /// <param name="onCompleted">Action to invoke upon graceful termination of the observable sequence.</param>
        /// <returns>IDisposable object used to unsubscribe from the observable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="onNext"/> or <paramref name="onCompleted"/> is null.</exception>
        public static IDisposable Subscribe<T>(this IObservable<T> source, Action<T> onNext, Action onCompleted)
        {
            if (source == null)
                throw new ArgumentNullException("source");
            if (onNext == null)
                throw new ArgumentNullException("onNext");
            if (onCompleted == null)
                throw new ArgumentNullException("onCompleted");

            //
            // [OK] Use of unsafe Subscribe: non-pretentious constructor for an observer; this overload is not to be used internally.
            //
            return source.Subscribe/*Unsafe*/(new AnonymousObserver<T>(onNext, Stubs.Throw, onCompleted));
        }

        /// <summary>
        /// Subscribes an element handler, an exception handler, and a completion handler to an observable sequence.
        /// </summary>
        /// <typeparam name="T">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Observable sequence to subscribe to.</param>
        /// <param name="onNext">Action to invoke for each element in the observable sequence.</param>
        /// <param name="onError">Action to invoke upon exceptional termination of the observable sequence.</param>
        /// <param name="onCompleted">Action to invoke upon graceful termination of the observable sequence.</param>
        /// <returns>IDisposable object used to unsubscribe from the observable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="onNext"/> or <paramref name="onError"/> or <paramref name="onCompleted"/> is null.</exception>
        public static IDisposable Subscribe<T>(this IObservable<T> source, Action<T> onNext, Action<Exception> onError, Action onCompleted)
        {
            if (source == null)
                throw new ArgumentNullException("source");
            if (onNext == null)
                throw new ArgumentNullException("onNext");
            if (onError == null)
                throw new ArgumentNullException("onError");
            if (onCompleted == null)
                throw new ArgumentNullException("onCompleted");

            //
            // [OK] Use of unsafe Subscribe: non-pretentious constructor for an observer; this overload is not to be used internally.
            //
            return source.Subscribe/*Unsafe*/(new AnonymousObserver<T>(onNext, onError, onCompleted));
        }

        /// <summary>
        /// Subscribes to the specified source, re-routing synchronous exceptions during invocation of the Subscribe method to the observer's OnError channel.
        /// This method is typically used when writing query operators.
        /// </summary>
        /// <typeparam name="T">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Observable sequence to subscribe to.</param>
        /// <param name="observer">Observer that will be passed to the observable sequence, and that will be used for exception propagation.</param>
        /// <returns>IDisposable object used to unsubscribe from the observable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="observer"/> is null.</exception>
        [EditorBrowsable(EditorBrowsableState.Advanced)]
        public static IDisposable SubscribeSafe<T>(this IObservable<T> source, IObserver<T> observer)
        {
            if (source == null)
                throw new ArgumentNullException("source");
            if (observer == null)
                throw new ArgumentNullException("observer");

            //
            // The following types are white-listed and should not exhibit exceptional behavior
            // for regular operation circumstances.
            //
            if (source is ObservableBase<T>)
                return source.Subscribe(observer);

#if !NO_PERF
            var producer = source as Producer<T>;
            if (producer != null)
                return producer.SubscribeRaw(observer, false);
#endif

            var d = Disposable.Empty;

            try
            {
                d = source.Subscribe(observer);
            }
            catch (Exception exception)
            {
                //
                // The effect of redirecting the exception to the OnError channel is automatic
                // clean-up of query operator state for a large number of cases. For example,
                // consider a binary and temporal query operator with the following Subscribe
                // behavior (implemented using the Producer pattern with a Run method):
                //
                //   public IDisposable Run(...)
                //   {
                //       var tm = _scheduler.Schedule(_due, Tick);
                //
                //       var df = _fst.SubscribeSafe(new FstObserver(this, ...));
                //       var ds = _snd.SubscribeSafe(new SndObserver(this, ...)); // <-- fails
                //
                //       return new CompositeDisposable(tm, df, ds);
                //   }
                //
                // If the second subscription fails, we're not leaving the first subscription
                // or the scheduled job hanging around. Instead, the OnError propagation to
                // the SndObserver should take care of a Dispose call to the observer's parent
                // object. The handshake between Producer and Sink objects will ultimately
                // cause disposal of the CompositeDisposable that's returned from the method.
                //
                observer.OnError(exception);
            }

            return d;
        }
    }
}