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using System;
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using Implab.Parallels;
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using System.Threading;
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using System.Reflection;
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using System.Diagnostics;
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namespace Implab {
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/// <summary>
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/// Abstract class for creation of custom one-shot thread safe events.
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/// </summary>
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/// <remarks>
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/// <para>
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/// An event is something that should happen in the future and the
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/// triggering of the event causes execution of some pending actions
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/// which are formely event handlers. One-shot events occur only once
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/// and any handler added after the event is triggered should run
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/// without a delay.
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/// </para>
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/// <para>
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/// The lifecycle of the one-shot event is tipically consists of following
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/// phases.
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/// <list>
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/// <description>Pending state. This is the initial state of the event. Any
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/// handler added to the event will be queued for the future execution.
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/// </description>
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/// <description>Transitional state. This is intermediate state between pending
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/// and fulfilled states, during this state internal initialization and storing
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/// of the result occurs.
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/// </description>
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/// <description>Fulfilled state. The event contains the result, all queued
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/// handlers are signalled to run and newly added handlers are executed
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/// immediatelly.
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/// </description>
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/// </list>
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/// </para>
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/// </remarks>
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public abstract class AbstractEvent<THandler> where THandler : class {
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const int PendingState = 0;
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const int TransitionalState = 1;
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const int ResolvedState = 2;
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volatile int m_state;
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THandler m_handler;
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SimpleAsyncQueue<THandler> m_extraHandlers;
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public bool IsResolved {
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get {
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return m_state > TransitionalState;
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}
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}
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#region state managment
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protected bool BeginTransit() {
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return PendingState == Interlocked.CompareExchange(ref m_state, TransitionalState, PendingState);
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}
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protected void CompleteTransit() {
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#if DEBUG
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if (TransitionalState != Interlocked.CompareExchange(ref m_state, ResolvedState, TransitionalState))
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throw new InvalidOperationException("Can't complete transition when the object isn't in the transitional state");
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#else
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m_state = ResolvedState;
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#endif
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Signal();
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}
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protected void WaitTransition() {
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if (m_state == TransitionalState) {
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SpinWait spin = new SpinWait();
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do {
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spin.SpinOnce();
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} while (m_state == TransitionalState);
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}
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}
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protected abstract void SignalHandler(THandler handler);
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void Signal() {
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THandler handler;
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while (TryDequeueHandler(out handler))
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SignalHandler(handler);
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}
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#endregion
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#region handlers managment
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protected void AddHandler(THandler handler) {
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if (IsResolved) {
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// the promise is in the resolved state, just invoke the handler
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SignalHandler(handler);
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} else {
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EnqueueHandler(handler);
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if (IsResolved && TryDequeueHandler(out handler))
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// if the promise have been resolved while we was adding the handler to the queue
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// we can't guarantee that someone is still processing it
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// therefore we need to fetch a handler from the queue and execute it
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// note that fetched handler may be not the one that we have added
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// even we can fetch no handlers at all :)
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SignalHandler(handler);
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}
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}
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void EnqueueHandler(THandler handler) {
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if (Interlocked.CompareExchange(ref m_handler, handler, null) != null) {
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if (m_extraHandlers == null)
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// compare-exchange will protect from loosing already created queue
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Interlocked.CompareExchange(ref m_extraHandlers, new SimpleAsyncQueue<THandler>(), null);
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m_extraHandlers.Enqueue(handler);
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}
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}
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bool TryDequeueHandler(out THandler handler) {
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handler = Interlocked.Exchange(ref m_handler, null);
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if (handler != null)
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return true;
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return m_extraHandlers != null && m_extraHandlers.TryDequeue(out handler);
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}
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#endregion
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}
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}
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