JUCE/modules/juce_events/broadcasters/juce_AsyncUpdater.cpp

/*
  ==============================================================================

   This file is part of the JUCE library.
   Copyright (c) 2017 - ROLI Ltd.

   JUCE is an open source library subject to commercial or open-source
   licensing.

   The code included in this file is provided under the terms of the ISC license
   http://www.isc.org/downloads/software-support-policy/isc-license. Permission
   To use, copy, modify, and/or distribute this software for any purpose with or
   without fee is hereby granted provided that the above copyright notice and
   this permission notice appear in all copies.

   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
   DISCLAIMED.

  ==============================================================================
*/

class AsyncUpdater::AsyncUpdaterMessage  : public CallbackMessage
{
public:
    AsyncUpdaterMessage (AsyncUpdater& au)  : owner (au) {}

    void messageCallback() override
    {
        if (shouldDeliver.compareAndSetBool (0, 1))
            owner.handleAsyncUpdate();
    }

    AsyncUpdater& owner;
    Atomic<int> shouldDeliver;

    JUCE_DECLARE_NON_COPYABLE (AsyncUpdaterMessage)
};

//==============================================================================
AsyncUpdater::AsyncUpdater()
{
    activeMessage = new AsyncUpdaterMessage (*this);
}

AsyncUpdater::~AsyncUpdater()
{
    // You're deleting this object with a background thread while there's an update
    // pending on the main event thread - that's pretty dodgy threading, as the callback could
    // happen after this destructor has finished. You should either use a MessageManagerLock while
    // deleting this object, or find some other way to avoid such a race condition.
    jassert ((! isUpdatePending())
              || MessageManager::getInstanceWithoutCreating() == nullptr
              || MessageManager::getInstanceWithoutCreating()->currentThreadHasLockedMessageManager());

    activeMessage->shouldDeliver.set (0);
}

void AsyncUpdater::triggerAsyncUpdate()
{
    // If you're calling this before (or after) the MessageManager is
    // running, then you're not going to get any callbacks!
    jassert (MessageManager::getInstanceWithoutCreating() != nullptr);

    if (activeMessage->shouldDeliver.compareAndSetBool (1, 0))
        if (! activeMessage->post())
            cancelPendingUpdate(); // if the message queue fails, this avoids getting
                                   // trapped waiting for the message to arrive
}

void AsyncUpdater::cancelPendingUpdate() noexcept
{
    activeMessage->shouldDeliver.set (0);
}

void AsyncUpdater::handleUpdateNowIfNeeded()
{
    // This can only be called by the event thread.
    jassert (MessageManager::getInstance()->currentThreadHasLockedMessageManager());

    if (activeMessage->shouldDeliver.exchange (0) != 0)
        handleAsyncUpdate();
}

bool AsyncUpdater::isUpdatePending() const noexcept
{
    return activeMessage->shouldDeliver.value != 0;
}