gecko/xpcom/threads/SyncRunnable.h

/* -*- Mode: C++; tab-width: 12; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef mozilla_SyncRunnable_h
#define mozilla_SyncRunnable_h

#include "nsThreadUtils.h"
#include "mozilla/Monitor.h"

namespace mozilla {

/**
 * This class will wrap a nsIRunnable and dispatch it to the main thread
 * synchronously. This is different from nsIEventTarget.DISPATCH_SYNC:
 * this class does not spin the event loop waiting for the event to be
 * dispatched. This means that you don't risk reentrance from pending
 * messages, but you must be sure that the target thread does not ever block
 * on this thread, or else you will deadlock.
 *
 * Typical usage:
 * nsRefPtr<SyncRunnable> sr = new SyncRunnable(new myrunnable...());
 * sr->DispatchToThread(t);
 *
 * We also provide a convenience wrapper:
 * SyncRunnable::DispatchToThread(new myrunnable...());
 *
 */
class SyncRunnable : public nsRunnable
{
public:
  SyncRunnable(nsIRunnable* r)
    : mRunnable(r)
    , mMonitor("SyncRunnable")
    , mDone(false)
  { }

  void DispatchToThread(nsIEventTarget* thread,
                        bool forceDispatch = false)
  {
    nsresult rv;
    bool on;

    if (!forceDispatch) {
      rv = thread->IsOnCurrentThread(&on);
      MOZ_ASSERT(NS_SUCCEEDED(rv));
      if (NS_SUCCEEDED(rv) && on) {
        mRunnable->Run();
        return;
      }
    }

    rv = thread->Dispatch(this, NS_DISPATCH_NORMAL);
    if (NS_SUCCEEDED(rv)) {
      mozilla::MonitorAutoLock lock(mMonitor);
      while (!mDone) {
        lock.Wait();
      }
    }
  }

  static void DispatchToThread(nsIEventTarget* thread,
                               nsIRunnable* r,
                               bool forceDispatch = false)
  {
    nsRefPtr<SyncRunnable> s(new SyncRunnable(r));
    s->DispatchToThread(thread, forceDispatch);
  }

protected:
  NS_IMETHODIMP Run()
  {
    mRunnable->Run();

    mozilla::MonitorAutoLock lock(mMonitor);
    MOZ_ASSERT(!mDone);

    mDone = true;
    mMonitor.Notify();

    return NS_OK;
  }

private:
  nsCOMPtr<nsIRunnable> mRunnable;
  mozilla::Monitor mMonitor;
  bool mDone;
};

} // namespace mozilla

#endif // mozilla_SyncRunnable_h