chromium之message_pump_win之二

  // MessagePump methods:
  virtual void ScheduleWork();
  virtual void ScheduleDelayedWork(const Time& delayed_work_time);

  virtual void DoRunLoop();

//-----------------------------------------------------------------------------
// MessagePumpForUI extends MessagePumpWin with methods that are particular to a
// MessageLoop instantiated with TYPE_UI.
//
// MessagePumpForUI implements a "traditional" Windows message pump. It contains
// a nearly infinite loop that peeks out messages, and then dispatches them.
// Intermixed with those peeks are callouts to DoWork for pending tasks, and
// DoDelayedWork for pending timers. When there are no events to be serviced,
// this pump goes into a wait state. In most cases, this message pump handles
// all processing.
//
// However, when a task, or windows event, invokes on the stack a native dialog
// box or such, that window typically provides a bare bones (native?) message
// pump.  That bare-bones message pump generally supports little more than a
// peek of the Windows message queue, followed by a dispatch of the peeked
// message.  MessageLoop extends that bare-bones message pump to also service
// Tasks, at the cost of some complexity.
//
// The basic structure of the extension (refered to as a sub-pump) is that a
// special message, kMsgHaveWork, is repeatedly injected into the Windows
// Message queue.  Each time the kMsgHaveWork message is peeked, checks are
// made for an extended set of events, including the availability of Tasks to
// run.
//
// After running a task, the special message kMsgHaveWork is again posted to
// the Windows Message queue, ensuring a future time slice for processing a
// future event.  To prevent flooding the Windows Message queue, care is taken
// to be sure that at most one kMsgHaveWork message is EVER pending in the
// Window‘s Message queue.
//
// There are a few additional complexities in this system where, when there are
// no Tasks to run, this otherwise infinite stream of messages which drives the
// sub-pump is halted.  The pump is automatically re-started when Tasks are
// queued.
//
// A second complexity is that the presence of this stream of posted tasks may
// prevent a bare-bones message pump from ever peeking a WM_PAINT or WM_TIMER.
// Such paint and timer events always give priority to a posted message, such as
// kMsgHaveWork messages.  As a result, care is taken to do some peeking in
// between the posting of each kMsgHaveWork message (i.e., after kMsgHaveWork
// is peeked, and before a replacement kMsgHaveWork is posted).
//
// NOTE: Although it may seem odd that messages are used to start and stop this
// flow (as opposed to signaling objects, etc.), it should be understood that
// the native message pump will *only* respond to messages.  As a result, it is
// an excellent choice.  It is also helpful that the starter messages that are
// placed in the queue when new task arrive also awakens DoRunLoop.
//

void MessagePumpForUI::ScheduleWork() {
  if (InterlockedExchange(&have_work_, 1))
    return;  // Someone else continued the pumping.

  // Make sure the MessagePump does some work for us.
  PostMessage(message_hwnd_, kMsgHaveWork, reinterpret_cast<WPARAM>(this), 0);
}

void MessagePumpForUI::ScheduleDelayedWork(const Time& delayed_work_time) {
  //
  // We would *like* to provide high resolution timers.  Windows timers using
  // SetTimer() have a 10ms granularity.  We have to use WM_TIMER as a wakeup
  // mechanism because the application can enter modal windows loops where it
  // is not running our MessageLoop; the only way to have our timers fire in
  // these cases is to post messages there.
  //
  // To provide sub-10ms timers, we process timers directly from our run loop.
  // For the common case, timers will be processed there as the run loop does
  // its normal work.  However, we *also* set the system timer so that WM_TIMER
  // events fire.  This mops up the case of timers not being able to work in
  // modal message loops.  It is possible for the SetTimer to pop and have no
  // pending timers, because they could have already been processed by the
  // run loop itself.
  //
  // We use a single SetTimer corresponding to the timer that will expire
  // soonest.  As new timers are created and destroyed, we update SetTimer.
  // Getting a spurrious SetTimer event firing is benign, as we‘ll just be
  // processing an empty timer queue.
  //
  delayed_work_time_ = delayed_work_time;

  int delay_msec = GetCurrentDelay();
  DCHECK(delay_msec >= 0);
  if (delay_msec < USER_TIMER_MINIMUM)
    delay_msec = USER_TIMER_MINIMUM;

  // Create a WM_TIMER event that will wake us up to check for any pending
  // timers (in case we are running within a nested, external sub-pump).
  SetTimer(message_hwnd_, reinterpret_cast<UINT_PTR>(this), delay_msec, NULL);
}

void MessagePumpForUI::DoRunLoop() {
  // IF this was just a simple PeekMessage() loop (servicing all possible work
  // queues), then Windows would try to achieve the following order according
  // to MSDN documentation about PeekMessage with no filter):
  //    * Sent messages
  //    * Posted messages
  //    * Sent messages (again)
  //    * WM_PAINT messages
  //    * WM_TIMER messages
  //
  // Summary: none of the above classes is starved, and sent messages has twice
  // the chance of being processed (i.e., reduced service time).

  for (;;) {
    // If we do any work, we may create more messages etc., and more work may
    // possibly be waiting in another task group.  When we (for example)
    // ProcessNextWindowsMessage(), there is a good chance there are still more
    // messages waiting.  On the other hand, when any of these methods return
    // having done no work, then it is pretty unlikely that calling them again
    // quickly will find any work to do.  Finally, if they all say they had no
    // work, then it is a good time to consider sleeping (waiting) for more
    // work.

    bool more_work_is_plausible = ProcessNextWindowsMessage();
    if (state_->should_quit)
      break;

    more_work_is_plausible |= state_->delegate->DoWork();
    if (state_->should_quit)
      break;

    more_work_is_plausible |=
        state_->delegate->DoDelayedWork(&delayed_work_time_);
    // If we did not process any delayed work, then we can assume that our
    // existing WM_TIMER if any will fire when delayed work should run.  We
    // don‘t want to disturb that timer if it is already in flight.  However,
    // if we did do all remaining delayed work, then lets kill the WM_TIMER.
    if (more_work_is_plausible && delayed_work_time_.is_null())
      KillTimer(message_hwnd_, reinterpret_cast<UINT_PTR>(this));
    if (state_->should_quit)
      break;

    if (more_work_is_plausible)
      continue;

    more_work_is_plausible = state_->delegate->DoIdleWork();
    if (state_->should_quit)
      break;

    if (more_work_is_plausible)
      continue;

    WaitForWork();  // Wait (sleep) until we have work to do again.
  }
}