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// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_
#define BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_
#include "base/base_export.h"
#include "base/basictypes.h"
#include "base/threading/non_thread_safe.h"
namespace base {
class TimeDelta;
class TimeTicks;
class BASE_EXPORT MessagePump : public NonThreadSafe {
public:
// Please see the comments above the Run method for an illustration of how
// these delegate methods are used.
class BASE_EXPORT Delegate {
public:
virtual ~Delegate() {}
// Called from within Run in response to ScheduleWork or when the message
// pump would otherwise call DoDelayedWork. Returns true to indicate that
// work was done. DoDelayedWork will still be called if DoWork returns
// true, but DoIdleWork will not.
virtual bool DoWork() = 0;
// Called from within Run in response to ScheduleDelayedWork or when the
// message pump would otherwise sleep waiting for more work. Returns true
// to indicate that delayed work was done. DoIdleWork will not be called
// if DoDelayedWork returns true. Upon return |next_delayed_work_time|
// indicates the time when DoDelayedWork should be called again. If
// |next_delayed_work_time| is null (per Time::is_null), then the queue of
// future delayed work (timer events) is currently empty, and no additional
// calls to this function need to be scheduled.
virtual bool DoDelayedWork(TimeTicks* next_delayed_work_time) = 0;
// Called from within Run just before the message pump goes to sleep.
// Returns true to indicate that idle work was done.
virtual bool DoIdleWork() = 0;
// Via the two required out pointers, returns the length of the Delegate's
// work queue and the length of time that the first item in the queue has
// been waiting to run. If the work queue is empty, the count and delay
// will both be zero.
// Note that this only counts the tasks in the ready-to-run queue and not
// the incoming queue that is used by other threads to post tasks. The
// latter queue requires holding a lock, which is deemed too expensive for
// instrumentation code. Under normal conditions, the incoming queue should
// be small or zero, but under heavy loads it may be much larger and
// |queue_count| may be up to 1/4 the size of the incoming queue.
virtual void GetQueueingInformation(size_t* queue_count,
TimeDelta* queueing_delay) {}
};
MessagePump();
virtual ~MessagePump();
// The Run method is called to enter the message pump's run loop.
//
// Within the method, the message pump is responsible for processing native
// messages as well as for giving cycles to the delegate periodically. The
// message pump should take care to mix delegate callbacks with native
// message processing so neither type of event starves the other of cycles.
//
// The anatomy of a typical run loop:
//
// for (;;) {
// bool did_work = DoInternalWork();
// if (should_quit_)
// break;
//
// did_work |= delegate_->DoWork();
// if (should_quit_)
// break;
//
// TimeTicks next_time;
// did_work |= delegate_->DoDelayedWork(&next_time);
// if (should_quit_)
// break;
//
// if (did_work)
// continue;
//
// did_work = delegate_->DoIdleWork();
// if (should_quit_)
// break;
//
// if (did_work)
// continue;
//
// WaitForWork();
// }
//
// Here, DoInternalWork is some private method of the message pump that is
// responsible for dispatching the next UI message or notifying the next IO
// completion (for example). WaitForWork is a private method that simply
// blocks until there is more work of any type to do.
//
// Notice that the run loop cycles between calling DoInternalWork, DoWork,
// and DoDelayedWork methods. This helps ensure that none of these work
// queues starve the others. This is important for message pumps that are
// used to drive animations, for example.
//
// Notice also that after each callout to foreign code, the run loop checks
// to see if it should quit. The Quit method is responsible for setting this
// flag. No further work is done once the quit flag is set.
//
// NOTE: Care must be taken to handle Run being called again from within any
// of the callouts to foreign code. Native message pumps may also need to
// deal with other native message pumps being run outside their control
// (e.g., the MessageBox API on Windows pumps UI messages!). To be specific,
// the callouts (DoWork and DoDelayedWork) MUST still be provided even in
// nested sub-loops that are "seemingly" outside the control of this message
// pump. DoWork in particular must never be starved for time slices unless
// it returns false (meaning it has run out of things to do).
//
virtual void Run(Delegate* delegate) = 0;
// Quit immediately from the most recently entered run loop. This method may
// only be used on the thread that called Run.
virtual void Quit() = 0;
// Schedule a DoWork callback to happen reasonably soon. Does nothing if a
// DoWork callback is already scheduled. This method may be called from any
// thread. Once this call is made, DoWork should not be "starved" at least
// until it returns a value of false.
virtual void ScheduleWork() = 0;
// Schedule a DoDelayedWork callback to happen at the specified time,
// cancelling any pending DoDelayedWork callback. This method may only be
// used on the thread that called Run.
virtual void ScheduleDelayedWork(const TimeTicks& delayed_work_time) = 0;
};
} // namespace base
#endif // BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_
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