1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 #ifndef BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_
6 #define BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_
8 #include "base/base_export.h"
9 #include "base/basictypes.h"
10 #include "base/message_loop/timer_slack.h"
11 #include "base/threading/non_thread_safe.h"
18 class BASE_EXPORT MessagePump : public NonThreadSafe {
20 // Please see the comments above the Run method for an illustration of how
21 // these delegate methods are used.
22 class BASE_EXPORT Delegate {
24 virtual ~Delegate() {}
26 // Called from within Run in response to ScheduleWork or when the message
27 // pump would otherwise call DoDelayedWork. Returns true to indicate that
28 // work was done. DoDelayedWork will still be called if DoWork returns
29 // true, but DoIdleWork will not.
30 virtual bool DoWork() = 0;
32 // Called from within Run in response to ScheduleDelayedWork or when the
33 // message pump would otherwise sleep waiting for more work. Returns true
34 // to indicate that delayed work was done. DoIdleWork will not be called
35 // if DoDelayedWork returns true. Upon return |next_delayed_work_time|
36 // indicates the time when DoDelayedWork should be called again. If
37 // |next_delayed_work_time| is null (per Time::is_null), then the queue of
38 // future delayed work (timer events) is currently empty, and no additional
39 // calls to this function need to be scheduled.
40 virtual bool DoDelayedWork(TimeTicks* next_delayed_work_time) = 0;
42 // Called from within Run just before the message pump goes to sleep.
43 // Returns true to indicate that idle work was done.
44 virtual bool DoIdleWork() = 0;
46 // Via the two required out pointers, returns the length of the Delegate's
47 // work queue and the length of time that the first item in the queue has
48 // been waiting to run. If the work queue is empty, the count and delay
50 // Note that this only counts the tasks in the ready-to-run queue and not
51 // the incoming queue that is used by other threads to post tasks. The
52 // latter queue requires holding a lock, which is deemed too expensive for
53 // instrumentation code. Under normal conditions, the incoming queue should
54 // be small or zero, but under heavy loads it may be much larger and
55 // |queue_count| may be up to 1/4 the size of the incoming queue.
56 virtual void GetQueueingInformation(size_t* queue_count,
57 TimeDelta* queueing_delay) {}
61 virtual ~MessagePump();
63 // The Run method is called to enter the message pump's run loop.
65 // Within the method, the message pump is responsible for processing native
66 // messages as well as for giving cycles to the delegate periodically. The
67 // message pump should take care to mix delegate callbacks with native
68 // message processing so neither type of event starves the other of cycles.
70 // The anatomy of a typical run loop:
73 // bool did_work = DoInternalWork();
77 // did_work |= delegate_->DoWork();
81 // TimeTicks next_time;
82 // did_work |= delegate_->DoDelayedWork(&next_time);
89 // did_work = delegate_->DoIdleWork();
99 // Here, DoInternalWork is some private method of the message pump that is
100 // responsible for dispatching the next UI message or notifying the next IO
101 // completion (for example). WaitForWork is a private method that simply
102 // blocks until there is more work of any type to do.
104 // Notice that the run loop cycles between calling DoInternalWork, DoWork,
105 // and DoDelayedWork methods. This helps ensure that none of these work
106 // queues starve the others. This is important for message pumps that are
107 // used to drive animations, for example.
109 // Notice also that after each callout to foreign code, the run loop checks
110 // to see if it should quit. The Quit method is responsible for setting this
111 // flag. No further work is done once the quit flag is set.
113 // NOTE: Care must be taken to handle Run being called again from within any
114 // of the callouts to foreign code. Native message pumps may also need to
115 // deal with other native message pumps being run outside their control
116 // (e.g., the MessageBox API on Windows pumps UI messages!). To be specific,
117 // the callouts (DoWork and DoDelayedWork) MUST still be provided even in
118 // nested sub-loops that are "seemingly" outside the control of this message
119 // pump. DoWork in particular must never be starved for time slices unless
120 // it returns false (meaning it has run out of things to do).
122 virtual void Run(Delegate* delegate) = 0;
124 // Quit immediately from the most recently entered run loop. This method may
125 // only be used on the thread that called Run.
126 virtual void Quit() = 0;
128 // Schedule a DoWork callback to happen reasonably soon. Does nothing if a
129 // DoWork callback is already scheduled. This method may be called from any
130 // thread. Once this call is made, DoWork should not be "starved" at least
131 // until it returns a value of false.
132 virtual void ScheduleWork() = 0;
134 // Schedule a DoDelayedWork callback to happen at the specified time,
135 // cancelling any pending DoDelayedWork callback. This method may only be
136 // used on the thread that called Run.
137 virtual void ScheduleDelayedWork(const TimeTicks& delayed_work_time) = 0;
139 // Sets the timer slack to the specified value.
140 virtual void SetTimerSlack(TimerSlack timer_slack);
145 #endif // BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_