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.
4
5#ifndef BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_
6#define BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_
7
8#include "base/base_export.h"
9#include "base/message_loop/timer_slack.h"
10#include "base/threading/non_thread_safe.h"
11
12namespace base {
13
14class TimeTicks;
15
16class BASE_EXPORT MessagePump : public NonThreadSafe {
17 public:
18  // Please see the comments above the Run method for an illustration of how
19  // these delegate methods are used.
20  class BASE_EXPORT Delegate {
21   public:
22    virtual ~Delegate() {}
23
24    // Called from within Run in response to ScheduleWork or when the message
25    // pump would otherwise call DoDelayedWork.  Returns true to indicate that
26    // work was done.  DoDelayedWork will still be called if DoWork returns
27    // true, but DoIdleWork will not.
28    virtual bool DoWork() = 0;
29
30    // Called from within Run in response to ScheduleDelayedWork or when the
31    // message pump would otherwise sleep waiting for more work.  Returns true
32    // to indicate that delayed work was done.  DoIdleWork will not be called
33    // if DoDelayedWork returns true.  Upon return |next_delayed_work_time|
34    // indicates the time when DoDelayedWork should be called again.  If
35    // |next_delayed_work_time| is null (per Time::is_null), then the queue of
36    // future delayed work (timer events) is currently empty, and no additional
37    // calls to this function need to be scheduled.
38    virtual bool DoDelayedWork(TimeTicks* next_delayed_work_time) = 0;
39
40    // Called from within Run just before the message pump goes to sleep.
41    // Returns true to indicate that idle work was done. Returning false means
42    // the pump will now wait.
43    virtual bool DoIdleWork() = 0;
44  };
45
46  MessagePump();
47  virtual ~MessagePump();
48
49  // The Run method is called to enter the message pump's run loop.
50  //
51  // Within the method, the message pump is responsible for processing native
52  // messages as well as for giving cycles to the delegate periodically.  The
53  // message pump should take care to mix delegate callbacks with native
54  // message processing so neither type of event starves the other of cycles.
55  //
56  // The anatomy of a typical run loop:
57  //
58  //   for (;;) {
59  //     bool did_work = DoInternalWork();
60  //     if (should_quit_)
61  //       break;
62  //
63  //     did_work |= delegate_->DoWork();
64  //     if (should_quit_)
65  //       break;
66  //
67  //     TimeTicks next_time;
68  //     did_work |= delegate_->DoDelayedWork(&next_time);
69  //     if (should_quit_)
70  //       break;
71  //
72  //     if (did_work)
73  //       continue;
74  //
75  //     did_work = delegate_->DoIdleWork();
76  //     if (should_quit_)
77  //       break;
78  //
79  //     if (did_work)
80  //       continue;
81  //
82  //     WaitForWork();
83  //   }
84  //
85  // Here, DoInternalWork is some private method of the message pump that is
86  // responsible for dispatching the next UI message or notifying the next IO
87  // completion (for example).  WaitForWork is a private method that simply
88  // blocks until there is more work of any type to do.
89  //
90  // Notice that the run loop cycles between calling DoInternalWork, DoWork,
91  // and DoDelayedWork methods.  This helps ensure that none of these work
92  // queues starve the others.  This is important for message pumps that are
93  // used to drive animations, for example.
94  //
95  // Notice also that after each callout to foreign code, the run loop checks
96  // to see if it should quit.  The Quit method is responsible for setting this
97  // flag.  No further work is done once the quit flag is set.
98  //
99  // NOTE: Care must be taken to handle Run being called again from within any
100  // of the callouts to foreign code.  Native message pumps may also need to
101  // deal with other native message pumps being run outside their control
102  // (e.g., the MessageBox API on Windows pumps UI messages!).  To be specific,
103  // the callouts (DoWork and DoDelayedWork) MUST still be provided even in
104  // nested sub-loops that are "seemingly" outside the control of this message
105  // pump.  DoWork in particular must never be starved for time slices unless
106  // it returns false (meaning it has run out of things to do).
107  //
108  virtual void Run(Delegate* delegate) = 0;
109
110  // Quit immediately from the most recently entered run loop.  This method may
111  // only be used on the thread that called Run.
112  virtual void Quit() = 0;
113
114  // Schedule a DoWork callback to happen reasonably soon.  Does nothing if a
115  // DoWork callback is already scheduled.  This method may be called from any
116  // thread.  Once this call is made, DoWork should not be "starved" at least
117  // until it returns a value of false.
118  virtual void ScheduleWork() = 0;
119
120  // Schedule a DoDelayedWork callback to happen at the specified time,
121  // cancelling any pending DoDelayedWork callback.  This method may only be
122  // used on the thread that called Run.
123  virtual void ScheduleDelayedWork(const TimeTicks& delayed_work_time) = 0;
124
125  // Sets the timer slack to the specified value.
126  virtual void SetTimerSlack(TimerSlack timer_slack);
127};
128
129}  // namespace base
130
131#endif  // BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_
132