base/message_loop/message_pump.h

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