base/message_pump.h

   1 // Copyright (c) 2011 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_PUMP_H_
   6 #define BASE_MESSAGE_PUMP_H_
   7 #pragma once
   8
   9 #include "base/base_api.h"
  10 #include "base/memory/ref_counted.h"
  11
  12 namespace base {
  13
  14 class TimeTicks;
  15
  16 class BASE_API MessagePump : public RefCountedThreadSafe<MessagePump> {
  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_API 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.
  42     virtual bool DoIdleWork() = 0;
  43   };
  44
  45   MessagePump();
  46   virtual ~MessagePump();
  47
  48   // The Run method is called to enter the message pump's run loop.
  49   //
  50   // Within the method, the message pump is responsible for processing native
  51   // messages as well as for giving cycles to the delegate periodically.  The
  52   // message pump should take care to mix delegate callbacks with native
  53   // message processing so neither type of event starves the other of cycles.
  54   //
  55   // The anatomy of a typical run loop:
  56   //
  57   //   for (;;) {
  58   //     bool did_work = DoInternalWork();
  59   //     if (should_quit_)
  60   //       break;
  61   //
  62   //     did_work |= delegate_->DoWork();
  63   //     if (should_quit_)
  64   //       break;
  65   //
  66   //     TimeTicks next_time;
  67   //     did_work |= delegate_->DoDelayedWork(&next_time);
  68   //     if (should_quit_)
  69   //       break;
  70   //
  71   //     if (did_work)
  72   //       continue;
  73   //
  74   //     did_work = delegate_->DoIdleWork();
  75   //     if (should_quit_)
  76   //       break;
  77   //
  78   //     if (did_work)
  79   //       continue;
  80   //
  81   //     WaitForWork();
  82   //   }
  83   //
  84   // Here, DoInternalWork is some private method of the message pump that is
  85   // responsible for dispatching the next UI message or notifying the next IO
  86   // completion (for example).  WaitForWork is a private method that simply
  87   // blocks until there is more work of any type to do.
  88   //
  89   // Notice that the run loop cycles between calling DoInternalWork, DoWork,
  90   // and DoDelayedWork methods.  This helps ensure that none of these work
  91   // queues starve the others.  This is important for message pumps that are
  92   // used to drive animations, for example.
  93   //
  94   // Notice also that after each callout to foreign code, the run loop checks
  95   // to see if it should quit.  The Quit method is responsible for setting this
  96   // flag.  No further work is done once the quit flag is set.
  97   //
  98   // NOTE: Care must be taken to handle Run being called again from within any
  99   // of the callouts to foreign code.  Native message pumps may also need to
 100   // deal with other native message pumps being run outside their control
 101   // (e.g., the MessageBox API on Windows pumps UI messages!).  To be specific,
 102   // the callouts (DoWork and DoDelayedWork) MUST still be provided even in
 103   // nested sub-loops that are "seemingly" outside the control of this message
 104   // pump.  DoWork in particular must never be starved for time slices unless
 105   // it returns false (meaning it has run out of things to do).
 106   //
 107   virtual void Run(Delegate* delegate) = 0;
 108
 109   // Quit immediately from the most recently entered run loop.  This method may
 110   // only be used on the thread that called Run.
 111   virtual void Quit() = 0;
 112
 113   // Schedule a DoWork callback to happen reasonably soon.  Does nothing if a
 114   // DoWork callback is already scheduled.  This method may be called from any
 115   // thread.  Once this call is made, DoWork should not be "starved" at least
 116   // until it returns a value of false.
 117   virtual void ScheduleWork() = 0;
 118
 119   // Schedule a DoDelayedWork callback to happen at the specified time,
 120   // cancelling any pending DoDelayedWork callback.  This method may only be
 121   // used on the thread that called Run.
 122   virtual void ScheduleDelayedWork(const TimeTicks& delayed_work_time) = 0;
 123 };
 124
 125 }  // namespace base
 126
 127 #endif  // BASE_MESSAGE_PUMP_H_