base/message_pump.h

   1 // Copyright (c) 2006-2008 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
   8 #include "base/ref_counted.h"
   9
  10 namespace base {
  11
  12 class Time;
  13
  14 class MessagePump : public RefCountedThreadSafe<MessagePump> {
  15  public:
  16   // Please see the comments above the Run method for an illustration of how
  17   // these delegate methods are used.
  18   class Delegate {
  19    public:
  20     virtual ~Delegate() {}
  21
  22     // Called from within Run in response to ScheduleWork or when the message
  23     // pump would otherwise call DoDelayedWork.  Returns true to indicate that
  24     // work was done.  DoDelayedWork will not be called if DoWork returns true.
  25     virtual bool DoWork() = 0;
  26
  27     // Called from within Run in response to ScheduleDelayedWork or when the
  28     // message pump would otherwise sleep waiting for more work.  Returns true
  29     // to indicate that delayed work was done.  DoIdleWork will not be called
  30     // if DoDelayedWork returns true.  Upon return |next_delayed_work_time|
  31     // indicates the time when DoDelayedWork should be called again.  If
  32     // |next_delayed_work_time| is null (per Time::is_null), then the queue of
  33     // future delayed work (timer events) is currently empty, and no additional
  34     // calls to this function need to be scheduled.
  35     virtual bool DoDelayedWork(Time* next_delayed_work_time) = 0;
  36
  37     // Called from within Run just before the message pump goes to sleep.
  38     // Returns true to indicate that idle work was done.
  39     virtual bool DoIdleWork() = 0;
  40   };
  41
  42   virtual ~MessagePump() {}
  43
  44   // The Run method is called to enter the message pump's run loop.
  45   //
  46   // Within the method, the message pump is responsible for processing native
  47   // messages as well as for giving cycles to the delegate periodically.  The
  48   // message pump should take care to mix delegate callbacks with native
  49   // message processing so neither type of event starves the other of cycles.
  50   //
  51   // The anatomy of a typical run loop:
  52   //
  53   //   for (;;) {
  54   //     bool did_work = DoInternalWork();
  55   //     if (should_quit_)
  56   //       break;
  57   //
  58   //     did_work |= delegate_->DoWork();
  59   //     if (should_quit_)
  60   //       break;
  61   //
  62   //     did_work |= delegate_->DoDelayedWork();
  63   //     if (should_quit_)
  64   //       break;
  65   //
  66   //     if (did_work)
  67   //       continue;
  68   //
  69   //     did_work = delegate_->DoIdleWork();
  70   //     if (should_quit_)
  71   //       break;
  72   //
  73   //     if (did_work)
  74   //       continue;
  75   //
  76   //     WaitForWork();
  77   //   }
  78   //
  79   // Here, DoInternalWork is some private method of the message pump that is
  80   // responsible for dispatching the next UI message or notifying the next IO
  81   // completion (for example).  WaitForWork is a private method that simply
  82   // blocks until there is more work of any type to do.
  83   //
  84   // Notice that the run loop cycles between calling DoInternalWork, DoWork,
  85   // and DoDelayedWork methods.  This helps ensure that neither work queue
  86   // starves the other.  This is important for message pumps that are used to
  87   // drive animations, for example.
  88   //
  89   // Notice also that after each callout to foreign code, the run loop checks
  90   // to see if it should quit.  The Quit method is responsible for setting this
  91   // flag.  No further work is done once the quit flag is set.
  92   //
  93   // NOTE: Care must be taken to handle Run being called again from within any
  94   // of the callouts to foreign code.  Native message pumps may also need to
  95   // deal with other native message pumps being run outside their control
  96   // (e.g., the MessageBox API on Windows pumps UI messages!).  To be specific,
  97   // the callouts (DoWork and DoDelayedWork) MUST still be provided even in
  98   // nested sub-loops that are "seemingly" outside the control of this message
  99   // pump.  DoWork in particular must never be starved for time slices unless
 100   // it returns false (meaning it has run out of things to do).
 101   //
 102   virtual void Run(Delegate* delegate) = 0;
 103
 104   // Quit immediately from the most recently entered run loop.  This method may
 105   // only be used on the thread that called Run.
 106   virtual void Quit() = 0;
 107
 108   // Schedule a DoWork callback to happen reasonably soon.  Does nothing if a
 109   // DoWork callback is already scheduled.  This method may be called from any
 110   // thread.  Once this call is made, DoWork should not be "starved" at least
 111   // until it returns a value of false.
 112   virtual void ScheduleWork() = 0;
 113
 114   // Schedule a DoDelayedWork callback to happen at the specified time,
 115   // cancelling any pending DoDelayedWork callback.  This method may only be
 116   // used on the thread that called Run.
 117   virtual void ScheduleDelayedWork(const Time& delayed_work_time) = 0;
 118 };
 119
 120 }  // namespace base
 121
 122 #endif  // BASE_MESSAGE_PUMP_H_