描述wxWidgets中事件处理的类型转化
wxWidgets是一个比较常用的UI界面库,我曾经试着使用wxWidgets写一个UI编辑工具,在此期间,学习了一些wxWidgets的知识。我对wxWidgets的绑定(Bind)比较好奇,想知道,wxWidgets是如何知道,我Bind的函数,是需要什么参数,所以查看了一些源代码,这里,将了解的知识写出来:
首先,给出绑定的相关源代码:
class wxEvtHandler : public wxObject,
public wxTrackable
{
// These functions are used for old, untyped, event handlers and don't
// check that the type of the function passed to them actually matches the
// type of the event. They also only allow connecting events to methods of
// wxEvtHandler-derived classes.
//
// The template Bind() methods below are safer and allow connecting
// events to arbitrary functions or functors -- but require compiler
// support for templates.
//
// Dynamic association of a member function handler with the event handler,
// winid and event type void Connect(int winid,
int lastId,
wxEventType eventType,
wxObjectEventFunction func,
wxObject *userData = NULL,
wxEvtHandler *eventSink = NULL)
{
DoBind(winid, lastId, eventType,
wxNewEventFunctor(eventType, func, eventSink),
userData);
} bool Disconnect(int winid,
int lastId,
wxEventType eventType,
wxObjectEventFunction func = NULL,
wxObject* userData = NULL,
wxEvtHandler* eventSink = NULL)
{
return DoUnbind(winid, lastId, eventType,
wxMakeEventFunctor(eventType, func, eventSink),
userData);
} // Bind a method of a class (called on the specified handler which must
// be convertible to this class) object to an event:
template <typename EventTag, typename Class, typename EventArg, typename EventHandler>
void Bind(const EventTag &eventType,
void (Class::*method)(EventArg &),
EventHandler *handler,
int winid = wxID_ANY,
int lastId = wxID_ANY,
wxObject* userData = NULL)
{
DoBind(winid, lastId, eventType,
wxNewEventFunctor(eventType, method, handler),
userData);
} template <typename EventTag, typename Class, typename EventArg, typename EventHandler>
bool Unbind(const EventTag &eventType,
void (Class::*method)(EventArg&),
EventHandler *handler,
int winid = wxID_ANY,
int lastId = wxID_ANY,
wxObject* userData = NULL)
{
return DoUnbind(winid, lastId, eventType,
wxMakeEventFunctor(eventType, method, handler),
userData);
} void wxEvtHandler::DoBind(int id,
int lastId,
wxEventType eventType,
wxEventFunctor *func,
wxObject *userData)
{
wxDynamicEventTableEntry *entry =
new wxDynamicEventTableEntry(eventType, id, lastId, func, userData); // Check
// ... if (!m_dynamicEvents)
m_dynamicEvents = new DynamicEvents; // We prefer to push back the entry here and then iterate over the vector
// in reverse direction in GetNextDynamicEntry() as it's more efficient
// than inserting the element at the front.
m_dynamicEvents->push_back(entry); // 该函数的以下部分可忽略
// Make sure we get to know when a sink is destroyed
wxEvtHandler *eventSink = func->GetEvtHandler();
if ( eventSink && eventSink != this )
{
wxEventConnectionRef *evtConnRef = FindRefInTrackerList(eventSink);
if ( evtConnRef )
evtConnRef->IncRef( );
else
new wxEventConnectionRef(this, eventSink);
}
}
};
我只贴出一部分,与我描述相关的内容,当然,很多时候会有些多余,不过,对于不太懂的地方,可以略过,对于比较重要的地方,我会比较详细的说明。wxWidgets使用的是通过模板来记录类型,然后,再将类型还原回来。
这里,我随便从wxWidgets中贴出一个Bind使用:
MyFrame::MyFrame(const wxString& title, const wxPoint& pos, const wxSize& size)
: wxFrame((wxFrame *)NULL, -1, title, pos, size)
{
wxPanel* mainPane = new wxPanel(this, wxID_ANY, wxDefaultPosition, wxDefaultSize, wxWANTS_CHARS);
mainPane->Bind(wxEVT_CHAR_HOOK, &MyFrame::OnKeyDown, this);
}
void MyFrame::OnKeyDown(wxKeyEvent& event)
{
wxMessageBox(wxString::Format("KeyDown: %i\n", (int)event.GetKeyCode()));
event.Skip();
}
根据wxWidgets源代码,可以知道wxEVT_CHAR_HOOK是const wxEventTypeTag<wxKeyEvent>,从Bind函数,可以知道,我们可以从两处知道OnKeyDown函数的参数是wxKeyEvent,一个是传入的参数wxEVT_CHAR_HOOK,一个是OnKeyDown。根据传参可以知道,实现只会和DoBind的wxEventType eventType,以及wxEventFunctor *func有关。wxDynamicEventTableEntry的构造函数,使用了eventType,以及func作为参数,那么类型转化会与wxDynamicEventTableEntry的实现有关吗?下面参考wxDynamicEventTableEntry的实现:
struct wxDynamicEventTableEntry : public wxEventTableEntryBase
{
wxDynamicEventTableEntry(int evType, int winid, int idLast,
wxEventFunctor* fn, wxObject *data)
: wxEventTableEntryBase(winid, idLast, fn, data),
m_eventType(evType)
{ } // not a reference here as we can't keep a reference to a temporary int
// created to wrap the constant value typically passed to Connect() - nor
// do we need it
int m_eventType; private:
wxDynamicEventTableEntry& operator=(const wxDynamicEventTableEntry&) = delete;
};
注意,这里evtType的类型是int,再向上看,其实DoBind第三个参数wxEventType实际类型就是int,也就是说,携带参数类型是wxKeyEvent的参数只有传入DoBind的wxEventFunctor *func。参看wxDynamicEventTableEntry的实现,这个结构甚至不是模板,所以不会携带类型信息。那么唯一的希望,就只有
wxNewEventFunctor(eventType, func, eventSink)构造的func指针所指示的对象,在DoBind中的wxEvtHandler *eventSink = func->GetEvtHandler();,eventSink并不携带类型信息。
现在我们要查看wxNewEventFunctor函数的实现:
template <typename EventTag, typename Class, typename EventArg, typename EventHandler>
inline wxEventFunctorMethod<EventTag, Class, EventArg, EventHandler> *
wxNewEventFunctor(const EventTag&, void (Class::*method)(EventArg&), EventHandler *handler)
{
return new wxEventFunctorMethod<EventTag, Class, EventArg, EventHandler>
(method, handler);
}
以及wxEventFunctorMethod的实现:
// functor forwarding the event to a method of the given object
//
template <typename EventTag, typename Class, typename EventArg, typename EventHandler>
class wxEventFunctorMethod :
public wxEventFunctor,
private wxPrivate::HandlerImpl
<
Class,
EventArg,
wxIsPublicyDerived<Class, wxEvtHandler>::value !=
>
{
public:
void operator()(wxEvtHandler* handler, wxEvent& event) override
{
Class *realHandler = m_handler;
if (!realHandler)
{
realHandler = this->ConvertFromEvtHandler(handler); // this is not supposed to happen but check for it nevertheless
wxCHECK_RET(realHandler, "invalid event handler");
} // the real (run-time) type of event is EventClass and we check in
// the ctor that EventClass can be converted to EventArg, so this cast
// is always valid
(realHandler->*method)(static_cast<EventArg&>(event));
}
};
请认真看一下上面的static_cast<EventArg&>,正是这里,wxWiidgets将event类型转变为了正确的函数需要的类型。
下面贴出,wxWidgets调用上述函数的地方:
// 下面简略摘抄一下,调用处理事件(wxEvent)的函数的位置:
bool wxEvtHandler::SearchDynamicEventTable(wxEvent& event)
{
DynamicEvents& dynamicEvents = *m_dynamicEvents; bool needToPruneDeleted = false; for (size_t n = dynamicEvents.size(); n; n--)
{
wxDynamicEventTableEntry* const entry = dynamicEvents[n-]; if (!entry)
{
needToPruneDeleted = true;
continue;
} if (event.GetEventType() == entry->m_eventType)
{
wxEvtHandler *handler = entry->m_fn->GetEvtHandler();
if (!handler)
handler = this;
if (ProcessEventIfMatched(*entry, handler, event))
{
return true;
}
}
} // 以下省略
};
bool wxEvtHandler::ProcessEventIfMatched(const wxEventTableEntryBase& entry,
wxEvtHandler *handler,
wxEvent& event)
{
int tableId1 = entry.m_id,
tableId2 = entry.m_lastId; // match only if the event type is the same and the id is either -1 in
// the event table (meaning "any") or the event id matches the id
// specified in the event table either exactly or by falling into
// the range between first and last
if ((tableId1 == wxID_ANY) ||
(tableId2 == wxID_ANY && tableId1 == event.GetId()) ||
(tableId2 != wxID_ANY &&
(event.GetId() >= tableId1 && event.GetId() <= tableId2)))
{
event.Skip(false);
event.m_callbackUserData = entry.m_callbackUserData; #if wxUSE_EXCEPTIONS
if (wxTheApp)
{
// call the handler via wxApp method which allows the user to catch
// any exceptions which may be throw by any handler in the program
// in one place
wxTheApp->CallEventHandler(handler, *entry.m_fn, event);
}
else
#endif // wxUSE_EXCEPTIONS
{
(*entry.m_fn)(handler, event);
} if (!event.GetSkipped())
return true;
} return false;
}
// 下面再查看一下wxAppConsoleBase的CallEventHandler函数
void wxAppConsoleBase::CallEventHandler(wxEvtHandler *handler,
wxEventFunctor& functor,
wxEvent& event) const
{
// If the functor holds a method then, for backward compatibility, call
// HandleEvent()
wxEventFunction eventFunction = functor.GetEvtMethod(); if (eventFunction)
HandleEvent(handler, eventFunction, event);
else
functor(handler, event);
}
void wxAppConsoleBase::HandleEvent(wxEvtHandler* handler,
wxEventFunction func,
wxEvent& event) const
{
// by default, simply call the handler
(handler->*func)(event);
}
到这里,应该说完了wxWidgets中处理事件类型中最主要的部分,这个也是C++模板用来保证类型安全的一个具体应用。C++模板是一种非常强大的工具,合理使用,可以使代码得到很大的优化。本来,想要通过自己写的一个简单例子来进一步说明的,不过考虑这个例子已经比较长了,所以,写在下一篇随笔上面了。
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