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 != 0
    >
{
    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-1];

    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。