added null pointer check and assert

[wxWidgets.git] / docs / latex / wx / evthand.tex

\section{\class{wxEvtHandler}}\label{wxevthandler}

A class that can handle events from the windowing system.
wxWindow (and therefore all window classes) are derived from
this class.

When events are received, wxEvtHandler invokes the method listed in the
event table using itself as the object.  When using multiple inheritance
it is imperative that the wxEvtHandler(-derived) class be the first
class inherited such that the "this" pointer for the overall object
will be identical to the "this" pointer for the wxEvtHandler portion.

\wxheading{Derived from}

\helpref{wxObject}{wxobject}

\wxheading{Include files}

<wx/event.h>

\wxheading{Library}

\helpref{wxBase}{librarieslist}

\wxheading{See also}

\overview{Event handling overview}{eventhandlingoverview}

\latexignore{\rtfignore{\wxheading{Members}}}

\membersection{wxEvtHandler::wxEvtHandler}\label{wxevthandlerctor}

\func{}{wxEvtHandler}{\void}

Constructor.

\membersection{wxEvtHandler::\destruct{wxEvtHandler}}\label{wxevthandlerdtor}

\func{}{\destruct{wxEvtHandler}}{\void}

Destructor. If the handler is part of a chain, the destructor will
unlink itself and restore the previous and next handlers so that they point to
each other.

\membersection{wxEvtHandler::AddPendingEvent}\label{wxevthandleraddpendingevent}

\func{virtual void}{AddPendingEvent}{\param{const wxEvent\& }{event}}

This function posts an event to be processed later.

\wxheading{Parameters}

\docparam{event}{Event to add to process queue.}

\wxheading{Remarks}

The difference between sending an event (using the
\helpref{ProcessEvent}{wxevthandlerprocessevent} method) and posting it is
that in the first case the event is processed before the function returns,
while in the second case, the function returns immediately and the event will
be processed sometime later (usually during the next event loop iteration).

A copy of {\it event} is made by the function, so the original can be deleted
as soon as function returns (it is common that the original is created on the
stack).  This requires that the \helpref{wxEvent::Clone}{wxeventclone} method
be implemented by {\it event} so that it can be duplicated and stored until
it gets processed.

This is also the method to call for inter-thread communication---it will
post events safely between different threads which means that this method is
thread-safe by using critical sections where needed.  In a multi-threaded
program, you often need to inform the main GUI thread about the status of
other working threads and such notification should be done using this method.

This method automatically wakes up idle handling if the underlying window 
system is currently idle and thus would not send any idle events. (Waking
up idle handling is done calling \helpref{::wxWakeUpIdle}{wxwakeupidle}.)

\membersection{wxEvtHandler::Connect}\label{wxevthandlerconnect}

\func{void}{Connect}{\param{int}{ id}, \param{int}{ lastId},
 \param{wxEventType }{eventType}, \param{wxObjectEventFunction}{ function},
 \param{wxObject*}{ userData = NULL}, \param{wxEvtHandler*}{ eventSink = NULL}}

\func{void}{Connect}{\param{int}{ id},
 \param{wxEventType }{eventType}, \param{wxObjectEventFunction}{ function},
 \param{wxObject*}{ userData = NULL}, \param{wxEvtHandler*}{ eventSink = NULL}}

\func{void}{Connect}{\param{wxEventType }{eventType}, \param{wxObjectEventFunction}{ function},
 \param{wxObject*}{ userData = NULL}, \param{wxEvtHandler*}{ eventSink = NULL}}

Connects the given function dynamically with the event handler, id and event type. This
is an alternative to the use of static event tables. See the 'event' or the old 'dynamic' sample for usage.

\wxheading{Parameters}

\docparam{id}{The identifier (or first of the identifier range) to be
associated with the event handler function. For the version not taking this
argument, it defaults to \texttt{wxID\_ANY}.}

\docparam{lastId}{The second part of the identifier range to be associated with the event handler function.}

\docparam{eventType}{The event type to be associated with this event handler.}

\docparam{function}{The event handler function. Note that this function should
be explicitly converted to the correct type which can be done using a macro
called \texttt{wxFooHandler} for the handler for any \texttt{wxFooEvent}.}

\docparam{userData}{Data to be associated with the event table entry.}

\docparam{eventSink}{Object whose member function should be called. If this is NULL,
\textit{this} will be used.}

\wxheading{Example}

\begin{verbatim}
  frame->Connect( wxID_EXIT,
    wxEVT_COMMAND_MENU_SELECTED,
    wxCommandEventHandler(MyFrame::OnQuit) );
\end{verbatim}

\perlnote{In wxPerl this function takes 4 arguments: \texttt{id,
lastid, type, method}; if \texttt{method} is \texttt{undef}, the
handler is disconnected.}

\membersection{wxEvtHandler::Disconnect}\label{wxevthandlerdisconnect}

\func{bool}{Disconnect}{\param{wxEventType }{eventType = wxEVT\_NULL}, \param{wxObjectEventFunction}{ function = NULL},
 \param{wxObject*}{ userData = NULL}, \param{wxEvtHandler*}{ eventSink = NULL}}

\func{bool}{Disconnect}{\param{int}{ id = \texttt{wxID\_ANY}},
 \param{wxEventType }{eventType = wxEVT\_NULL}, \param{wxObjectEventFunction}{ function = NULL},
 \param{wxObject*}{ userData = NULL}, \param{wxEvtHandler*}{ eventSink = NULL}}

\func{bool}{Disconnect}{\param{int}{ id}, \param{int}{ lastId = \texttt{wxID\_ANY}},
 \param{wxEventType }{eventType = wxEVT\_NULL}, \param{wxObjectEventFunction}{ function = NULL},
 \param{wxObject*}{ userData = NULL}, \param{wxEvtHandler*}{ eventSink = NULL}}

Disconnects the given function dynamically from the event handler, using the specified
parameters as search criteria and returning true if a matching function has been
found and removed. This method can only disconnect functions which have been added
using the \helpref{wxEvtHandler::Connect}{wxevthandlerconnect} method. There is no way
to disconnect functions connected using the (static) event tables.

\wxheading{Parameters}

\docparam{id}{The identifier (or first of the identifier range) associated with the event handler function.}

\docparam{lastId}{The second part of the identifier range associated with the event handler function.}

\docparam{eventType}{The event type associated with this event handler.}

\docparam{function}{The event handler function.}

\docparam{userData}{Data associated with the event table entry.}

\docparam{eventSink}{Object whose member function should be called.}

\perlnote{In wxPerl this function takes 3 arguments: \texttt{id,
lastid, type}.}

\membersection{wxEvtHandler::GetClientData}\label{wxevthandlergetclientdata}

\func{void* }{GetClientData}{\void}

Gets user-supplied client data.

\wxheading{Remarks}

Normally, any extra data the programmer wishes to associate with the object
should be made available by deriving a new class with new data members.

\wxheading{See also}

\helpref{wxEvtHandler::SetClientData}{wxevthandlersetclientdata}

\membersection{wxEvtHandler::GetClientObject}\label{wxevthandlergetclientobject}

\constfunc{wxClientData*}{GetClientObject}{\void}

Get a pointer to the user-supplied client data object.

\wxheading{See also}

\helpref{wxEvtHandler::SetClientObject}{wxevthandlersetclientobject},
\helpref{wxClientData}{wxclientdata}

\membersection{wxEvtHandler::GetEvtHandlerEnabled}\label{wxevthandlergetevthandlerenabled}

\func{bool}{GetEvtHandlerEnabled}{\void}

Returns true if the event handler is enabled, false otherwise.

\wxheading{See also}

\helpref{wxEvtHandler::SetEvtHandlerEnabled}{wxevthandlersetevthandlerenabled}

\membersection{wxEvtHandler::GetNextHandler}\label{wxevthandlergetnexthandler}

\func{wxEvtHandler*}{GetNextHandler}{\void}

Gets the pointer to the next handler in the chain.

\wxheading{See also}

\helpref{wxEvtHandler::SetNextHandler}{wxevthandlersetnexthandler},\rtfsp
\helpref{wxEvtHandler::GetPreviousHandler}{wxevthandlergetprevioushandler},\rtfsp
\helpref{wxEvtHandler::SetPreviousHandler}{wxevthandlersetprevioushandler},\rtfsp
\helpref{wxWindow::PushEventHandler}{wxwindowpusheventhandler},\rtfsp
\helpref{wxWindow::PopEventHandler}{wxwindowpopeventhandler}

\membersection{wxEvtHandler::GetPreviousHandler}\label{wxevthandlergetprevioushandler}

\func{wxEvtHandler*}{GetPreviousHandler}{\void}

Gets the pointer to the previous handler in the chain.

\wxheading{See also}

\helpref{wxEvtHandler::SetPreviousHandler}{wxevthandlersetprevioushandler},\rtfsp
\helpref{wxEvtHandler::GetNextHandler}{wxevthandlergetnexthandler},\rtfsp
\helpref{wxEvtHandler::SetNextHandler}{wxevthandlersetnexthandler},\rtfsp
\helpref{wxWindow::PushEventHandler}{wxwindowpusheventhandler},\rtfsp
\helpref{wxWindow::PopEventHandler}{wxwindowpopeventhandler}

\membersection{wxEvtHandler::ProcessEvent}\label{wxevthandlerprocessevent}

\func{virtual bool}{ProcessEvent}{\param{wxEvent\& }{event}}

Processes an event, searching event tables and calling zero or more suitable event handler function(s).

\wxheading{Parameters}

\docparam{event}{Event to process.}

\wxheading{Return value}

true if a suitable event handler function was found and executed, and the function did not
call \helpref{wxEvent::Skip}{wxeventskip}.

\wxheading{Remarks}

Normally, your application would not call this function: it is called in the wxWidgets
implementation to dispatch incoming user interface events to the framework (and application).

However, you might need to call it if implementing new functionality (such as a new control) where
you define new event types, as opposed to allowing the user to override virtual functions.

An instance where you might actually override the {\bf ProcessEvent} function is where you want
to direct event processing to event handlers not normally noticed by wxWidgets. For example,
in the document/view architecture, documents and views are potential event handlers.
When an event reaches a frame, {\bf ProcessEvent} will need to be called on the associated
document and view in case event handler functions are associated with these objects.
The property classes library (wxProperty) also overrides {\bf ProcessEvent} for similar reasons.

The normal order of event table searching is as follows:

\begin{enumerate}\itemsep=0pt
\item If the object is disabled (via a call to \helpref{wxEvtHandler::SetEvtHandlerEnabled}{wxevthandlersetevthandlerenabled})
the function skips to step (6).
\item If the object is a wxWindow, {\bf ProcessEvent} is recursively called on the window's\rtfsp
\helpref{wxValidator}{wxvalidator}. If this returns true, the function exits.
\item {\bf SearchEventTable} is called for this event handler. If this fails, the base
class table is tried, and so on until no more tables exist or an appropriate function was found,
in which case the function exits.
\item The search is applied down the entire chain of event handlers (usually the chain has a length
of one). If this succeeds, the function exits.
\item If the object is a wxWindow and the event is a wxCommandEvent, {\bf ProcessEvent} is
recursively applied to the parent window's event handler. If this returns true, the function exits.
\item Finally, {\bf ProcessEvent} is called on the wxApp object.
\end{enumerate}

\wxheading{See also}

\helpref{wxEvtHandler::SearchEventTable}{wxevthandlersearcheventtable}

\membersection{wxEvtHandler::SearchEventTable}\label{wxevthandlersearcheventtable}

\func{virtual bool}{SearchEventTable}{\param{wxEventTable\& }{table}, \param{wxEvent\& }{event}}

Searches the event table, executing an event handler function if an appropriate one
is found.

\wxheading{Parameters}

\docparam{table}{Event table to be searched.}

\docparam{event}{Event to be matched against an event table entry.}

\wxheading{Return value}

true if a suitable event handler function was found and executed, and the function did not
call \helpref{wxEvent::Skip}{wxeventskip}.

\wxheading{Remarks}

This function looks through the object's event table and tries to find an entry
that will match the event.

An entry will match if:

\begin{enumerate}\itemsep=0pt
\item The event type matches, and
\item the identifier or identifier range matches, or the event table entry's identifier is zero.
\end{enumerate}

If a suitable function is called but calls \helpref{wxEvent::Skip}{wxeventskip}, this function will
fail, and searching will continue.

\wxheading{See also}

\helpref{wxEvtHandler::ProcessEvent}{wxevthandlerprocessevent}

\membersection{wxEvtHandler::SetClientData}\label{wxevthandlersetclientdata}

\func{void}{SetClientData}{\param{void* }{data}}

Sets user-supplied client data.

\wxheading{Parameters}

\docparam{data}{Data to be associated with the event handler.}

\wxheading{Remarks}

Normally, any extra data the programmer wishes to associate with 
the object should be made available by deriving a new class
with new data members. You must not call this method and
\helpref{SetClientObject}{wxevthandlersetclientobject} on the
same class - only one of them.

\wxheading{See also}

\helpref{wxEvtHandler::GetClientData}{wxevthandlergetclientdata}

\membersection{wxEvtHandler::SetClientObject}\label{wxevthandlersetclientobject}

\func{void}{SetClientObject}{\param{wxClientData* }{data}}

Set the client data object. Any previous object will be deleted.

\wxheading{See also}

\helpref{wxEvtHandler::GetClientObject}{wxevthandlergetclientobject},
\helpref{wxClientData}{wxclientdata}

\membersection{wxEvtHandler::SetEvtHandlerEnabled}\label{wxevthandlersetevthandlerenabled}

\func{void}{SetEvtHandlerEnabled}{\param{bool }{enabled}}

Enables or disables the event handler.

\wxheading{Parameters}

\docparam{enabled}{true if the event handler is to be enabled, false if it is to be disabled.}

\wxheading{Remarks}

You can use this function to avoid having to remove the event handler from the chain, for example
when implementing a dialog editor and changing from edit to test mode.

\wxheading{See also}

\helpref{wxEvtHandler::GetEvtHandlerEnabled}{wxevthandlergetevthandlerenabled}

\membersection{wxEvtHandler::SetNextHandler}\label{wxevthandlersetnexthandler}

\func{void}{SetNextHandler}{\param{wxEvtHandler* }{handler}}

Sets the pointer to the next handler.

\wxheading{Parameters}

\docparam{handler}{Event handler to be set as the next handler.}

\wxheading{See also}

\helpref{wxEvtHandler::GetNextHandler}{wxevthandlergetnexthandler},\rtfsp
\helpref{wxEvtHandler::SetPreviousHandler}{wxevthandlersetprevioushandler},\rtfsp
\helpref{wxEvtHandler::GetPreviousHandler}{wxevthandlergetprevioushandler},\rtfsp
\helpref{wxWindow::PushEventHandler}{wxwindowpusheventhandler},\rtfsp
\helpref{wxWindow::PopEventHandler}{wxwindowpopeventhandler}

\membersection{wxEvtHandler::SetPreviousHandler}\label{wxevthandlersetprevioushandler}

\func{void}{SetPreviousHandler}{\param{wxEvtHandler* }{handler}}

Sets the pointer to the previous handler.

\wxheading{Parameters}

\docparam{handler}{Event handler to be set as the previous handler.}

\wxheading{See also}

\helpref{wxEvtHandler::GetPreviousHandler}{wxevthandlergetprevioushandler},\rtfsp
\helpref{wxEvtHandler::SetNextHandler}{wxevthandlersetnexthandler},\rtfsp
\helpref{wxEvtHandler::GetNextHandler}{wxevthandlergetnexthandler},\rtfsp
\helpref{wxWindow::PushEventHandler}{wxwindowpusheventhandler},\rtfsp
\helpref{wxWindow::PopEventHandler}{wxwindowpopeventhandler}