docs/latex/wx/tipc.tex

   1 \section{Interprocess communication overview}\label{ipcoverview}
   2
   3 Classes: \helpref{wxServer}{wxddeserver},
   4 \helpref{wxConnection}{wxddeconnection},
   5 \helpref{wxClient}{wxddeclient}
   6 %\helpref{wxTCPServer}{wxtcpserver}, \helpref{wxTCPConnection}{wxtcpconnection},
   7 %\helpref{wxTCPClient}{wxtcpclient}
   8
   9 wxWidgets has a number of different classes to help with
  10 interprocess communication and network programming. This section
  11 only discusses one family of classes -- the DDE-like protocol --
  12 but here's a list of other useful classes:
  13
  14 \begin{itemize}\itemsep=0pt
  15 \item \helpref{wxSocketEvent}{wxsocketevent},
  16 \helpref{wxSocketBase}{wxsocketbase},
  17 \helpref{wxSocketClient}{wxsocketclient},
  18 \helpref{wxSocketServer}{wxsocketserver}: classes for the low-level TCP/IP API.
  19 \item \helpref{wxProtocol}{wxprotocol}, \helpref{wxURL}{wxurl}, \helpref{wxFTP}{wxftp}, \helpref{wxHTTP}{wxhttp}: classes
  20 for programming popular Internet protocols.
  21 \end{itemize}
  22
  23 wxWidgets' DDE-like protocol is a high-level protocol based on
  24 Windows DDE. There are two implementations of this DDE-like
  25 protocol: one using real DDE running on Windows only, and another
  26 using TCP/IP (sockets) that runs on most platforms. Since the API
  27 and virtually all of the behaviour is the same apart from the
  28 names of the classes, you should find it easy to switch between
  29 the two implementations.
  30
  31 Notice that by including {\tt <wx/ipc.h>} you may define
  32 convenient synonyms for the IPC classes: {\tt wxServer} for either
  33 {\tt wxDDEServer} or {\tt wxTCPServer} depending on whether
  34 DDE-based or socket-based implementation is used and the same
  35 thing for {\tt wxClient} and {\tt wxConnection}.
  36
  37 By default, DDE implementation is used under Windows. DDE works
  38 only within one computer. If you want to use IPC between
  39 different workstations you should define {\tt
  40 wxUSE\_DDE\_FOR\_IPC} as $0$ before including this header -- this
  41 will force using TCP/IP implementation even under Windows.
  42
  43 The following description refers to wx... but remember that the
  44 equivalent wxTCP... and wxDDE... classes can be used in much the
  45 same way.
  46
  47 Three classes are central to the DDE-like API:
  48
  49 \begin{enumerate}\itemsep=0pt
  50 \item wxClient. This represents the client application, and is used
  51 only within a client program.
  52 \item wxServer. This represents the server application, and is used
  53 only within a server program.
  54 \item wxConnection. This represents the connection from the
  55 client to the server - both the client and the server use an
  56 instance of this class, one per connection. Most DDE transactions
  57 operate on this object.
  58 \end{enumerate}
  59
  60 Messages between applications are usually identified by three
  61 variables: connection object, topic name and item name.  A data
  62 string is a fourth element of some messages. To create a
  63 connection (a conversation in Windows parlance), the client
  64 application uses wxClient::MakeConnection to send a message to the
  65 server object, with a string service name to identify the server
  66 and a topic name to identify the topic for the duration of the
  67 connection. Under Unix, the service name may be either an integer
  68 port identifier in which case an Internet domain socket will be
  69 used for the communications or a valid file name (which shouldn't
  70 exist and will be deleted afterwards) in which case a Unix domain
  71 socket is created.
  72
  73 {\bf SECURITY NOTE:} Using Internet domain sockets if extremely insecure for
  74 IPC as there is absolutely no access control for them, use Unix domain sockets
  75 whenever possible!
  76
  77 The server then responds and either vetoes the connection or
  78 allows it. If allowed, both the server and client objects create
  79 wxConnection objects which persist until the connection is
  80 closed. The connection object is then used for sending and
  81 receiving subsequent messages between client and server -
  82 overriding virtual functions in your class derived from
  83 wxConnection allows you to handle the DDE messages.
  84
  85 To create a working server, the programmer must:
  86
  87 \begin{enumerate}\itemsep=0pt
  88 \item Derive a class from wxConnection, providing handlers for various messages sent to the server
  89 side of a wxConnection (e.g. OnExecute, OnRequest, OnPoke). Only
  90 the handlers actually required by the application need to be
  91 overridden.
  92 \item Derive a class from wxServer, overriding OnAcceptConnection
  93 to accept or reject a connection on the basis of the topic
  94 argument. This member must create and return an instance of the
  95 derived connection class if the connection is accepted.
  96 \item Create an instance of your server object and call Create to
  97 activate it, giving it a service name.
  98 \end{enumerate}
  99
 100 To create a working client, the programmer must:
 101
 102 \begin{enumerate}\itemsep=0pt
 103 \item Derive a class from wxConnection, providing handlers for various
 104 messages sent to the client side of a wxConnection (e.g.
 105 OnAdvise). Only the handlers actually required by the application
 106 need to be overridden.
 107 \item Derive a class from wxClient, overriding OnMakeConnection to
 108 create and return an instance of the derived connection class.
 109 \item Create an instance of your client object.
 110 \item When appropriate, create a new connection using
 111 \helpref{wxClient::MakeConnection}{wxddeclientmakeconnection},
 112 with arguments host name (processed in Unix only, use `localhost'
 113 for local computer), service name, and topic name for this
 114 connection. The client object will call
 115 \helpref{OnMakeConnection}{wxddeclientonmakeconnection} to create
 116 a connection object of the derived class if the connection is
 117 successful.
 118 \item Use the wxConnection member functions to send messages to the server.
 119 \end{enumerate}
 120
 121 \subsection{Data transfer}
 122
 123 These are the ways that data can be transferred from one
 124 application to another. These are methods of wxConnection.
 125
 126 \begin{itemize}\itemsep=0pt
 127 \item {\bf Execute:} the client calls the server with a data string representing
 128 a command to be executed. This succeeds or fails, depending on the
 129 server's willingness to answer. If the client wants to find the result
 130 of the Execute command other than success or failure, it has to explicitly
 131 call Request.
 132 \item {\bf Request:} the client asks the server for a particular data string
 133 associated with a given item string. If the server is unwilling to
 134 reply, the return value is NULL. Otherwise, the return value is a string
 135 (actually a pointer to the connection buffer, so it should not be
 136 deallocated by the application).
 137 \item {\bf Poke:} The client sends a data string associated with an item
 138 string directly to the server. This succeeds or fails.
 139 \item {\bf Advise:} The client asks to be advised of any change in data
 140 associated with a particular item. If the server agrees, the server will
 141 send an OnAdvise message to the client along with the item and data.
 142 \end{itemize}
 143
 144 The default data type is wxCF\_TEXT (ASCII text), and the default data
 145 size is the length of the null-terminated string. Windows-specific data
 146 types could also be used on the PC.
 147
 148 \subsection{Examples}
 149
 150 See the sample programs {\it server}\/ and {\it client}\/ in the IPC
 151 samples directory.  Run the server, then the client. This demonstrates
 152 using the Execute, Request, and Poke commands from the client, together
 153 with an Advise loop: selecting an item in the server list box causes
 154 that item to be highlighted in the client list box.
 155
 156 \subsection{More DDE details}
 157
 158 A wxClient object initiates the client part of a client-server
 159 DDE-like (Dynamic Data Exchange) conversation (available in both
 160 Windows and Unix).
 161
 162 To create a client which can communicate with a suitable server,
 163 you need to derive a class from wxConnection and another from
 164 wxClient. The custom wxConnection class will receive
 165 communications in a `conversation' with a server.  and the custom
 166 wxServer is required so that a user-overridden
 167 \helpref{wxDDEClient::OnMakeConnection}{wxddeclientonmakeconnection}
 168 member can return a wxDDEConnection of the required class, when a
 169 connection is made.
 170
 171 For example:
 172
 173 \begin{verbatim}
 174 class MyConnection: public wxConnection {
 175  public:
 176   MyConnection(void)::wxConnection() {}
 177   ~MyConnection(void) { }
 178   bool OnAdvise(const wxString& topic, const wxString& item, char *data, int size, wxIPCFormat format)
 179   { wxMessageBox(topic, data); }
 180 };
 181
 182 class MyClient: public wxClient {
 183  public:
 184   MyClient(void) {}
 185   wxConnectionBase *OnMakeConnection(void) { return new MyConnection; }
 186 };
 187
 188 \end{verbatim}
 189
 190 Here, {\bf MyConnection} will respond to
 191 \helpref{OnAdvise}{wxddeconnectiononadvise} messages sent by the
 192 server by displaying a message box.
 193
 194 When the client application starts, it must create an instance of
 195 the derived wxClient. In the following, command line arguments
 196 are used to pass the host name (the name of the machine the
 197 server is running on) and the server name (identifying the server
 198 process). Calling
 199 \helpref{wxDDEClient::MakeConnection}{wxddeclientmakeconnection}\rtfsp
 200 implicitly creates an instance of {\bf MyConnection} if the
 201 request for a connection is accepted, and the client then
 202 requests an {\it Advise} loop from the server (an Advise loop is
 203 where the server calls the client when data has changed).
 204
 205 \begin{verbatim}
 206   wxString server = "4242";
 207   wxString hostName;
 208   wxGetHostName(hostName);
 209
 210   // Create a new client
 211   MyClient *client = new MyClient;
 212   connection = (MyConnection *)client->MakeConnection(hostName, server, "IPC TEST");
 213
 214   if (!connection)
 215   {
 216     wxMessageBox("Failed to make connection to server", "Client Demo Error");
 217     return NULL;
 218   }
 219   connection->StartAdvise("Item");
 220 \end{verbatim}
 221
 222 Note that it is no longer necessary to call wxDDEInitialize or wxDDECleanUp, since
 223 wxWidgets will do this itself if necessary.
 224