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577 lines
21 KiB
577 lines
21 KiB
/*
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A dummy source file for documenting the library.
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Copied from HOWTO with small syntactic changes.
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*/
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/**
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\mainpage The DCOP Desktop COmmunication Protocol library
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DCOP is a simple IPC/RPC mechanism built to operate over sockets.
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Either unix domain sockets or TCP/IP sockets are supported. DCOP is
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built on top of the Inter Client Exchange (ICE) protocol, which comes
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standard as a part of X11R6 and later. It also depends on Qt, but
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beyond that it does not require any other libraries. Because of this,
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it is extremely lightweight, enabling it to be linked into all Trinity
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applications with low overhead.
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\section model Model:
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The model is simple. Each application using DCOP is a client. They
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communicate to each other through a DCOP server, which functions like
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a traffic director, dispatching messages/calls to the proper
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destinations. All clients are peers of each other.
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Two types of actions are possible with DCOP: "send and forget"
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messages, which do not block, and "calls," which block waiting for
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some data to be returned.
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Any data that will be sent is serialized (also referred to as marshalling
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in CORBA speak) using the built-in QDataStream operators available in all
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of the Qt classes. This is fast and easy. In fact it's so little work
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that you can easily write the marshalling code by hand. In addition,
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there's a simple IDL-like compiler available (dcopidl and dcopidl2cpp)
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that generates stubs and skeletons for you. Using the dcopidl compiler
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has the additional benefit of type safety.
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The manual method is covered first, followed by the automatic IDL method.
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\section establish Establishing the Connection:
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KApplication has gained a method called \p KApplication::dcopClient()
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which returns a pointer to a DCOPClient instance. The first time this
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method is called, the client class will be created. DCOPClients have
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unique identifiers attached to them which are based on what
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KApplication::name() returns. In fact, if there is only a single
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instance of the program running, the appId will be equal to
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KApplication::name().
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To actually enable DCOP communication to begin, you must use
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\p DCOPClient::attach(). This will attempt to attach to the DCOP server.
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If no server is found or there is any other type of error,
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DCOPClient::attach() will return false. KApplication will catch a dcop
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signal and display an appropriate error message box in that case.
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After connecting with the server via DCOPClient::attach(), you need to
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register this appId with the server so it knows about you. Otherwise,
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you are communicating anonymously. Use the
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DCOPClient::registerAs(const QCString &name) to do so. In the simple
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case:
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\code
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appId = client->registerAs(kapp->name());
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\endcode
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If you never retrieve the DCOPClient pointer from KApplication, the
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object will not be created and thus there will be no memory overhead.
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You may also detach from the server by calling DCOPClient::detach().
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If you wish to attach again you will need to re-register as well. If
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you only wish to change the ID under which you are registered, simply
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call DCOPClient::registerAs() with the new name.
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KUniqueApplication automatically registers itself to DCOP. If you
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are using KUniqueApplication you should not attach or register
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yourself, this is already done. The appId is by definition
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equal to \p kapp->name(). You can retrieve the registered DCOP client
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by calling \p kapp->dcopClient().
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\section sending_data Sending Data to a Remote Application:
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To actually communicate, you have one of two choices. You may either
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call the "send" or the "call" method. Both methods require three
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identification parameters: an application identifier, a remote object,
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a remote function. Sending is asynchronous (i.e. it returns immediately)
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and may or may not result in your own application being sent a message at
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some point in the future. Then "send" requires one and "call" requires
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two data parameters.
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The remote object must be specified as an object hierarchy. That is,
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if the toplevel object is called \p fooObject and has the child
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\p barObject, you would reference this object as \p fooObject/barObject.
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Functions must be described by a full function signature. If the
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remote function is called \p doIt, and it takes an int, it would be
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described as \p doIt(int). Please note that the return type is not
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specified here, as it is not part of the function signature (or at
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least the C++ understanding of a function signature). You will get
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the return type of a function back as an extra parameter to
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DCOPClient::call(). See the section on call() for more details.
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In order to actually get the data to the remote client, it must be
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"serialized" via a QDataStream operating on a QByteArray. This is how
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the data parameter is "built". A few examples will make clear how this
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works.
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Say you want to call \p doIt as described above, and not block (or wait
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for a response). You will not receive the return value of the remotely
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called function, but you will not hang while the RPC is processed either.
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The return value of DCOPClient::send() indicates whether DCOP communication
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succeeded or not.
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\code
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QByteArray data;
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QDataStream arg(data, IO_WriteOnly);
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arg << 5;
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if (!client->send("someAppId", "fooObject/barObject", "doIt(int)",
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data))
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qDebug("there was some error using DCOP.");
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\endcode
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OK, now let's say we wanted to get the data back from the remotely
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called function. You have to execute a DCOPClient::call() instead of a
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DCOPClient::send(). The returned value will then be available in the
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data parameter "reply". The actual return value of call() is still
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whether or not DCOP communication was successful.
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\code
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QByteArray data, replyData;
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QCString replyType;
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QDataStream arg(data, IO_WriteOnly);
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arg << 5;
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if (!client->call("someAppId", "fooObject/barObject", "doIt(int)",
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data, replyType, replyData))
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qDebug("there was some error using DCOP.");
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else {
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QDataStream reply(replyData, IO_ReadOnly);
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if (replyType == "QString") {
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QString result;
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reply >> result;
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print("the result is: %s",result.latin1());
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} else
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qDebug("doIt returned an unexpected type of reply!");
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}
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\endcode
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\section receiving_data Receiving Data via DCOP:
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Currently the only real way to receive data from DCOP is to multiply
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inherit from the normal class that you are inheriting (usually some
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sort of QWidget subclass or QObject) as well as the DCOPObject class.
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DCOPObject provides one very important method: DCOPObject::process().
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This is a pure virtual method that you must implement in order to
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process DCOP messages that you receive. It takes a function
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signature, QByteArray of parameters, and a reference to a QByteArray
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for the reply data that you must fill in.
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Think of DCOPObject::process() as a sort of dispatch agent. In the
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future, there will probably be a precompiler for your sources to write
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this method for you. However, until that point you need to examine
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the incoming function signature and take action accordingly. Here is
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an example implementation.
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\code
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bool BarObject::process(const QCString &fun, const QByteArray &data,
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QCString &replyType, QByteArray &replyData)
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{
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if (fun == "doIt(int)") {
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QDataStream arg(data, IO_ReadOnly);
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int i; // parameter
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arg >> i;
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QString result = self->doIt (i);
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QDataStream reply(replyData, IO_WriteOnly);
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reply << result;
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replyType = "QString";
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return true;
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} else {
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qDebug("unknown function call to BarObject::process()");
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return false;
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}
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}
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\endcode
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\section receiving_calls Receiving Calls and processing them:
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If your applications is able to process incoming function calls
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right away the above code is all you need. When your application
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needs to do more complex tasks you might want to do the processing
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out of 'process' function call and send the result back later when
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it becomes available.
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For this you can ask your DCOPClient for a transactionId. You can
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then return from the 'process' function and when the result is
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available finish the transaction. In the mean time your application
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can receive incoming DCOP function calls from other clients.
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Such code could like this:
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\code
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bool BarObject::process(const QCString &fun, const QByteArray &data,
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QCString &, QByteArray &)
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{
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if (fun == "doIt(int)") {
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QDataStream arg(data, IO_ReadOnly);
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int i; // parameter
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arg >> i;
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QString result = self->doIt(i);
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DCOPClientTransaction *myTransaction;
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myTransaction = kapp->dcopClient()->beginTransaction();
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// start processing...
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// Calls slotProcessingDone when finished.
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startProcessing( myTransaction, i);
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return true;
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} else {
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qDebug("unknown function call to BarObject::process()");
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return false;
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}
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}
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slotProcessingDone(DCOPClientTransaction *myTransaction, const QString &result)
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{
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QCString replyType = "QString";
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QByteArray replyData;
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QDataStream reply(replyData, IO_WriteOnly);
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reply << result;
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kapp->dcopClient()->endTransaction( myTransaction, replyType, replyData );
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}
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\endcode
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\section dcopidl Using the dcopidl compiler:
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dcopidl makes setting up a DCOP server easy. Instead of having to implement
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the process() method and unmarshalling (retrieving from QByteArray) parameters
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manually, you can let dcopidl create the necessary code on your behalf.
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This also allows you to describe the interface for your class in a
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single, separate header file.
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Writing an IDL file is very similar to writing a normal C++ header. An
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exception is the keyword 'ASYNC'. It indicates that a call to this
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function shall be processed asynchronously. For the C++ compiler, it
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expands to 'void'.
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Example:
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\code
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#ifndef MY_INTERFACE_H
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#define MY_INTERFACE_H
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#include <dcopobject.h>
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class MyInterface : virtual public DCOPObject
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{
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K_DCOP
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k_dcop:
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virtual ASYNC myAsynchronousMethod(QString someParameter) = 0;
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virtual QRect mySynchronousMethod() = 0;
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};
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#endif
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\endcode
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As you can see, you're essentially declaring an abstract base class, which
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virtually inherits from DCOPObject.
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If you're using the standard Trinity build scripts, then you can simply
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add this file (which you would call MyInterface.h) to your sources
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directory. Then you edit your Makefile.am, adding 'MyInterface.skel'
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to your SOURCES list and MyInterface.h to include_HEADERS.
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The build scripts will use dcopidl to parse MyInterface.h, converting
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it to an XML description in MyInterface.kidl. Next, a file called
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MyInterface_skel.cpp will automatically be created, compiled and
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linked with your binary.
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The next thing you have to do is to choose which of your classes will
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implement the interface described in MyInterface.h. Alter the inheritance
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of this class such that it virtually inherits from MyInterface. Then
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add declarations to your class interface similar to those on MyInterface.h,
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but virtual, not pure virtual.
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Example:
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\code
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class MyClass: public QObject, virtual public MyInterface
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{
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Q_OBJECT
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public:
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MyClass();
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~MyClass();
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ASYNC myAsynchronousMethod(QString someParameter);
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QRect mySynchronousMethod();
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};
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\endcode
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\note (Qt issue) Remember that if you are inheriting from QObject, you must
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place it first in the list of inherited classes.
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In the implementation of your class' ctor, you must explicitly initialize
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those classes from which you are inheriting from. This is, of course, good
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practice, but it is essential here as you need to tell DCOPObject the name of
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the interface which your are implementing.
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Example:
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\code
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MyClass::MyClass()
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: QObject(),
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DCOPObject("MyInterface")
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{
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// whatever...
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}
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\endcode
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Now you can simply implement the methods you have declared in your interface,
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exactly the same as you would normally.
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Example:
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\code
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void MyClass::myAsynchronousMethod(QString someParameter)
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{
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qDebug("myAsyncMethod called with param `" + someParameter + "'");
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}
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\endcode
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It is not necessary (though very clean) to define an interface as an
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abstract class of its own, like we did in the example above. We could
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just as well have defined a k_dcop section directly within MyClass:
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\code
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class MyClass: public QObject, virtual public DCOPObject
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{
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Q_OBJECT
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K_DCOP
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public:
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MyClass();
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~MyClass();
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k_dcop:
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ASYNC myAsynchronousMethod(QString someParameter);
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QRect mySynchronousMethod();
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};
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\endcode
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In addition to skeletons, dcopidl2cpp also generate stubs. Those make
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it easy to call a DCOP interface without doing the marshalling
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manually. To use a stub, add MyInterface.stub to the SOURCES list of
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your Makefile.am. The stub class will then be called MyInterface_stub.
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\section iuc Inter-user communication:
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Sometimes it might be interesting to use DCOP between processes
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belonging to different users, e.g. a frontend process running
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with the user's id, and a backend process running as root.
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To do this, two steps have to be taken:
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a) both processes need to talk to the same DCOP server
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b) the authentication must be ensured
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For the first step, you simply pass the server address (as
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found in .DCOPserver) to the second process. For the authentication,
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you can use the ICEAUTHORITY environment variable to tell the
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second process where to find the authentication information.
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(Note that this implies that the second process is able to
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read the authentication file, so it will probably only work
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if the second process runs as root. If it should run as another
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user, a similar approach to what tdesu does with xauth must
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be taken. In fact, it would be a very good idea to add DCOP
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support to tdesu!)
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For example
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ICEAUTHORITY=~user/.ICEauthority tdesu root -c kcmroot -dcopserver `cat ~user/.DCOPserver`
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will, after tdesu got the root password, execute kcmroot as root, talking
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to the user's dcop server.
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NOTE: DCOP communication is not encrypted, so please do not
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pass important information around this way.
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\section protocol DCOP Protocol description:
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A DCOPSend message does not expect any reply.
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\code
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data: << fromId << toId << objId << fun << dataSize + data[dataSize]
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\endcode
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A DCOPCall message can get a DCOPReply, a DCOPReplyFailed
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or a DCOPReplyWait message in response.
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\code
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data: << fromId << toId << objId << fun << dataSize + data[dataSize]
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\endcode
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DCOPReply is the successful reply to a DCOPCall message
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\code
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data: << fromId << toId << replyType << replyDataSize + replyData[replyDataSize]
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\endcode
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DCOPReplyFailed indicates failure of a DCOPCall message
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\code
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data: << fromId << toId
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\endcode
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DCOPReplyWait indicates that a DCOPCall message is successfully
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being processed but that response will come later.
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\code
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data: << fromId << toId << transactionId
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\endcode
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DCOPReplyDelayed is the successful reply to a DCOPCall message
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after a DCOPReplyWait message.
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\code
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data: << fromId << toId << transactionId << replyType << replyData
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\endcode
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DCOPFind is a message much like a "call" message. It can however
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be send to multiple objects within a client. If a function in a
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object that is being called returns a boolean with the value "true",
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a DCOPReply will be send back containing the DCOPRef of the object
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who returned "true".
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All c-strings (fromId, toId, objId, fun and replyType), are marshalled with
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their respective length as 32 bit unsigned integer first:
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\code
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data: length + string[length]
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\endcode
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\note This happens automatically when using QCString on a QDataStream.
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\section Deadlock protection and reentrancy
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When a DCOP call is made, the dcop client will be monitoring the
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dcop connection for the reply on the call. When an incoming call is
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received in this period, it will normally not be processed but queued
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until the outgoing call has been fully handled.
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However, the above scenario would cause deadlock if the incoming call
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was directly or indirectly a result of the outgoing call and the reply
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on the outgoing call is waiting for the result of the incoming call.
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(E.g. a circular call such as client A calling client B, with client B
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calling client A)
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To prevent deadlock in this case, DCOP has a call tracing mechanism that
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detects circular calls. When it detects an incoming circular call that
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would otherwise be queued and as a result cause deadlock, it will handle
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the incoming call immediately instead of queueing it. This means that the
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incoming call may be processed at a point in the code where an outgoing
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DCOP call is made. An application should be aware of this kind of
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reentrancy. A special case of this is when a DCOP client makes a call
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to itself, such calls are always handled directly.
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Call tracing works by appending a key to each outgoing call. When a client
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receives an incoming call while waiting for a response on an outgoing call,
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it will check if the key of the incoming call is equal to the key used for
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the last outgoing call. If the keys are equal a circular call has been
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detected.
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The key used by clients is 0 if they have not yet received any key. In this
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case the server will send them back a unique key that they should use in
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further calls. If a client makes an outgoing call in response to an incoming
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call it will use the key of the incoming call for the outgoing call instead
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of the key that was received from the server.
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A key value of 1 has a special meaning and is used for non-call messages
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such as DCOPSend, DCOPReplyFailed and DCOP signals.
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A key value of 2 has a special meaning and is used for priority calls.
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When a dcop clien is in priority call mode, it will only handle incoming
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calls that have a key value of 2.
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NOTE: If client A and client B would call each other simultaneously there
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is still a risk of deadlock because both calls would have unique keys and
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both clients would decide to queue the incoming call until they receive
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a response on their outgoing call.
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\section dcop_signals DCOP Signals:
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Sometimes a component wants to send notifications via DCOP to other
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components but does not know which components will be interested in these
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notifications. One could use a broadcast in such a case but this is a very
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crude method. For a more sophisticated method DCOP signals have been invented.
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DCOP signals are very similair to Qt signals, there are some differences
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though. A DCOP signal can be connected to a DCOP function. Whenever the DCOP
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signal gets emitted, the DCOP functions to which the signal is connected are
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being called. DCOP signals are, just like Qt signals, one way. They do not
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provide a return value. For declaration of dcop signals, the keyword
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\p k_dcop_signals is provided. A declaration looks like this:
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\code
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class Example : virtual public DCOPClient
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{
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K_DCOP
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k_dcop:
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// some ordinary dcop methods here
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|
...
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|
k_dcop_signals:
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// our dcop signal
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void clientDied(pid_t pid);
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|
...
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|
}
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\endcode
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|
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A DCOP signal originates from a DCOP Object/DCOP Client combination (sender).
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|
It can be connected to a function of another DCOP Object/DCOP Client
|
|
combination (receiver).
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|
|
|
\note There are two major differences between connections of Qt signals and
|
|
connections of DCOP signals. In DCOP, unlike Qt, a signal connections can
|
|
have an anonymous sender and, unlike Qt, a DCOP signal connection can be
|
|
non-volatile.
|
|
|
|
With DCOP one can connect a signal without specifying the sending DCOP Object
|
|
or DCOP Client. In that case signals from any DCOP Object and/or DCOP Client
|
|
will be delivered. This allows the specification of certain events without
|
|
tying oneself to a certain object that implementes the events.
|
|
|
|
Another DCOP feature are so called non-volatile connections. With Qt signal
|
|
connections, the connection gets deleted when either sender or receiver of
|
|
the signal gets deleted. A volatile DCOP signal connection will behave the
|
|
same. However, a non-volatile DCOP signal connection will not get deleted
|
|
when the sending object gets deleted. Once a new object gets created with
|
|
the same name as the original sending object, the connection will be restored.
|
|
There is no difference between the two when the receiving object gets deleted,
|
|
in that case the signal connection will always be deleted.
|
|
|
|
A receiver can create a non-volatile connection while the sender doesn't (yet)
|
|
exist. An anonymous DCOP connection should always be non-volatile.
|
|
|
|
The following example shows how KLauncher emits a signal whenever it notices
|
|
that an application that was started via KLauncher terminates:
|
|
|
|
\code
|
|
QByteArray params;
|
|
QDataStream stream(params, IO_WriteOnly);
|
|
stream << pid;
|
|
kapp->dcopClient()->emitDCOPSignal("clientDied(pid_t)", params);
|
|
\endcode
|
|
|
|
The task manager of the Trinity panel connects to this signal. It uses an
|
|
anonymous connection (it doesn't require that the signal is being emitted
|
|
by KLauncher) that is non-volatile:
|
|
|
|
\code
|
|
connectDCOPSignal(0, 0, "clientDied(pid_t)", "clientDied(pid_t)", false);
|
|
\endcode
|
|
|
|
It connects the clientDied(pid_t) signal to its own clientDied(pid_t) DCOP
|
|
function. In this case the signal and the function to call have the same name.
|
|
This isn't needed as long as the arguments of both signal and receiving function
|
|
match. The receiving function may ignore one or more of the trailing arguments
|
|
of the signal. E.g. it is allowed to connect the clientDied(pid_t) signal to
|
|
a clientDied(void) DCOP function.
|
|
|
|
|
|
\section conclusion Conclusion:
|
|
|
|
Hopefully this document will get you well on your way into the world of
|
|
inter-process communication with Trinity! Please direct all comments and/or
|
|
suggestions to the Trinity Core Developers List \<kde-core-devel@kde.org\>.
|
|
|
|
*/
|