.NET/ASP.NET/C#/VB.NET PDF Document SDK

We tell WCF this by setting the SessionMode property to SessionModeRequired Note that this doesn t actually switch on sessions; it merely says that anything that wants to communicate using this contract had better do so with sessions enabled Remember, the contract is separate from implementations that conform to the contract The effect of this setting is that WCF will produce an error if you try to use this contract without enabling sessions; we ll see how to enable sessions by modifying the client and server configuration files once we ve finished modifying the code A session will be established the first time a client connects to a service, which presents our application with another problem WCF won t send a message until it has something to send, so our chat client will first connect to the service when we send our first note.

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(Creating an instance of the ChatServiceProxy does not connect nothing goes over the network until the first time you try to invoke an operation) But we want clients to be able to receive notes straight away, without being required to post one first So we need a way for clients to announce their presence to the service without sending a note That s why Example 13-9 adds a Connect method And we ve also provided a Disconnect method for clients to announce that they are leaving so that the chat server doesn t attempt to send notes to clients that are no longer there (Without this, the server would get an exception the next time it tried to send a message.

To add this translation to the project, simply add the following line to the project file: TRANSLATIONS += sdi_sv_SE.ts You can add any number of translations to a project by adding new TRANSLATION += lines as appropriate. You can also specify several translations at once by separating them by spaces or tabs.

Although it would notice that the clients had gone, an explicit disconnect is a bit neater it also makes it possible to tell the difference between users who deliberately leave the conversation and users who get cut off due to problems) We now need to update the server to implement the modified contract, and to track the clients..

In general, the WS-* family of web service protocols avoids depending on HTTP. This may seem like a peculiar tendency for web service standards, but a lot of the organizations involved in creating these specifications wanted the message formats to be useful in message-queue-based systems as well as HTTP. So in general, they tend to avoid transport-specific mechanisms.

Integrating this with the Atlas-based web form you saw earlier and enabling it for asynchronous updates, Ajax style, is easy. Figure 11-18 shows the designer for the web form.

Our service is going to need to maintain a list of connected clients so that it can notify every client when it receives each note. We can store the list as private data in our service class, but since that one list needs to be available across all sessions, we need to tell WCF that we only ever want it to create one instance of that class. WCF offers several different modes for creating instances of your service class. It can create one per client session that s useful when you want per-session state. But in our case, all notes get sent to everyone, so the only interesting state is global. Since our application state is global, we don t have much use for per-client instances here. WCF can also create a new instance of your service class for every single request if you don t hold any state in the service class itself this is a reasonable thing to do. But in our case, we want one instance for the lifetime of the service. We can indicate this like so:

[ServiceBehavior( InstanceContextMode=InstanceContextMode.Single, ConcurrencyMode=ConcurrencyMode.Reentrant)] public class ChatService : IChatService {

Listing 15-8. A CMake project file for a basic Qt application PROJECT( basics ) SET( basics_SOURCES main.cpp mainwindow.cpp otherdialog.cpp preferencedialog.cpp ) SET( basics_HEADERS mainwindow.h otherdialog.h preferencedialog.h ) SET( basics_FORMS otherdialog.ui preferencedialog.ui ) FIND_PACKAGE( Qt4 REQUIRED ) INCLUDE( ${QT_USE_FILE} ) QT4_WRAP_CPP( basics_HEADERS_MOC ${basics_HEADERS} ) QT4_WRAP_UI( basics_FORMS_HEADERS ${basics_FORMS} ) INCLUDE_DIRECTORIES( ${CMAKE_CURRENT_BINARY_DIR} ) ADD_DEFINITIONS( ${QT_DEFINITIONS} ) ADD_EXECUTABLE( basics ${basics_SOURCES} ${basics_HEADERS_MOC} ${basics_FORMS_HEADERS} ) TARGET_LINK_LIBRARIES( basics ${QT_LIBRARIES} )

We added a ServiceBehavior attribute to the code to specify this single-instance behavior. Notice that we also asked for a ConcurrencyMode of Reentrant. This tells WCF to have our service work on requests for only one session at a time if requests from multiple clients come in simultaneously, WCF will service them one after another. This is convenient as it means that as long as any single client does only one thing at a time, we don t need to write any code to ensure the thread safety of our state handling.