.NET/Java PDF, Tiff, Barcode SDK Library

An alternative to the single-instance context mode would have been to store our state in a static field. This would share the data across all clients, which is what we need. But then we d be on our own for thread safety. The ConcurrencyMode property applies only to any particular instance of the service, so if you don t choose the single-instance mode, WCF will let different instances of your service execute simultaneously. In practice, real applications are likely to need to do their own thread synchronization. Here we re relying on clients making only one call at a time, which might work in a small, controlled example but is a risky thing to do if you don t completely trust your client machines. (Even with only one session at a time, a single client session could invoke multiple operations simultaneously.) You may be wondering why we didn t use ConcurrencyMode.Single, which enforces a completely strict one-at-a-time model. Unfortunately, that turns out to prevent you from calling back into clients while you re in the middle of handling a call from a client a blocking outbound call from a nonreentrant singlethreaded context presents an opportunity for deadlocks, so WCF forbids it.

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Next, we ll add a field to hold the state a collection of currently connected clients:

To build the project file shown in Listing 15-8 you need to understand how CMake is executed Before you look at the command line options, have a look at the features that CMake offers You can run CMake from the source tree, just like QMake, leaving the intermediate files and the results in the source tree It is also possible to run CMake from a distant directory, resulting in a clean source tree This means that intermediate files and result files such as applications and libraries will not appear in the source tree By keeping the source tree clean from these files, you can put the entire source tree under version control at all times.

private Dictionary<IChatClient, string> clientsAndNames = new Dictionary<IChatClient, string>();

This is a dictionary where the key type is the client callback interface we defined earlier. The value is the client s name. To see how this gets used, here s the Connect implementation:

The UpdatePanel control contains a label, which is a server-side control that allows you to set its contents. You will be setting it to be an <img> tag and pointing the image source at the PH.aspx page so that it will return the graphic into the pane wrapped by the UpdatePanel control, thus achieving an asynchronous update.

public bool Connect(string name) { if (clientsAndNames.ContainsValue(name)) { // Name already in use, so refuse connection return false; } IChatClient clientCallback = OperationContext.Current.GetCallbackChannel<IChatClient>(); // clientsAndNames is shared state, but we're not locking // here, because we're relying on ConcurrentMode.Reentrant // to give us messages one at a time. clientsAndNames.Add(clientCallback, name); Console.WriteLine(name + " connected"); } return true;

This means that you do not have to clean out any unwanted files when adding your source to version control, and you avoid the risk of adding intermediate files to the version control system that you might be using by accident You can also use CMake to create projects for many different build systems The different systems are targeted by using different generators On Linux and Mac OS X, the default generator usually works This generator targets the GCC system On Windows it can be necessary to specify which generator to use If you are using the open source version of Qt together with MinGW, the generator to use is MinGW Makefiles You do this using the -G command line option more about this later Other supported build systems include various Microsoft compilers, Borland, Watcom, MSYS, and generic Unix Makefiles.

The first thing we do is check that the username is unique. Now that we re maintaining a list of connected clients, we re in a position to prevent multiple users from picking the same name. If a new user is trying to sign up with a duplicate name, we return false. (A return code here makes more sense than an exception because this isn t really an exceptional condition.) If the name looks OK, we retrieve the client callback interface with the following expression:

OperationContext.Current.GetCallbackChannel<IChatClient>()

the operation that your code is handling right now. One of the services it provides is the ability to retrieve the callback interface when a duplex contract is in use. GetCallbackChannel returns a proxy object similar to the one the client uses to talk to the service, but this proxy goes in the other direction it invokes operations on the client that called our Connect method. We just add this to the dictionary of connected clients, associating it with the client s chosen name, and then return true to indicate that we re happy that the user s name wasn t previously in use and that we have accepted the user s connection.

Next, let s look at the modified PostNote:

When running CMake, it is important to have all the tools that you are planning to use in your PATH environment variable, including your compiler and Qt tools (such as uic and moc) These tools are usually in the path; if not, CMake will tell you what it cannot find..