MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

socketsSample.displayStatus("Client sent: " + string + ".");
}, onError);
The DataWriter.storeAsync call is what actually writes the data to the stream in the socket. If you set
a breakpoint here and on both messagereceived event handlers, you’ll then see that storeAsync
generates a message to the server side, hitting onServerMessageReceived in js/startListener.js. This will
then write the message back to the socket, which will hit onMessage-Received in js/connectToListener.js,
which displays the message. (And to complete the process, Scenario 4 gives you a button to call the
socket’s close method.)
The sample does everything with the same app on localhost to make it easier to see how the process
works. Typically, of course, the server will be running on another machine entirely, but the steps of
setting up a listener apply just the same. As noted in Chapter 13, localhost connections work only on a
machine with a developer license and will not work for apps acquired through the Windows Store.
Stream Sockets
In contrast to datagram sockets, streaming data over sockets uses the Transmission Control Protocol
(TCP). The hallmark of TCP is accurate and reliable delivery—it guarantees that the bytes received are
the same as the bytes that were sent: when a packet is sent across the network, TCP will attempt to
retransmit the packet if there are problems along the way. This is why it’s part of TCP/IP, which gives us
the World Wide Web, email, file transfers, and lots more. HTTP, SMTP, and the Session Initiation
Protocol (SIP) are also built on TCP. In all cases, clients and servers just see a nice reliable stream of data
flowing from one end to the other.
Unlike datagram sockets, for which we have a single class in WinRT for both sides of the relationship,
stream sockets are more distinctive to match the unique needs of the client and server roles. On the
client side is Windows.Networking.Sockets.StreamSocket; on the server it’s StreamSocketListener.
Starting with the latter, the StreamSocketListener object looks quite similar to the DatagramSocket
we’ve just covered in the previous section, with these methods, properties, and events:
• information Provides a StreamSocketListenerInformation object containing a localPort
string.
• control Provides a StreamSocketListenerControl object with a qualityOfService property.
• connectionreceived An event that’s fired when a connection is made to the listener. Its event
arguments are a StreamSocketListenerConnectionReceivedEventArgs that contains a single
property, socket. This is the StreamSocket for the client, in which is an outputStream property
where the listener can obtain the data stream.
• bindEndpointAsync and bindServiceNameAsync Binds the listener to a HostName and service
name, or binds just a service name.
close Terminates connections and aborts pending operations.
680