[转] Delphi Socket Architecture
1 - Socket Programming in DelphiSockets were first introduced under Unix. Windows later, much later, followed. Since Windows has no easy multitasking library, a consortium defined a WindowsSocket specification which could use the Windows messages for socket scheduling. Third parties offered libraries (Trumpet was one of those). Later Windowsincluded its own WinSock version. And later Windows offered threading, which allows blocking sockets without message notifications. Therefore it is a small wonder that Delphi had to change its VCL components several times to allow networking. To make a long story short:
This paper will present the client and server socket component organization, as well as a small file exchange application. 2 - The ScktComp unit2.1 - Required ClassesCommunication between a Server and a Client needs:
To review what is really needed, have a look at the Socket Programming article. In any case, a component library will naturally include:
However there are several aspects:
2.2 - The ScktComp UnitThe tClientSocket and tServerSocket have been place in a single 90 K unit, which contains 12 classes. Splitting those into 6 units helped us better understand the relationships between those classes. Basically there are two layers:
2.3 - The WinSock encapsulation
To add threading capabilities, the following CLASSES (17 K) have been added:
As usual, threading add its share of complexity: there is a thread cache, one has to wait for thread termination etc. And to read and write socket as streams, the following tStream descendent is included:
In addition, when we use threads, there are some access protection to take care of:
2.4 - The User classesThe user is mainly concerned with
For this purpose, we have the following CLASSes:
2.5 - The UML class diagramBase on the previous analysis, we can draw the following diagram:
Note that
2.6 - ScktComp usageThe ScktComp is used in the following VCL units:
There is a Chat demo application in the DEMO directory of Delphi. This is a very elegant sample, since all the people taking part in a chat use the same application. The user either start with "Listen" or "Connect" and the tForm1 contains an IsServer Boolean which tells whether the application should use the tServerSocket or thetClientSocket. And all is contained in 7 K. I would be reluctant though to use this application as a Socket programming first example, since most Socket application are disymmetric by nature: there is a Server application handling server tasks, and a separate Client application used by all the clients dedicated to client business. The server receives an HTML page requests and uses CGI, ISAPI, ASP etc to build the page and send it over to the Client. On the other side, the Client receives the page and renders it, analyzing Style Sheets parameters and monkeying around with Visual Basic Script, Java Script or ActiveX components. Quite different jobs. The tClientSocket and tServerSocket are also published on the Palette to let us build Socket application which:
We will now build a simple file transfer application as an example. 3 - Socket File Transfer3.1 - The GoalThe Client will send a file name, and the Server will send the file back. To avoid typing errors, the Client selects the file names in a tFileListBox (so in effect he has already access to the files, but let's pretend that he cannot grab them directly). The Server loads the file and uses Send to transfer the file. 3.2 - The ClientThe Client application uses a tClientSocket. We have the following possibilities:
The tClientSocket.OnRead will be notified of data reception. The Client first reads the file size (4 bytes) and then reads the packets in a loop, until the reception buffer is empty. If the file was not received, the loop is exited, and the next OnRead will fetch the next packets. 3.3 - The ServerThe "listen" and "disconnect" buttons allow to start or stop the Server. The requested file will be read using a tFileStream and sent over to the client with tServerSocket.SendStream or tServerSocket.SendBuff. Since several Clientstransfer might overlap, we have to use a separate tFileStream for each incoming Client. We chose to save those tFileStreams as pointers in the tListBox.Objects. The key we chose to identify each Client is the ServerClientSocket handle. Here is the tServerSocket.OnAccept handler:
And when the fd_read notification arrives, we use the following tServerSocket.OnRead handler:
This procedure is unnecessarily complex, because we wanted to use packet transfers with delay in order to display several Clients in action. If we use a zero delay, only SendStream is called. Notice however that Delphi frees the tFileStream when the transfer is over. We personally prefer to let the unit which calls Create also call Free, but this is not the case here. 3.4 - Transfer exampleHere is a snapshot of 2 clients downloading files from the server: In this example
In this case, since the server uses an infinite loop until all the file is sent, we will observe that:
If we wanted to evenly spread the sending between clients, we should
4 - Download the SourcesYou can download the Client and Server projects (not very useful, but anyway...):
Those .ZIP files contain:
Those .ZIP
To use the .ZIP:
To remove the .ZIP simply delete the folder. As usual:
5 - The authorFelix John COLIBRI works at the Pascal Institute. Starting with Pascal in 1979, he then became involved with Object Oriented Programming, Delphi, Sql, Tcp/Ip, Html, UML. Currently, he is mainly active in the area of custom software development, Delphi Consulting and Delph training, and is a frequent speaker at Borland Developer Conferences. His web site features tutorials, technical papers about programming with full downloadable source code, and the description and calendar of forthcoming Delphi, Interbase, Asp.Net, Ado.Net and OOP / UML training sessions. |
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