Creation of own service of updating: how to load files from the site

 

For this purpose in CA-Visual Objects (CA-VO) there is a wonderful library «Internet». For operation according to the ftp protocol – we will use the class «cFTP», and for http – «cHTTP». Let's say I already have a site. I don't want to use loading according to the FTP-protocol. Means, I need to use the class «cHTTP». It seems to me that the description and examples of operation with the Internet in CA-VO it isn't enough. As speak in Russia: «без пол литра – не разобраться». I.e., to understand how it works - it is necessary to drink something alcoholic.

I don't drink alcohol. Therefore, I looked for information necessary to me at forums. Very quickly I understood that in loading of files there are certain difficulties. Otherwise, why programmers try to improve and correct «Internet»-library all the time. At first the Australian Geoff Schaller programmer was engaged in it. And then is proceeded in «IC2» (see CA-Visual Objects: Third-Party Products, point 21).

 

On a photo: Geoff Schaller

 

The most unpleasant for beginners is an absence of good examples and the detailed description. If you don't have it - you is guaranteed you will spend a lot of time. With such situation we will be helped by SDK CA-VO.

 

I want to warn my "followers" directly: in my opinion, the code in SDK can sometimes not correspond to a real code in CA-VO. It strongly complicates understanding of the task. I.e., some errors which I found there, disappeared when instead of SDK library I used standard (not SDK) «Internet»-library. For example, in a «Help» syntax is specified:

 

CHttp{ <cCaption>, <n> }  => SELF

Init( <cCaption>, <n> )  => SELF

 

Actually, in CHttp:Init() there are three parameters (see SDK, «Internet SDK»):

 

METHOD Init( cCaption, n, lStat ) CLASS CHttp

 

Unfortunately, the third parameter isn't described. Give we will look at it. «lStat» actually is responsible for use of critical sections. TRUE is to use critical sections, FALSE – No. The author of SDK uses own mechanism of operation with sections and considers that you also use it (by default, lStat = TRUE). Therefore, by operation with SDK if you don't plan to use critical sections, you should write: cHTTP{ cCaption, n, FALSE }. Or you should rewrite SDK. The most ridiculous, in the worker (not SDK) «Internet»-library this problem isn't present. This problem exists only in SDK!

 

If you don't want to work in SDK with critical sections, I suggest making a subclass:

 

 

 

 

 

Here "csWSA" is a global variable of my critical section. And you can have another. I remind, in a normal code (not for operation with SDK) is it isn't necessary, moreover – it is contraindicated. It only for operation with SDK («Internet SDK»)!!!

 

Let's return back a little. Why I refused idea of use of ftp? First, I wanted to consider more difficult option. It can be useful if your site doesn't support loading through the FTP-protocol.

Secondly, use of HTTP allows use safer methods of communication. Thirdly, there are standard means for scanning of the files which are on ftp, and for http - No. I want that users received the required files only after processing of the form on the site. Plus, we can make very simple check of settings in Windows: if we can come into the Internet through IE – we can make loading (updating).

 

Yes… Somehow I precipitately refused idea of use of ftp… My further research showed that in standard library «Internet» there are no suitable means for operation with files according to the http-protocol. In the childhood I liked to watch newsreels. In one of them there was such phrase (I translate into English): «The nutlet of knowledge is solid. But, to us will help to split it newsreel "I want to know everything!"» And good music! What music do you listen to? And I (it is direct now) – «RPWL».

 

Plan such:

 

Well, and when closing we will do everything on the contrary.

 

Let's see how and what is done in the library «Internet SDK». The first that we will see is a lack of control of actions. I.e., to us won't prevent to add check of opening of Internet session and connection with a Host (site). Plus, the analysis of the SDK code shows weak processing of closing of processes. It means that it will be better for us if we remake these slices of a code (because, we have no warranties that in standard library these issues are resolved correctly).

 

Now we have to take another important decision: how we will control and report the causes of malfunctions. I decided to use standard reception – if Windows considers that Internet session isn't open – I will simply report about it, without specifying the reason. Clearing up of the reasons is a task of the administrator. We don't write the diagnostic program? The reasons can be much: Windows (Firewall), devices (a board, the modem, a router, a cable), provider…

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

So, how to open – we know how to close – already wrote. It remains to learn how to download the files.

-         How many it is already possible to scoff at readers?

-         It is necessary to work a little more and to rewrite the standard GetFile() method. Now we will pick up to it «suit» and «shoes»:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I want to warn impatient testers: be attentive! My code isn't ideal and doesn't consider many nuances. For example, the method of obtaining files doesn't consider existence of such opportunity, as URL redirection. I.e., if the user receives a certain link which actually will redirect him to other place where he will receive following (perhaps, correct) – my GetFile() method won't work.

 

 

 

Example "on snack":

 

 

 

 

 

 

 

      Good luck!