HWatch (Function)

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WLanguage procedure executed in the main thread
WLanguage procedure executed in a secondary thread
Special cases

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HWatch (Function)

In french: HSurveille

Available only with this kind of connection

Warning
From version 2024, HTrack is kept for backward compatibility. This function has been replaced with HWatch.

Starts watching write actions (add, modify or delete) performed on a file in an HFSQL server. In case of modification, a specific procedure can be triggered.

This watch mechanism can be stopped with HWatchStop.

Example

// Starts watching the Order data file
// The ProcessOrder procedure is called whenever a record is added to the data file
HWatch(Order, ProcessOrder, hRecNumAll, hsAdd)

Syntax

<Result> = HWatch(<Data file> , <WLanguage procedure> [, <Record number> [, <Action to watch> [, <Execution mode>]]])

<Result>: Boolean

True if the data file watch mechanism is enabled,
False otherwise. To get more details on the error, use ErrorInfo.

<Data file>: Optional character string

Name of HFSQL data file to use. The data file is associated with a connection and with a server.
If this parameter is an empty string (""), HWatch manipulates the last data file used by the last HFSQL function (function starting with "H").

<WLanguage procedure>: Procedure name

Name of the WLanguage procedure ("callback") called when a change is made in the specified data file.
For more details on this procedure, see Parameters of the procedure used by HWatch.

<Record number>: Optional integer

Number of the record to watch.
hRecNumAll constant to watch all the records in the data file.

<Action to watch>: Optional constant (or combination of constants)

Actions to watch for which the procedure will be started:
hsAdd Monitors the operations for adding records.
hsCross Monitors the operations for crossing records.
hsDeletion Monitors the operations for deleting records.
hsModification Monitors the operations for modifying records.

<Execution mode>: Optional constant

WLanguage procedure execution mode:
hSecondaryThread Execute the WLanguage procedure in a secondary thread.

By default, the WLanguage procedure is executed in the main thread.

Remarks

WLanguage procedure executed in the main thread

By default, this WLanguage procedure is called in the main thread of the application, with the HFSQL context that corresponds to the procedure:

If the procedure is a local procedure of a window that uses an independent context, the context of the window will be used.
If the procedure belongs to a component that uses an independent context, the context of the component will be used.

Caution:
The procedure associated with HWatch can be called at any time in the application.
If the procedure reads records or moves between records in the data files, this will have an impact on the application (like the timers).
It may therefore be necessary to use:

HSavePosition and HRestorePosition to save the contexts of the files used.
a variable of type Alias to perform operations on alias files.

Multitask must not be used in the procedure code.

For more details on this procedure, see Parameters of the procedure used by HWatch.

WLanguage procedure executed in a secondary thread

If the WLanguage procedure is called in a secondary thread:

the HFSQL context is copied the first time HWatch is called. Only the directories containing the data files in HFSQL Classic mode and/or the connections in HFSQL Client/Server mode are stored.
Remark: If HChangeDir is called between two calls to the HWatch function, HChangeDir is ignored. Example:
```
HWatch(AFile, hSecondaryThread)
// The HChangeDir function is ignored
HChangeDir(AnotherFile)
HWatch(AnotherFile, hSecondaryThread)
```
UI elements (windows, controls, etc.) cannot be manipulated in the secondary thread. When a secondary thread must interact with the user or update the UI, it must use a process started from the main thread. This process can correspond to:
- a global procedure of the project or a local procedure (of a window, etc.) called by ExecuteMainThread,
- the "Request for refreshing the display" event of a window run by RequestRefreshUI.

For more details on this procedure, see Parameters of the procedure used by HWatch.

Special cases

Watching is defined for a data file and a connection:

If the physical name or the connection associated with the logical file name changes, the file watched by the server remains the one specified during the call to HWatch.
If the connection is closed, the watch mechanism is automatically stopped.
Watching continues if the connection is lost then restored by automatic reconnection.

To stop watching the data file, use HWatchStop.

Windows service:
HWatch and HWatchStop can be used in a Windows service.
When HWatch is used in a Windows service, it can only be tested on the deployed service, and not when testing the service via the "Go" option.

Business / UI classification: Business Logic

Component: wd290hf.dll