The DIRWATCHER control

With the release of version 10.1 a new control type called DIRWATCHER (“Directory Watcher”) has been added to OpenInsight. This is a fairly simple control which allows you to monitor one or more directories on your system and then receive notifications when the contents are changed.

Using the control is very straightforward:

  • Use the WATCHDIR method to add a directory to monitor for changes.
  • Handle the CHANGED event to receive notifications of directory changes.
  • Use the STOP method to stop monitoring directories.

We’ll take a quick look at each of these methods and events below along with a couple of important properties:

The WATCHDIR method

This method allows you to specify a directory to monitor along with some optional flags. It may be called multiple times to watch more than one directory.

bSuccess = Exec_Method( CtrlEntID, "WATCHDIR", DirName, bSubtree, Flags )
ParameterDescription
DirName(required) Specifies the name of the directory to watch. It should be a fully qualified, non-relative directory path.
bSubtree(optional) Set to TRUE$ to monitor all sub-directories beneath DirName as well. Defaults to FALSE$.
Flags(optional) A bit-mask value containing a set of flags that denote the events to monitor in the directories. It defaults to the following flags:

FILE_NOTIFY_CHANGE_FILE_NAME$
FILE_NOTIFY_CHANGE_LAST_WRITE$
FILE_NOTIFY_CHANGE_CREATION$

The flag values are specified in the MSWIN_FILENOTIFY_EQUATES insert record,

This method returns TRUE$ if successful, or FALSE$ otherwise.

The STOP method

This method stops the control monitoring its specified directories.

bSuccess = Exec_Method( CtrlEntID, "STOP" )

This method returns TRUE$ if successful, or FALSE$ otherwise.

(Note – to resume directory monitoring after the STOP method has been called the WATCHDIR method(s) must be executed again).

The CHANGED event

This event is raised when changes have been detected in the monitored directories.

bForward = CHANGED( NewData )

This event passes a single parameter called NewData which contains an @vm-delimited list of changed items (i.e. notifications). Each item in the list comprises an “action code” and the name and path of the affected file, delimited by an @svm.

Action codes are defined in the MSWIN_FILENOTIFY_EQUATES insert record like so:

   equ FILE_ACTION_ADDED$               to 0x00000001   
   equ FILE_ACTION_REMOVED$             to 0x00000002   
   equ FILE_ACTION_MODIFIED$            to 0x00000003   
   equ FILE_ACTION_RENAMED_OLD_NAME$    to 0x00000004   
   equ FILE_ACTION_RENAMED_NEW_NAME$    to 0x00000005 

Remarks

If a monitored directory experiences a high volume of changes (such as copying or removing thousands of files) it could generate a correspondingly high number of CHANGED events, which in turn could produce an adverse affect on your application and slow it down. In order to deal with this potential issue it is possible to “bundle up” multiple notifications with the NOTIFYTHRESHOLD property into a single CHANGED event so they may be processed more efficiently.

The NOTIFYTHRESHOLD property

The NOTIFYTHRESHOLD property is an integer that specifies the maximum number of notifications that should be bundled before a CHANGED event is raised.

CurrVal = Get_Property( CtrlEntID, "NOTIFYTHRESHOLD" )
PrevVal = Set_Property( CtrlEntID, "NOTIFYTHRESHOLD", NewVal )

By default it is set to 100.

The NOTIFYTIMER property

The NOTIFYTIMER property is an integer that specifies the number of milliseconds before a CHANGED event is raised if the NOTIFYTHRESHOLD property value is not met.

CurrVal = Get_Property( CtrlEntID, "NOTIFYTIMER" )
PrevVal = Set_Property( CtrlEntID, "NOTIFYTIMER", NewVal )

By default it is set to 50 (ms).

Remarks

If the NOTIFYTHRESHOLD property is set to a value greater than 1 then the control will try to bundle that number of notifications together before raising a CHANGED event. However, when this is set to a high value it is possible that the threshold may not be reached in a timely fashion and the CHANGED event not actually raised.

E.g. If the NOTIFYTHRESHOLD is set to 1000, and only 200 notifications are received then the CHANGED event would not be raised.

To prevent this problem the NOTIFYTIMER property may be used to specify the amount of time after receiving the last notification before a CHANGED event is raised even if the NOTIFYTHRESHOLD is not met.

E.g. in the example above, if the control had a NOTIFYTIMER of 50, then a CHANGED event would be raised 50ms after the last notification (200) was received, even though the NOTIFYTHRESHOLD of 1000 has not actually been met.

Developer Notes

The DIRWATCHER control is intended as a “non-visual” control and should probably be hidden at runtime in your own applications. However, it is actually derived from a normal STATIC control so all of the properties and methods that apply to a STATIC apply to the DIRWATCHER as well, and you may use them as normal if you wish.

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