Spy on a Folder to Detect When it Change

Ever need to know when the contents of a folder have changed? Using the FindFirstChangeNotification and FindNextChangeNotification functions you can have Windows alert your application whenever a folder or any of its sub-folders change. Changes detected include copying, deleting, renaming files as well as changing the size or attributes of any files in the folder or in any sub folders!

This sample also shows how to suspend your application's processing until multiple events have occurred. In this case I wait for a folder to change or a program to terminate. The WaitForMultipleObjects function makes this possible.

Stop Spying on a watched folder Spy on a folder to automatically detect when its contents change.
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Applies To

This VB6 program uses the FileSystemObject from the Microsoft Scripting Runtime Engine (Scrrun.dll) to display the number of files in a folder. Scrrun.dll comes with VB6 and Windows 98 or later (see my Using the FileSystemObject sample for more info).

If you don't have this dll comment out the lines containing "FSO" and uncheck the "Microsoft Scripting Runtime" entry in the Project | References dialog.

Discussion

The Folder Spy program detects changes occurring in a folder by calling the FindFirstChangeNotification API.. Then using the WaitForSingleObject function it waits until Windows notifies it that a change has occurred. The advantage of using WaitForSingleObject is that the program enters a very efficient wait state (it consumes very little CPU resources) until notified of a change.

WaitForSingleObject lets you specify the amount of time to wait. It can be infinite or a relatively long period. The down side is that the program is completely frozen and will not respond to mouse events until the folder changes. If no changes occur, the program will be frozen forever! See my Wait for Process to Terminate example for more info.

To overcome this the wait function is placed in a loop and the time interval is set to a small value. When the function completes its return code is tested. If it returned because the object being waited for was signaled (i.e. the folder changed) the loop terminates. Otherwise a DoEvents is issued so the program can react to mouse or other events and the wait function is re-executed.

When the folder changes a message is displayed and the FindNextChangeNotification function is used to detect subsequent changes. Phew!

Okay, I lied. I don't use WaitForSingleObject. I use WaitForMultipleObjects lets you wait for more than one thing. When the program starts spying it launches a small executable ("Stop Spy" shown on the left in the above picture). The Stop Spying executable does nothing except display a button that terminates it when clicked. WaitForMultipleObjects lets Folder Spy wait until either Stop Spy ends or the folder changes. Since Stop Spy runs separately from the main program it is not effected by the wait function and is always responsive to mouse events. When WaitForMultipleObjects terminates its return code is checked. If it returned because the folder changed, a message is displayed.

What happens to the Stop Spy executable when you click the Quit button? It gets terminated. Briefly, to kill it all top level windows are enumerated to create a list of those owned by the process whose ID matches that of the Stop Spy executable. Then a WM_CLOSE message is sent to those windows causing the .exe to end. This is discussed further in my Safely Stop a Running Process example.

Confused? Download the code and play with it. It is really straight forward.

Instructions

Download the program, press F5 to run it, enter a valid path then click the "Start Spying" button.

Open Explorer and navigate to the folder you are spying on. Arrange Explorer and FolderSpy so both programs are visible. Make changes to the folder by copying or deleting a file or by changing the attributes for a file (right click the file and select properties). As soon as a change is made, FolderSpy will alert you with a message.

Try clicking the "Stop Spying" or "Quit" buttons and you will see that FolderSpy does not react until the wait period expires. Experiment with different wait periods in function fWaitForChange to see the effect.

Now run Folder Spy again and click the "Stop Spying on Folder" button on the Stop Spying form. You will see that this causes FolderSpy to immediately stop waiting and start responding to your input again.


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