Execute script on wallpaper change event

Simple - you don't.

There's nothing in AppleScript that will respond to this kind of trigger.

The closest you could come would be a background script that periodically checks the current desktop picture and compares it to what was seen previously. Any difference indicates that the desktop picture changed and you can react accordingly.

In other words, your script can poll the system to find the current setting, but the OS won't notify your script directly.

Use a listener script that does not timeout, something like this.

Im not if this one will timeout or not though.




[code]
set change_status to false
repeat
with timeout of 0 seconds
tell application "Finder" to set current_pic to desktop picture
delay 3
tell application "Finder" to set recheck_name to desktop picture
if current_pic ≠ recheck_name then set change_status to true
if change_status = true then do_stuff()
if current_pic = recheck_name then set change_status to false
end timeout
end repeat





on do_stuff()
display dialog "DO STUFF HERE BECAUSE THE DESKTOP PICTURE WAS CHANGED"
end do_stuff

Objective-C sample

-(void)applicationActivity:(NSNotification *)notification
{
 NSRunningApplication *app = [[notification userInfo] objectForKey:@"NSWorkspaceApplicationKey"];
 if(app.localizedName isEqualToString:@"ScreenSaverEngine")
 {
 // Your code here
 } 
}
-(void)awakeFromNib 
{
 [[[NSWorkspace sharedWorkspace] notificationCenter] 
 addObserver:self
 selector:@selector(applicationActivity:)
 name:NSWorkspaceDidActivateApplicationNotification 
 object:nil];
}

Your understanding/use of with timeout in incorrect.

with timeout tells AppleScript how long to wait for applications to execute the specified commands. Therefore in your example:

with timeout of 0 seconds
tell application "Finder" to set current_pic to desktop picture

You're saying 'wait indefinitely for the Finder to get the current desktop picture'. Under normal circumstances that task would take a few milliseconds, at worst, so setting a timeout doesn't make much sense. You're also checking the desktop picture twice per iteration, which is approximately double the amount of work you need to do.

Your script would be better written using an idle handler, like:

global old_pic

on run
tell application "Finder" to set old_pic to desktop picture as alias
end run

on idle
tell application "Finder" to set current_pic to desktop picture as alias
if current_pic is not old_pic then
display dialog "Desktop pattern is different"
set old_pic to current_pic
end if
return 10 -- check again in 10 seconds
end idle

In this way the desktop picture that's current when the script is launched is stored in a global variable. For each iteration of the idle handler the script gets the then-current desktop pattern and compares it against the original (so it only gets the desktop pattern once per iteration).
If they differ then the script posts a dialog (could be any action you want to take) and resets old_pic to reference the current desktop picture (so that next time around it compares the desktop to what we have now, not the original one when the script launched).
Finally the return 10 tells AppleScript to check again in 10 seconds (could be any time interval you like, depending on how soon you want to detect changes).

The ObjC solution is neater since it adds a hook into the system to call your code when the system environment changes. Not as clear/legible as AppleScript, of course, but it doesn't offer an even more elegant solution.

Agree. Makes more sense.

Just a thought.

Why cant you skip the Desktop Screensaver app to change your picture and use applecript to change your picture every 30 seconds. This way you dont need a listener.

See the applescript's dictionary of "System Events", the section for "Desktop Suite" for commands.

hi everyone

check this app

http://developer.apple.com/mac/library/samplecode/EventMonitorTest/listing2.html

this app give u the ability to capture and look into systme events

BUTTTTT

it is not made specifically for desktop wallpaper change event

try to figure out this section of code

// get the event class
switch ( GetEventClass( inEvent ) )
{
case kEventClassKeyboard:
className = "Keyboard";
// get the keyboard event type

source code at http://developer.apple.com/mac/library/samplecode/EventMonitorTest/

i hope the GetEventClass( inEvent ) returns something in case of a wallpaper change

Inform me also when u figure this out.

just as another example...
you would have to save this as an Applescript-Application with "stay open"-option checked.