How do I programmatically change system-wide desktop picture?

Question

I'm using the following code to set desktop picture:

NSURL* newImage = [[NSURL alloc] initFileURLWithPath:@"/Users/name/Pictures/test.png"];
[[NSWorkspace sharedWorkspace] setDesktopImageURL:newImage forScreen:screen options:nil error:&nserror];

It works fine and the desktop picture changed as I required. But it does not change the system-wide desktop preferences (for example, change picture every 30 minutes). How can I change the system-wide desktop preferences?

Answer 1

Take a look at Preferences and Settings Programming Guide. This should help.

Edit:
Here is the sample code:

NSString* newImgPath = @"/Users/cody/Desktop/stuff/imgs/Black_mac.png";

NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];
NSMutableDictionary* desktopDict = [NSMutableDictionary dictionaryWithDictionary:[defaults persistentDomainForName:@"com.apple.desktop"]];
NSMutableDictionary* bgDict = [desktopDict objectForKey:@"Background"];
NSMutableDictionary* spaces = [bgDict objectForKey:@"spaces"];
[spaces enumerateKeysAndObjectsUsingBlock:^(NSString* key, NSMutableDictionary* obj, BOOL *stop) {
    [obj enumerateKeysAndObjectsUsingBlock:^(id key, NSMutableDictionary* prefs, BOOL *stop) {
        [prefs setObject:newImgPath forKey:@"ImageFilePath"];
        [prefs setObject:newImgPath forKey:@"NewImageFilePath"];
        [prefs setObject:@"Never" forKey:@"Change"];
    }];
}];

//NSLog(@"%@", desktopDict);

[defaults setPersistentDomain:desktopDict forName:@"com.apple.desktop"];
if ([defaults synchronize] == NO)
    NSLog(@"synchronize failed");

// Restart dock
system ("/usr/bin/killall Dock");