How to detect dark mode in Yosemite to change the status bar menu icon - osx-yosemite

Question

The status bar app icon has to be changed when dark mode is enabled in Yosemite.
How to detect if dark mode has been enabled ? Is there any notification for the same ?
Is it better to display another image or change the alpha value of existing Image ?
Need inputs on which is the better way to go ??

Answer 1

You should make use of template images wherever possible because they allow your UI to automatically adapt to changes made by the system (at least when there's not a bug in the system... http://indiestack.com/2014/10/yosemites-dark-mode/). But in the case where you might use a custom view in the status bar and cannot take advantage of a template image, you can manually check for dark mode and adapt your UI accordingly.
You can check whether or not dark mode is enabled by retrieving a key from the user's global preferences, like this:
NSDictionary *dict = [[NSUserDefaults standardUserDefaults] persistentDomainForName:NSGlobalDomain];
id style = [dict objectForKey:#"AppleInterfaceStyle"];
BOOL darkModeOn = ( style && [style isKindOfClass:[NSString class]] && NSOrderedSame == [style caseInsensitiveCompare:#"dark"] );
At least for the first release of Yosemite, the key is not present when dark mode is disabled, but the key is present and returns the string value #"Dark" when dark mode is enabled. I added the case insensitive compare because I have seen preference keys change their case between system releases, and this adds a little insurance against that.
To monitor the current state of the setting, you register as an observer of a distributed notification (within an appropriate method), like this:
[[NSDistributedNotificationCenter defaultCenter] addObserver:self selector:#selector(darkModeChanged:) name:#"AppleInterfaceThemeChangedNotification" object:nil];
And you create a method to act as the message selector for the notification, like this:
-(void)darkModeChanged:(NSNotification *)notif
{
NSLog(#"Dark mode changed");
}

Answer 2

The Status bar icon needs to be a template image.
Just set the setTemplate:Yes to NSImage.
And when switched to dark mode , the vibrancy should apply.

Answer 3

As noted by bergdesign, you need to observe the system wide notification and read the persistent global domain.
We made a class to simplify handling changes to the Dark Mode settings:
https://github.com/weAreYeah/WAYTheDarkSide
It becomes as easy as...
[WAYTheDarkSide welcomeApplicationWithBlock:^{
// Enabling Dark Mode
[someWindow setAppearance:[NSAppearance appearanceNamed:NSAppearanceNameVibrantDark]];
[someVisualEffectView setMaterial:NSVisualEffectMaterialDark];
} immediately:YES];
and
[WAYTheDarkSide outcastApplicationWithBlock:^{
// Disabling Dark Mode
[someWindow setAppearance:[NSAppearance appearanceNamed:NSAppearanceNameVibrantLight]];
[someVisualEffectView setMaterial:NSVisualEffectMaterialLight];
} immediately:YES];
Hope this helps :)