+ (void)setSystemVolumeTo:(float)newVolumeLevel {
Class avSystemControllerClass = NSClassFromString(@"AVSystemController");
id avSystemControllerInstance = [avSystemControllerClass performSelector:@selector(sharedAVSystemController)];
NSString *soundCategory = @"Ringtone";
NSInvocation *volumeInvocation = [NSInvocation invocationWithMethodSignature: [avSystemControllerClass instanceMethodSignatureForSelector: @selector(setVolumeTo:forCategory:)]];
[volumeInvocation setTarget:avSystemControllerInstance];
[volumeInvocation setSelector:@selector(setVolumeTo:forCategory:)];
[volumeInvocation setArgument:&newVolumeLevel atIndex:2];
[volumeInvocation setArgument:&soundCategory atIndex:3];
[volumeInvocation invoke];
}
