Use NSWorkspaceDidWakeNotification to activate method?

Question

I created a simple app to learn how to use NSWorkspaceWillSleepNotification and NSWorkspaceDidWakeNotification. My goal is to call a method when the computer sleeps and wakes. The app I created will change each label accordingly. After building the app, I launch it from my desktop. After the application is launched, I put the computer to sleep. When the computer wakes the labels in the application do not change. I added IBAction buttons to the window to make sure that the labels would change. When buttons are pressed the labels do indeed change. But I want something like this to happen automatically upon sleep and wake. What am I doing wrong?

#import "Controller.h"

@implementation Controller

- (void)fileNotifications {

    [[[NSWorkspace sharedWorkspace] notificationCenter] addObserver: self 
                                                           selector: @selector(makeSleep:) 
                                                               name: NSWorkspaceWillSleepNotification 
                                                             object: nil];

    [[[NSWorkspace sharedWorkspace] notificationCenter] addObserver: self 
                                                           selector: @selector(makeWake:) 
                                                               name: NSWorkspaceDidWakeNotification 
                                                             object: nil];
}

- (IBAction)makeSleep:(id)sender {
    [sleepLabel setStringValue:@"Computer Slept!"];
}

- (IBAction)makeWake:(id)sender {
    [wakeLabel setStringValue:@"Computer Woke!"];
}


@end

Answer 1

Instead of [[NSWorkspace sharedWorkspace] notificationCenter] try using [NSNotificationCenter defaultCenter]

like this:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(makeSleep:) NSWorkspaceWillSleepNotification object:nil ];

and

[[NSNotificationCenter defaultCenter] addObserver:self @selector(makeWake:) NSWorkspaceDidWakeNotification object:nil ];

The above is incorrect, see https://developer.apple.com/library/mac/qa/qa1340/_index.html

Using [[NSWorkspace sharedWorkspace] notificationCenter] is necessary.

You should add observers upon - (void)awakeFromNib method.