diff --git a/AppleCommon/GBViewMetal.m b/AppleCommon/GBViewMetal.m
index 6a45dab..088d402 100644
--- a/AppleCommon/GBViewMetal.m
+++ b/AppleCommon/GBViewMetal.m
@@ -1,7 +1,9 @@
 #import <CoreImage/CoreImage.h>
 #import "GBViewMetal.h"
 #pragma clang diagnostic ignored "-Wpartial-availability"
-
+#if !TARGET_OS_IPHONE
+#import "../Cocoa/NSObject+DefaultsObserver.h"
+#endif
 
 static const vector_float2 rect[] =
 {
@@ -74,8 +76,13 @@ static const vector_float2 rect[] =
                                                   options:MTLResourceStorageModeShared];
     
     output_resolution = (simd_float2){view.drawableSize.width, view.drawableSize.height};
+    /* TODO: NSObject+DefaultsObserver can replace the less flexible `addDefaultObserver` in iOS */
+#if TARGET_OS_IPHONE
     [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(loadShader) name:@"GBFilterChanged" object:nil];
     [self loadShader];
+#else
+    [self observeStandardDefaultsKey:@"GBFilter" selector:@selector(loadShader)];
+#endif
 }
 
 - (void) loadShader
diff --git a/Cocoa/Document.m b/Cocoa/Document.m
index 9826bd4..911244a 100644
--- a/Cocoa/Document.m
+++ b/Cocoa/Document.m
@@ -13,6 +13,7 @@
 #import "GBPaletteEditorController.h"
 #import "GBObjectView.h"
 #import "GBPaletteView.h"
+#import "NSObject+DefaultsObserver.h"
 
 #define likely(x)   GB_likely(x)
 #define unlikely(x) GB_unlikely(x)
@@ -298,16 +299,6 @@ static void debuggerReloadCallback(GB_gameboy_t *gb)
     GB_set_palette(&_gb, [GBPaletteEditorController userPalette]);
 }
 
-- (void) updateBorderMode
-{
-    _borderModeChanged = true;
-}
-
-- (void) updateRumbleMode
-{
-    GB_set_rumble_mode(&_gb, [[NSUserDefaults standardUserDefaults] integerForKey:@"GBRumbleMode"]);
-}
-
 - (void) initCommon
 {
     GB_init(&_gb, [self internalModel]);
@@ -317,22 +308,52 @@ static void debuggerReloadCallback(GB_gameboy_t *gb)
     GB_set_log_callback(&_gb, (GB_log_callback_t) consoleLog);
     GB_set_input_callback(&_gb, (GB_input_callback_t) consoleInput);
     GB_set_async_input_callback(&_gb, (GB_input_callback_t) asyncConsoleInput);
-    GB_set_color_correction_mode(&_gb, (GB_color_correction_mode_t) [[NSUserDefaults standardUserDefaults] integerForKey:@"GBColorCorrection"]);
-    GB_set_light_temperature(&_gb, [[NSUserDefaults standardUserDefaults] doubleForKey:@"GBLightTemperature"]);
-    GB_set_interference_volume(&_gb, [[NSUserDefaults standardUserDefaults] doubleForKey:@"GBInterferenceVolume"]);
-    GB_set_border_mode(&_gb, (GB_border_mode_t) [[NSUserDefaults standardUserDefaults] integerForKey:@"GBBorderMode"]);
     [self updatePalette];
     GB_set_rgb_encode_callback(&_gb, rgbEncode);
     GB_set_camera_get_pixel_callback(&_gb, cameraGetPixel);
     GB_set_camera_update_request_callback(&_gb, cameraRequestUpdate);
-    GB_set_highpass_filter_mode(&_gb, (GB_highpass_mode_t) [[NSUserDefaults standardUserDefaults] integerForKey:@"GBHighpassFilter"]);
-    GB_set_rewind_length(&_gb, [[NSUserDefaults standardUserDefaults] integerForKey:@"GBRewindLength"]);
-    GB_set_rtc_mode(&_gb, [[NSUserDefaults standardUserDefaults] integerForKey:@"GBRTCMode"]);
     GB_apu_set_sample_callback(&_gb, audioCallback);
     GB_set_rumble_callback(&_gb, rumbleCallback);
     GB_set_infrared_callback(&_gb, infraredStateChanged);
     GB_set_debugger_reload_callback(&_gb, debuggerReloadCallback);
-    [self updateRumbleMode];
+    
+    GB_gameboy_t *gb = &_gb;
+    __unsafe_unretained Document *weakSelf = self;
+    
+    [self observeStandardDefaultsKey:@"GBColorCorrection" withBlock:^(NSNumber *value) {
+        GB_set_color_correction_mode(gb, value.unsignedIntValue);
+    }];
+    
+    [self observeStandardDefaultsKey:@"GBLightTemperature" withBlock:^(NSNumber *value) {
+        GB_set_light_temperature(gb, value.doubleValue);
+    }];
+    
+    [self observeStandardDefaultsKey:@"GBInterferenceVolume" withBlock:^(NSNumber *value) {
+        GB_set_interference_volume(gb, value.doubleValue);
+    }];
+    
+    GB_set_border_mode(&_gb, (GB_border_mode_t) [[NSUserDefaults standardUserDefaults] integerForKey:@"GBBorderMode"]);
+    [self observeStandardDefaultsKey:@"GBBorderMode" withBlock:^(NSNumber *value) {
+        _borderModeChanged = true;
+    }];
+    
+    [self observeStandardDefaultsKey:@"GBHighpassFilter" withBlock:^(NSNumber *value) {
+        GB_set_highpass_filter_mode(gb, value.unsignedIntValue);
+    }];
+    
+    [self observeStandardDefaultsKey:@"GBRewindLength" withBlock:^(NSNumber *value) {
+        [weakSelf performAtomicBlock:^{
+            GB_set_rewind_length(gb, value.unsignedIntValue);
+        }];
+    }];
+    
+    [self observeStandardDefaultsKey:@"GBRTCMode" withBlock:^(NSNumber *value) {
+        GB_set_rtc_mode(gb, value.unsignedIntValue);
+    }];
+    
+    [self observeStandardDefaultsKey:@"GBRumbleMode" withBlock:^(NSNumber *value) {
+        GB_set_rumble_mode(gb, value.unsignedIntValue);
+    }];
 }
 
 - (void) updateMinSize
@@ -766,82 +787,54 @@ static unsigned *multiplication_table_for_frequency(unsigned frequency)
                       accessibilityDescription:@"Print"];
     }
     
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(updateHighpassFilter)
-                                                 name:@"GBHighpassFilterChanged"
-                                               object:nil];
-    
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(updateColorCorrectionMode)
-                                                 name:@"GBColorCorrectionChanged"
-                                               object:nil];
-    
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(updateLightTemperature)
-                                                 name:@"GBLightTemperatureChanged"
-                                               object:nil];
-    
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(updateInterferenceVolume)
-                                                 name:@"GBInterferenceVolumeChanged"
-                                               object:nil];
-    
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(updateFrameBlendingMode)
-                                                 name:@"GBFrameBlendingModeChanged"
-                                               object:nil];
     
     [[NSNotificationCenter defaultCenter] addObserver:self
                                              selector:@selector(updatePalette)
                                                  name:@"GBColorPaletteChanged"
                                                object:nil];
-
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(updateBorderMode)
-                                                 name:@"GBBorderModeChanged"
-                                               object:nil];
     
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(updateRumbleMode)
-                                                 name:@"GBRumbleModeChanged"
-                                               object:nil];
+    __unsafe_unretained Document *weakSelf = self;
+    [self observeStandardDefaultsKey:@"GBFrameBlendingMode"
+                           withBlock:^(NSNumber *value) {
+        weakSelf.view.frameBlendingMode = (GB_frame_blending_mode_t)value.unsignedIntValue;
+    }];
     
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(updateRewindLength)
-                                                 name:@"GBRewindLengthChanged"
-                                               object:nil];
+    [self observeStandardDefaultsKey:@"GBDMGModel" withBlock:^(id newValue) {
+        weakSelf->_modelsChanging = true;
+        if (weakSelf->_currentModel == MODEL_DMG) {
+            [weakSelf reset:nil];
+        }
+        weakSelf->_modelsChanging = false;
+    }];
     
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(updateRTCMode)
-                                                 name:@"GBRTCModeChanged"
-                                               object:nil];
-
+    [self observeStandardDefaultsKey:@"GBSGBModel" withBlock:^(id newValue) {
+        weakSelf->_modelsChanging = true;
+        if (weakSelf->_currentModel == MODEL_SGB) {
+            [weakSelf reset:nil];
+        }
+        weakSelf->_modelsChanging = false;
+    }];
     
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(dmgModelChanged)
-                                                 name:@"GBDMGModelChanged"
-                                               object:nil];
+    [self observeStandardDefaultsKey:@"GBCGBModel" withBlock:^(id newValue) {
+        weakSelf->_modelsChanging = true;
+        if (weakSelf->_currentModel == MODEL_CGB) {
+            [weakSelf reset:nil];
+        }
+        weakSelf->_modelsChanging = false;
+    }];
     
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(sgbModelChanged)
-                                                 name:@"GBSGBModelChanged"
-                                               object:nil];
-    
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(cgbModelChanged)
-                                                 name:@"GBCGBModelChanged"
-                                               object:nil];
-    
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(agbModelChanged)
-                                                 name:@"GBAGBModelChanged"
-                                               object:nil];
+    [self observeStandardDefaultsKey:@"GBAGBModel" withBlock:^(id newValue) {
+        weakSelf->_modelsChanging = true;
+        if (weakSelf->_currentModel == MODEL_AGB) {
+            [weakSelf reset:nil];
+        }
+        weakSelf->_modelsChanging = false;
+    }];
     
     
-    [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(updateVolume)
-                                                 name:@"GBVolumeChanged"
-                                               object:nil];
+    [self observeStandardDefaultsKey:@"GBVolume" withBlock:^(id newValue) {
+        weakSelf->_volume = [[NSUserDefaults standardUserDefaults] doubleForKey:@"GBVolume"];
+    }];
         
     if ([[NSUserDefaults standardUserDefaults] boolForKey:@"EmulateDMG"]) {
         _currentModel = MODEL_DMG;
@@ -2173,96 +2166,6 @@ static bool is_path_writeable(const char *path)
     }];
 }
 
-- (void) updateVolume
-{
-    _volume = [[NSUserDefaults standardUserDefaults] doubleForKey:@"GBVolume"];
-}
-
-- (void) updateHighpassFilter
-{
-    if (GB_is_inited(&_gb)) {
-        GB_set_highpass_filter_mode(&_gb, (GB_highpass_mode_t) [[NSUserDefaults standardUserDefaults] integerForKey:@"GBHighpassFilter"]);
-    }
-}
-
-- (void) updateColorCorrectionMode
-{
-    if (GB_is_inited(&_gb)) {
-        GB_set_color_correction_mode(&_gb, (GB_color_correction_mode_t) [[NSUserDefaults standardUserDefaults] integerForKey:@"GBColorCorrection"]);
-    }
-}
-
-- (void) updateLightTemperature
-{
-    if (GB_is_inited(&_gb)) {
-        GB_set_light_temperature(&_gb, [[NSUserDefaults standardUserDefaults] doubleForKey:@"GBLightTemperature"]);
-    }
-}
-
-- (void) updateInterferenceVolume
-{
-    if (GB_is_inited(&_gb)) {
-        GB_set_interference_volume(&_gb, [[NSUserDefaults standardUserDefaults] doubleForKey:@"GBInterferenceVolume"]);
-    }
-}
-
-- (void) updateFrameBlendingMode
-{
-    self.view.frameBlendingMode = (GB_frame_blending_mode_t) [[NSUserDefaults standardUserDefaults] integerForKey:@"GBFrameBlendingMode"];
-}
-
-- (void) updateRewindLength
-{
-    [self performAtomicBlock:^{
-        if (GB_is_inited(&_gb)) {
-            GB_set_rewind_length(&_gb, [[NSUserDefaults standardUserDefaults] integerForKey:@"GBRewindLength"]);
-        }
-    }];
-}
-
-- (void) updateRTCMode
-{
-    if (GB_is_inited(&_gb)) {
-        GB_set_rtc_mode(&_gb, [[NSUserDefaults standardUserDefaults] integerForKey:@"GBRTCMode"]);
-    }
-}
-
-- (void)dmgModelChanged
-{
-    _modelsChanging = true;
-    if (_currentModel == MODEL_DMG) {
-        [self reset:nil];
-    }
-    _modelsChanging = false;
-}
-
-- (void)sgbModelChanged
-{
-    _modelsChanging = true;
-    if (_currentModel == MODEL_SGB) {
-        [self reset:nil];
-    }
-    _modelsChanging = false;
-}
-
-- (void)cgbModelChanged
-{
-    _modelsChanging = true;
-    if (_currentModel == MODEL_CGB) {
-        [self reset:nil];
-    }
-    _modelsChanging = false;
-}
-
-- (void)agbModelChanged
-{
-    _modelsChanging = true;
-    if (_currentModel == MODEL_AGB) {
-        [self reset:nil];
-    }
-    _modelsChanging = false;
-}
-
 - (void)setFileURL:(NSURL *)fileURL
 {
     [super setFileURL:fileURL];
diff --git a/Cocoa/GBGLShader.m b/Cocoa/GBGLShader.m
index 920226b..1d39090 100644
--- a/Cocoa/GBGLShader.m
+++ b/Cocoa/GBGLShader.m
@@ -163,7 +163,7 @@ void main(void) {\n\
 
     /* OpenGL is black magic. Closing one view causes others to be completely black unless we reload their shaders */
     /* We're probably not freeing thing in the right place. */
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBFilterChanged" object:nil];
+    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBFilterChanged$DefaultsObserver" object:nil];
 }
 
 + (GLuint)shaderWithContents:(NSString*)contents type:(GLenum)type
diff --git a/Cocoa/GBOpenGLView.m b/Cocoa/GBOpenGLView.m
index 0ce9047..aa401e8 100644
--- a/Cocoa/GBOpenGLView.m
+++ b/Cocoa/GBOpenGLView.m
@@ -1,5 +1,6 @@
 #import "GBOpenGLView.h"
 #import "GBView.h"
+#import "NSObject+DefaultsObserver.h"
 #import <OpenGL/gl.h>
 
 @implementation GBOpenGLView
@@ -27,13 +28,12 @@
 
 - (instancetype)initWithFrame:(NSRect)frameRect pixelFormat:(NSOpenGLPixelFormat *)format
 {
-    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(filterChanged) name:@"GBFilterChanged" object:nil];
+    __unsafe_unretained GBOpenGLView *weakSelf = self;
+    [self observeStandardDefaultsKey:@"GBFilter" withBlock:^(id newValue) {
+        weakSelf.shader = nil;
+        [weakSelf setNeedsDisplay:true];
+
+    }];
     return [super initWithFrame:frameRect pixelFormat:format];
 }
-
-- (void) filterChanged
-{
-    self.shader = nil;
-    [self setNeedsDisplay:true];
-}
 @end
diff --git a/Cocoa/GBPreferencesWindow.m b/Cocoa/GBPreferencesWindow.m
index 52a6d4f..32461f7 100644
--- a/Cocoa/GBPreferencesWindow.m
+++ b/Cocoa/GBPreferencesWindow.m
@@ -360,14 +360,12 @@ static inline NSString *keyEquivalentString(NSMenuItem *item)
 {
     [[NSUserDefaults standardUserDefaults] setObject:[[self class] filterList][[sender indexOfSelectedItem]]
                                               forKey:@"GBFilter"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBFilterChanged" object:nil];
 }
 
 - (IBAction)highpassFilterChanged:(id)sender
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender indexOfSelectedItem])
                                               forKey:@"GBHighpassFilter"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBHighpassFilterChanged" object:nil];
 }
 
 
@@ -399,43 +397,36 @@ static inline NSString *keyEquivalentString(NSMenuItem *item)
 {
     [[NSUserDefaults standardUserDefaults] setBool: [(NSButton *)sender state] != NSOnState
                                             forKey:@"GBAspectRatioUnkept"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBAspectChanged" object:nil];
 }
 
 - (IBAction)colorCorrectionChanged:(id)sender
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender selectedItem].tag)
                                               forKey:@"GBColorCorrection"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBColorCorrectionChanged" object:nil];
 }
 
 - (IBAction)lightTemperatureChanged:(id)sender
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender doubleValue] / 256.0)
                                               forKey:@"GBLightTemperature"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBLightTemperatureChanged" object:nil];
 }
 
 - (IBAction)interferenceVolumeChanged:(id)sender
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender doubleValue] / 256.0)
                                               forKey:@"GBInterferenceVolume"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBInterferenceVolumeChanged" object:nil];
 }
 
 - (IBAction)volumeChanged:(id)sender
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender doubleValue] / 256.0)
                                               forKey:@"GBVolume"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBVolumeChanged" object:nil];
 }
 
 - (IBAction)franeBlendingModeChanged:(id)sender
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender indexOfSelectedItem])
                                               forKey:@"GBFrameBlendingMode"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBFrameBlendingModeChanged" object:nil];
-    
 }
 
 - (void)updatePalettesMenu
@@ -490,14 +481,12 @@ static inline NSString *keyEquivalentString(NSMenuItem *item)
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender selectedItem].tag)
                                               forKey:@"GBBorderMode"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBBorderModeChanged" object:nil];
 }
 
 - (IBAction)rumbleModeChanged:(id)sender
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender selectedItem].tag)
                                               forKey:@"GBRumbleMode"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBRumbleModeChanged" object:nil];
 }
 
 - (IBAction)hotkey1Changed:(id)sender
@@ -516,15 +505,12 @@ static inline NSString *keyEquivalentString(NSMenuItem *item)
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender selectedTag])
                                               forKey:@"GBRewindLength"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBRewindLengthChanged" object:nil];
 }
 
 - (IBAction)rtcModeChanged:(id)sender
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender indexOfSelectedItem])
                                               forKey:@"GBRTCMode"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBRTCModeChanged" object:nil];
-
 }
 
 - (IBAction)changeAutoUpdates:(id)sender
@@ -813,7 +799,6 @@ static inline NSString *keyEquivalentString(NSMenuItem *item)
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender selectedTag])
                                               forKey:@"GBDMGModel"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBDMGModelChanged" object:nil];
 
 }
 
@@ -821,21 +806,18 @@ static inline NSString *keyEquivalentString(NSMenuItem *item)
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender selectedTag])
                                               forKey:@"GBSGBModel"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBSGBModelChanged" object:nil];
 }
 
 - (IBAction)cgbModelChanged:(id)sender
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender selectedTag])
                                               forKey:@"GBCGBModel"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBCGBModelChanged" object:nil];
 }
 
 - (IBAction)agbModelChanged:(id)sender
 {
     [[NSUserDefaults standardUserDefaults] setObject:@([sender selectedTag])
                                               forKey:@"GBAGBModel"];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"GBAGBModelChanged" object:nil];
 }
 
 - (IBAction)reloadButtonsData:(id)sender
diff --git a/Cocoa/GBView.m b/Cocoa/GBView.m
index ae15813..f6dcf2e 100644
--- a/Cocoa/GBView.m
+++ b/Cocoa/GBView.m
@@ -5,6 +5,7 @@
 #import "GBViewMetal.h"
 #import "GBButtons.h"
 #import "NSString+StringForKey.h"
+#import "NSObject+DefaultsObserver.h"
 #import "Document.h"
 
 #define JOYSTICK_HIGH 0x4000
@@ -138,7 +139,10 @@ static const uint8_t workboy_vk_to_key[] = {
 {
     [self registerForDraggedTypes:[NSArray arrayWithObjects: NSFilenamesPboardType, nil]];
     
-    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(ratioKeepingChanged) name:@"GBAspectChanged" object:nil];
+    __unsafe_unretained GBView *weakSelf = self;
+    [self observeStandardDefaultsKey:@"GBAspectRatioUnkept" withBlock:^(id newValue) {
+        [weakSelf setFrame:weakSelf.superview.frame];
+    }];
     tracking_area = [ [NSTrackingArea alloc] initWithRect:(NSRect){}
                                                   options:NSTrackingMouseEnteredAndExited | NSTrackingActiveAlways | NSTrackingInVisibleRect | NSTrackingMouseMoved
                                                     owner:self
@@ -161,11 +165,6 @@ static const uint8_t workboy_vk_to_key[] = {
     });
 }
 
-- (void) ratioKeepingChanged
-{
-    [self setFrame:self.superview.frame];
-}
-
 - (void)dealloc
 {
     if (mouse_hidden) {
diff --git a/Cocoa/NSObject+DefaultsObserver.h b/Cocoa/NSObject+DefaultsObserver.h
new file mode 100644
index 0000000..18469f8
--- /dev/null
+++ b/Cocoa/NSObject+DefaultsObserver.h
@@ -0,0 +1,6 @@
+#import <AppKit/AppKit.h>
+
+@interface NSObject (DefaultsObserver)
+- (void)observeStandardDefaultsKey:(NSString *)key withBlock:(void(^)(id newValue))block;
+- (void)observeStandardDefaultsKey:(NSString *)key selector:(SEL)selector;
+@end
diff --git a/Cocoa/NSObject+DefaultsObserver.m b/Cocoa/NSObject+DefaultsObserver.m
new file mode 100644
index 0000000..8dac912
--- /dev/null
+++ b/Cocoa/NSObject+DefaultsObserver.m
@@ -0,0 +1,72 @@
+#import "NSObject+DefaultsObserver.h"
+#import <AppKit/AppKit.h>
+#import <objc/runtime.h>
+
+@interface GBUserDefaultsObserverHelper : NSObject
+@end
+
+@implementation GBUserDefaultsObserverHelper
++ (void)observeValueForKeyPath:(NSString *)keyPath
+                      ofObject:(id)object
+                        change:(NSDictionary<NSKeyValueChangeKey,id> *)change
+                       context:(void *)context
+{
+    [[NSNotificationCenter defaultCenter] postNotificationName:[keyPath stringByAppendingString:@"Changed$DefaultsObserver"]
+                                                        object:nil
+                                                      userInfo:@{
+                                                          @"value": change[NSKeyValueChangeNewKey]
+                                                      }];
+}
+
++ (void)startObservingKey:(NSString *)key
+{
+    if (!NSThread.isMainThread) {
+        dispatch_sync(dispatch_get_main_queue(), ^{
+            [self startObservingKey:key];
+        });
+        return;
+    }
+    static NSMutableSet *set = nil;
+    if (!set) {
+        set = [NSMutableSet set];
+    }
+    if ([set containsObject:key]) return;
+    [set addObject:key];
+    [[NSUserDefaults standardUserDefaults] addObserver:(id)self
+                                            forKeyPath:key
+                                               options:NSKeyValueObservingOptionNew
+                                               context:nil];
+}
+@end
+
+@implementation NSObject (DefaultsObserver)
+- (void)observeStandardDefaultsKey:(NSString *)key selector:(SEL)selector
+{
+    __weak id weakSelf = self;
+    [self observeStandardDefaultsKey:key
+                           withBlock:^(id newValue) {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
+        [weakSelf performSelector:selector withObject:newValue];
+#pragma clang diagnostic pop
+    }];
+}
+
+- (void)observeStandardDefaultsKey:(NSString *)key withBlock:(void(^)(id newValue))block
+{
+    NSString *notificationName = [key stringByAppendingString:@"Changed$DefaultsObserver"];
+    objc_setAssociatedObject(self, sel_registerName(notificationName.UTF8String), block, OBJC_ASSOCIATION_RETAIN);
+    [[NSNotificationCenter defaultCenter] addObserver:self
+                                             selector:@selector(standardDefaultsKeyChanged:)
+                                                 name:notificationName
+                                               object:nil];
+    [GBUserDefaultsObserverHelper startObservingKey:key];
+    block([[NSUserDefaults standardUserDefaults] objectForKey:key]);
+}
+
+- (void)standardDefaultsKeyChanged:(NSNotification *)notification
+{
+    SEL selector = sel_registerName(notification.name.UTF8String);
+    ((void(^)(id))objc_getAssociatedObject(self, selector))(notification.userInfo[@"value"]);
+}
+@end