Question

Does anyone know if it’s possible to create a listener to detect when a Macs system volume changes? If so do you know where I can find an example on how to accomplish this?

Answer 1

Concept

So, hopefully there’s a better way to deal with this, but this is a concept:

Spin off a new thread to watch the volume attribute found in AudioToolbox. Then have it call a callback of some kind.

#import "AppDelegate.h"
#define kVolumeKey @"currentvolumeforvolumemonitorkey"

@implementation AppDelegate

@synthesize window = _window;

/* Credit to CocoaDev Starts Now */
/* http://www.cocoadev.com/index.pl?SoundVolume */
+(AudioDeviceID)defaultOutputDeviceID
{
    AudioDeviceID   outputDeviceID = kAudioObjectUnknown;
    
    // get output device device
    UInt32 propertySize = 0;
    OSStatus status = noErr;
    AudioObjectPropertyAddress propertyAOPA;
    propertyAOPA.mScope = kAudioObjectPropertyScopeGlobal;
    propertyAOPA.mElement = kAudioObjectPropertyElementMaster;
    propertyAOPA.mSelector = kAudioHardwarePropertyDefaultOutputDevice;
    
    if (!AudioHardwareServiceHasProperty(kAudioObjectSystemObject, &propertyAOPA))
    {
        NSLog(@"Cannot find default output device!");
        return outputDeviceID;
    }
    
    propertySize = sizeof(AudioDeviceID);
    
    status = AudioHardwareServiceGetPropertyData(kAudioObjectSystemObject, &propertyAOPA, 0, NULL, &propertySize, &outputDeviceID);
    
    if(status) 
    {
        NSLog(@"Cannot find default output device!");
    }
    return outputDeviceID;
}

+(float)volume 
{
    Float32         outputVolume;
    
    UInt32 propertySize = 0;
    OSStatus status = noErr;
    AudioObjectPropertyAddress propertyAOPA;
    propertyAOPA.mElement = kAudioObjectPropertyElementMaster;
    propertyAOPA.mSelector = kAudioHardwareServiceDeviceProperty_VirtualMasterVolume;
    propertyAOPA.mScope = kAudioDevicePropertyScopeOutput;
    
    AudioDeviceID outputDeviceID = [AppDelegate defaultOutputDeviceID];
    
    if (outputDeviceID == kAudioObjectUnknown)
    {
        NSLog(@"Unknown device");
        return 0.0;
    }
    
    if (!AudioHardwareServiceHasProperty(outputDeviceID, &propertyAOPA))
    {
        NSLog(@"No volume returned for device 0x%0x", outputDeviceID);
        return 0.0;
    }
    
    propertySize = sizeof(Float32);
    
    status = AudioHardwareServiceGetPropertyData(outputDeviceID, &propertyAOPA, 0, NULL, &propertySize, &outputVolume);
    
    if (status)
    {
        NSLog(@"No volume returned for device 0x%0x", outputDeviceID);
        return 0.0;
    }
    
    if (outputVolume < 0.0 || outputVolume > 1.0) return 0.0;
    
    return outputVolume;
}
/* Thanks CocoaDev! */


- (void)dealloc
{
    [super dealloc];
}

- (void)volumeDidChangeToLevel: (CGFloat)level
{
    NSLog(@"LEVEL: %f", level);
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0),
    ^{
        [[NSUserDefaults standardUserDefaults] setFloat: [[self class] volume] forKey: kVolumeKey];
        [[NSUserDefaults standardUserDefaults] synchronize];
        for(CGFloat volLevel = [[self class] volume];; volLevel = [[self class] volume])
        {
             if (volLevel != [[NSUserDefaults standardUserDefaults] floatForKey: kVolumeKey])
             {
                 [[NSUserDefaults standardUserDefaults] setFloat: [[self class] volume] forKey: kVolumeKey];
                 [[NSUserDefaults standardUserDefaults] synchronize];
                 
                 dispatch_async(dispatch_get_main_queue(),
                 ^{
                     [self volumeDidChangeToLevel: volLevel];
                 });
             }
        }
    });
}

@end

If this seems to be an approach you’d like, you might want to consider setting up an object with a delegate callback or a block or something.

Caveat

This solution requires a single background thread to watch for changes constantly. It should be pretty low-key, because it’s such a simple check, but it bears mentioning.