Using IOHIDManager to Retrieve Key Modifier Events

Question

I'm trying to use the IOHIDManager to get modifier key events, because Cocoa flagsChanged events aren’t enough (it's hard to distinguish between press / release, left / right if both are omitted, etc.) Here is the code where I create the manager and register the reverse call.

IOHIDManagerRef hidManager = IOHIDManagerCreate(kCFAllocatorDefault, kIOHIDOptionsTypeNone); if (CFGetTypeID(hidManager) != IOHIDManagerGetTypeID()) return 1; CFMutableDictionaryRef capsLock = myCreateDeviceMatchingDictionary(0x07, 0x39); CFMutableDictionaryRef lctrl = myCreateDeviceMatchingDictionary(0x07, 0xE0); CFMutableDictionaryRef lshift = myCreateDeviceMatchingDictionary(0x07, 0xE1); CFMutableDictionaryRef lalt = myCreateDeviceMatchingDictionary(0x07, 0xE2); CFMutableDictionaryRef lsuper = myCreateDeviceMatchingDictionary(0x07, 0xE3); CFMutableDictionaryRef rctrl = myCreateDeviceMatchingDictionary(0x07, 0xE4); CFMutableDictionaryRef rshift = myCreateDeviceMatchingDictionary(0x07, 0xE5); CFMutableDictionaryRef ralt = myCreateDeviceMatchingDictionary(0x07, 0xE6); CFMutableDictionaryRef rsuper = myCreateDeviceMatchingDictionary(0x07, 0xE7); CFMutableDictionaryRef matchesList[] = { capsLock, lctrl, lshift, lalt, lsuper, rctrl, rshift, ralt, rsuper }; CFArrayRef matches = CFArrayCreate(kCFAllocatorDefault, (const void **)matchesList, 9, NULL); IOHIDManagerSetDeviceMatchingMultiple(hidManager, matches); IOHIDManagerRegisterInputValueCallback(hidManager, myHandleModifiersCallback, NULL); IOHIDManagerScheduleWithRunLoop(hidManager, CFRunLoopGetMain(), kCFRunLoopDefaultMode); IOHIDManagerOpen(hidManager, kIOHIDOptionsTypeNone); 

However, the callback never starts. Did I miss something?

I do not fully understand the HID usage pages, so I did not know whether to use a shared desktop page (0x01) with a keyboard usage identifier (06) or a keyboard / keyboard page (0x07) with usage identifiers for individual keys. Maybe something is connected with this?

Answer 1

I get it. The way to do this is to use the full-featured keyboard (0x01) (06) (and the keyboard (07)) to use with the IOHIDManagerSetDeviceMatchingMultiple, and then the Keyboard / Keyboard Usage (0x07) material gets called back.

For example, to configure HIDManager for all keyboards / keyboards, you can do something like:

 IOHIDManagerRef hidManager = IOHIDManagerCreate(kCFAllocatorDefault, kIOHIDOptionsTypeNone); CFMutableDictionaryRef keyboard = myCreateDeviceMatchingDictionary(0x01, 6); CFMutableDictionaryRef keypad = myCreateDeviceMatchingDictionary(0x01, 7); CFMutableDictionaryRef matchesList[] = { keyboard, keypad, }; CFArrayRef matches = CFArrayCreate(kCFAllocatorDefault, (const void **)matchesList, 2, NULL); IOHIDManagerSetDeviceMatchingMultiple(hidManager, matches); IOHIDManagerRegisterInputValueCallback(hidManager, myHIDKeyboardCallback, NULL); IOHIDManagerScheduleWithRunLoop(hidManager, CFRunLoopGetMain(), kCFRunLoopDefaultMode); IOHIDManagerOpen(hidManager, kIOHIDOptionsTypeNone); 

Where myCreateDeviceMatchingDictionary is something like:

 CFMutableDictionaryRef myCreateDeviceMatchingDictionary(UInt32 usagePage, UInt32 usage) { CFMutableDictionaryRef ret = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, &kCFTypeDictionaryKeyCallBacks, &kCFTypeDictionaryValueCallBacks); if (!ret) return NULL; CFNumberRef pageNumberRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &usagePage ); if (!pageNumberRef) { CFRelease(ret); return NULL; } CFDictionarySetValue(ret, CFSTR(kIOHIDDeviceUsagePageKey), pageNumberRef); CFRelease(pageNumberRef); CFNumberRef usageNumberRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &usage); if (!usageNumberRef) { CFRelease(ret); return NULL; } CFDictionarySetValue(ret, CFSTR(kIOHIDDeviceUsageKey), usageNumberRef); CFRelease(usageNumberRef); return ret; } 

And myHIDKeyboardCallback looks something like this:

 void myHIDKeyboardCallback(void *context, IOReturn result, void *sender, IOHIDValueRef value) { IOHIDElementRef elem = IOHIDValueGetElement(value); if (IOHIDElementGetUsagePage(elem) != 0x07) return; uint32_t scancode = IOHIDElementGetUsage(elem); if (scancode < 4 || scancode > 231) return; long pressed = IOHIDValueGetIntegerValue(value); // ... Do something ... } 

Note that the callback appears to be called several times for each click or release, but with usage identifiers outside the normal range, which means "if (scancode <4 || scancode> 231)".

Answer 2

Thanks for answering your question.

instead of the if-statement in myHIDKeyboardCallback, which checks scancode <4 or scancode> 231, you can use IOHIDManagerSetInputValueMatching .

 // before IOHIDManagerOpen int usageMin = 4; CFNumberRef minNumberRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &usageMin); CFDictionarySetValue(inputValueFilter, CFSTR(kIOHIDElementUsageMinKey), minNumberRef); CFRelease(minNumberRef); int usageMax = 231; CFNumberRef maxNumberRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &usageMax); CFDictionarySetValue(inputValueFilter, CFSTR(kIOHIDElementUsageMaxKey), maxNumberRef); CFRelease(maxNumberRef); IOHIDManagerSetInputValueMatching(hidManager, inputValueFilter); 

this is more LOC than a simple if statement, but you get a cleaner callback.