macos - Using IOHIDManager to Get Modifier Key Events

Question

I'm trying to use IOHIDManager to get modifier key events because Cocoa flagsChanged events are lacking (difficult to differentiate between press/release, left/right if both are down, etc.) Here's the code where I create the manager and register the callback.

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 is never being run. Am I missing anything?

I don't fully understand HID usage pages, so I didn't know whether or not to use Generic Desktop Page (0x01) with the keyboard usage ID (06) or the Keyboard/Keypad Page (0x07) with Usage IDs for the individual keys. Maybe that has something to do with it?

Answer 1

I figured it out. The way to do it is to use the Generic Desktop Page (0x01) Keyboard (06) (and Keypad (07) for completeness) for use with IOHIDManagerSetDeviceMatchingMultiple, and then the input value callback gets Keyboard/Keypad Usage Page (0x07) stuff.

For example, to setup a an HIDManager for all keyboards/keypads, one could 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 is something like:

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 seems to be called multiple times per press or release but with usage IDs outside the normal range, which is what the "if (scancode < 4 || scancode > 231)" is for.