[SOLVED] Is it possible/safe to change the OSX mouse system preferences settings such as Tracking Speed programmatically?

Is it possible/safe to change the OSX mouse system preferences settings such as Tracking Speed programmatically?

I did some research and it looks like this is stored in the IORegistry under ":/IOResources/HIDSystem" as "HIDMouseAcceleration".

Can these be set from a user program using the IORegistry API?

Any other ways to programmatically change the mouse system preferences settings?

My choice of programming language is C. The approach needs to work on OS X v10.11+.

Thanks.

Solution

Yes, it's possible. Here's some rough code to do it:

io_object_t hidSystemParametersConnection = IO_OBJECT_NULL;

// We're looking for a service of the IOHIDSystem class
CFMutableDictionaryRef classesToMatch = IOServiceMatching("IOHIDSystem");
if (!classesToMatch)
    /* handle failure */;

// The following call implicitly releases classesToMatch
io_iterator_t matchingServicesIterator = IO_OBJECT_NULL;
IOReturn ret = IOServiceGetMatchingServices(kIOMasterPortDefault, classesToMatch, &matchingServicesIterator);
if (ret != kIOReturnSuccess)
    /* handle failure */;

io_object_t service;
while ((service = IOIteratorNext(matchingServicesIterator)))
{
    // Open the parameters connection to the HIDSystem service
    ret = IOServiceOpen(service, mach_task_self(), kIOHIDParamConnectType, &hidSystemParametersConnection);
    IOObjectRelease(service);

    if (ret == kIOReturnSuccess && hidSystemParametersConnection != IO_OBJECT_NULL)
        break;
}

IOObjectRelease(matchingServicesIterator);

CFTypeRef value;
ret = IOHIDCopyCFTypeParameter(hidSystemParametersConnection, CFSTR(kIOHIDPointerAccelerationKey), &value);
if (ret != kIOReturnSuccess || !value)
    /* handle failure */;

if (CFGetTypeID(value) != CFNumberGetTypeID())
{
    CFRelease(value);
    /* handle wrong type */
}

NSNumber* accel = CFBridgingRelease(value);
double newAccel = accel.doubleValue / 2;

ret = IOHIDSetCFTypeParameter(hidSystemParametersConnection, CFSTR(kIOHIDPointerAccelerationKey), (__bridge CFTypeRef)@(newAccel));
if (ret != kIOReturnSuccess)
    /* handle failure */;

IOServiceClose(hidSystemParametersConnection);

The various parameter keys are defined and lightly documented in IOKit.framework/Headers/hid/IOHIDProperties.h and IOKit.framework/Headers/hidsystem/IOHIDParameter.h. I would test thoroughly with as much different hardware and configurations in System Preferences as you can, to see exactly which parameters are relevant and what their values mean.