Software Development

On twitter, the question was posed if the Ink System Preferences pane could be installed without having a supported graphics tablet.

I immediately jumped to answer: YES, there is!
I remembered because I’ve had a need for this myself, way back when the OS X sandbox was introduced.

Why I Know This

Every now and then, I get an eMail from a graphics tablet user, describing how they suddenly aren’t able to drag files out of Yoink anymore. No warning. No apparent signs to why Yoink wouldn’t do its duty anymore. And all the console would say is “sandbox: deny hid-control”.
Thankfully, now, I know what it means and I can give users directions on what to do. However, after all these years, Apple still hasn’t done anything (not one thing!) about this bug in the sandbox, and it’s insanely irritating.

For me to be able to confirm that it had in fact to do with OS X’s handwriting recognition, aka Ink, i had to find a way to activate it, without actually having a graphics tablet at my disposal.

I tried doing the obvious thing (navigating to /System/Library/PreferencePanes/ and double-clicking Ink), but that only resulted in this error message:

Ink Install Error

Install The Ink System Preferences Pane

If you, too, need to test some stuff with Ink, even though you currently don’t have a graphics tablet handy, here’s how to do it.

After a lot of trial and error, this is the working solution I came up with:

  1. In Finder, navigate to /System/Library/CoreServices/Menu Extras
  2. Double-click onto Ink.menu

This item will appear in the right part of your menu bar (notice how it’s perfectly updated for the dark appearance of the menu bar… NOT) :

Ink menu bar icon

Now, click on Ink Preferences… and it will launch System Preferences for you, installing the Ink preference pane:

Ink preference pane

A restart of System Preferences removes the preference pane again, though you’ll continue to be able to install it using the menu bar item.
Should you have the wish to remove Ink from your menu bar, hold down the command key (⌘) on your keyboard and drag the item off, then release the mouse button. The poof-animation will appear, indicating that the item has been successfully removed.

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In iOS Simulator, you can, via a menu item, quickly simulate memory pressure for the simulated device to see if your app behaves correctly under low memory conditions (releasing caches, cleaning up resources, and so on).

Recently, I had the need for it on the Mac (where such a menu item is not available), while working on an XPC service for Yoink, to see if it terminated properly under certain conditions.

Meet the memory_pressure tool

OS X comes with the Terminal, and with the Terminal come some awesome tools. Among them is memory_pressure. Its man page says it all:

Apply real or simulate memory pressure on the system.

Usage of memory_tool

sudo memory_pressure -S -l critical

What this does is simulate (-S) a memory pressure of a critical level (-l critical).
sudo is needed, otherwise it will probably fail with an “Operation not permitted” error.

memory_tool’s options

-l:   Two levels of pressure are supported: ‘warn’ and ‘critical’.
-p:  For real pressure, this lets you define how much memory should remain free, in percent of total memory.
-S:  Simulate memory pressure, don’t really allocate memory.
-s:   If applying real pressure, this is the duration to wait before allocating or freeing memory.
       If it’s simulated pressure, the system will be maintained at an artificial memory level for this duration.

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In an effort to show preferences for configuring Force Touch in my apps (in particular, Yoink) only when a Force Touch device is actually available, I had to find a way to figure out how to detect Force Touch devices.

If this was iOS, I’d be done by now

On iOS, Apple provides a simple API for this:

UIForceTouchCapabilityUnavailable and UIForceTouchCapabilityAvailable

which you can check by calling UIView’s – (UIForceTouchCapability)traitCollection;. Lo and behold, a simple API like this is sadly not available on OS X. On the Mac, you have to do it yourself.

IOKit is where it’s at

Using IOKit, you can sort of set up a set of properties you’re looking for in a device and see if it returns anything. This does not only include an external or internal keyboard, mouse or trackpad, but also graphic cards, for example. To find out what the right properties are, I downloaded Apple’s Hardware IO Tools for Xcode 7.1 from their developer downloads site (Apple Developer account required) and launched the app IORegistryExplorer.

Digging down the IO Registry

I do have a Magic Trackpad 2 “attached” to my Mac via Bluetooth, so I tried searching for the term “Trackpad”, and sure enough, I saw my internal and external ones:

IORegistry TrackpadsThe Trackpads attached to my Mac, either via Bluetooth or internally.

Having found the Magic Trackpad 2, the next step is to see what properties it offers and if they are unique to the class of the device:

Trackpad PropertiesJackpot!

Sure enough, there it is – ForceSupported: True. I could not find such a key in the internal trackpad’s properties, hinting that it might be exclusive to devices that do support Force Touch. There’s also an entry for “Manufacturer”, which is “Apple Inc.”. Perfect.

Looking for Devices with IOKit

Now all I have to do is filtering devices by the Manufacturer – “Apple Inc.” -, iterate over the resulting devices, and filter out devices that have a DefaultMultitouchProperties key, containing a ForceSupported key with a value of true. If such a device is found, it means a device with Force Touch capabilities is available.

Code Listing #1

In this method, I create a dictionary mDict that is used to find matching devices. In this case, I’m looking for devices with the Manufacturer set to “Apple Inc.”. I query for possible devices using IOServiceGetMatchingServices. I can then iterate over the returned io_iterator_t iterator and recursively over the children in the core of all of this: – (BOOL)_containsForceTouchDevice:io_iterator_t)iterator;.

Code Listing #2

Here, we iterate recursively over iterator’s objects, checking for the DefaultMultitouchProperties key and, subsequently the ForceSupported key (and value).

Testing it with other Trackpads

The code you see above is final. However, in a previous version, all I could use to test it with was my Magic Trackpad 2 – an internal Force Touch Trackpad (the likes of which the new MacBooks and MacBook Pros feature) was not available to me directly. So I sent the first draft of the code to my friend (and fellow developer) Maurice Kelly (@mauricerkelly on twitter) who was kind enough to volunteer as my “test subject”; and – of course – it didn’t work. Turns out I shouldn’t assume the vendorID to be the same (which I used in the first draft just like the Manufacturer to filter the possible results). After leaving it out, it worked fine over all currently available Force Touch Trackpads.

Hot (Un-)Plugging

Sometimes you might want to get notified when devices are plugged in to or unplugged from the Mac. In Yoink, I’d like to display preferences specific to Force Touch if according hardware is available, but not show them if there isn’t any hardware connected that supports it. Also, as a nice touch, I’d like to hide the preferences if according hardware is disconnected and show it again when it is connected. We can accomplish this like that:

Code Listing #3

Sandbox

The code works just as well in the Sandbox environment, these entitlements have to be set, though, for the notifications to work:

Sandbox Entitlements

com.apple.security.device.usb and com.apple.security.device.bluetooth

Future Proof?

I’m not sure this code is future proof. The basic APIs will stay around, sure, but it all depends on the keys DefaultMultitouchProperties, ForceSupported and their according values which are not defined in the IOKit header files, which I unsuccessfully searched for constants pertaining to force/pressure. So, depending on this, the code might break with future versions of the Force Touch Trackpad and I’ll have to keep testing it as Apple releases new hardware. Nevertheless, I’m quite happy with the code and it works very well.

Getting the Source Code

I’ve uploaded a sample project to my server, you can download it here. The category on NSApplication in which this is available can be downloaded from Github. The example app is pretty simple:

Force Touch Device Detection Example App Screenshot

At launch, it asks if a Force Touch capable device is available and displays an according message. If subsequently the availability status changes, the message will be updated accordingly.

If you’d like to get in touch, you can mail me or write me on twitter. I’m looking forward to hearing from you 🙂

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