Motion actuation

Success criterion 2.5.4 covers functions triggered by moving the device or by moving toward it: shaking a phone to undo, tilting it to turn a page, waving a hand past a proximity sensor, raising a wrist to wake a watch. The criterion asks two things of any such trigger, and the second is the one people forget. The function must also be reachable through conventional controls, and the motion trigger must be able to be switched off. Both, not either. A gesture with an on-screen button but no way to stop the gesture firing still fails, and so does a gesture that can be disabled but has no button behind it.

The alternative exists because motion is an input a great many people cannot produce. A phone clamped to a wheelchair, a tablet on a stand, a device held in a mount on a desk, or a hand that cannot lift and shake the whole thing all leave the motion route unreachable, which makes the plain control the only route, so it has to be there. That is the same argument pointer gestures and dragging alternative make one level down about paths and drags: the operable function is the requirement, and a particular way of performing it never is.

The off switch is there for the opposite failure, motion nobody meant. A tremor, a wheelchair over a kerb, a bus, a walk, or a phone in a coat pocket all produce accelerometer data that reads as a deliberate shake, and the reader whose hand shakes hardest is the one who gets the undo prompt most often. An interruption that arrives repeatedly, unbidden, from a gesture the reader cannot stop making is a hard barrier, not an annoyance, which is why the criterion treats the ability to turn it off as being as important as the alternative control. Do not rely on the operating system to have the setting: iOS has one for shake to undo, and a shake your own code detects from raw motion events is yours to switch off.

Two edges. The criterion exempts motion that is essential to the function, which is why a pedometer, a spirit level, and a camera’s own shake detection are not caught by it, and it exempts triggers implemented through an accessibility-supported interface, since assistive technology can already reach those. And it is not a rule against motion input. A shake, a tilt, or a wrist raise is a fine shortcut for the people who can make it and want it, in exactly the way a keyboard shortcut is: layered on a route everyone has, and switchable, never the only door.