Defines the behaviour required of a module that wishes to act as the backing data store for a given HomeKit characteristic

Simple Value Store

To implement a value store for a simple value whose value does not change asynchronously, you must implement the get_value/1 and put_value/2 callbacks. These callbacks each take a set of opts (specified in the initial configuration passed to HAP.start_link/1) to allow your implementation to discriminate between various values within the same HAP.ValueStore module.

Supporting Asynchronous Notifications

To support notifying HomeKit of changes to an accessory's characteristics (such as a user pressing a button or a flood sensor detecting water), implementations of HAP.ValueStore may choose to implement the optional set_change_token/2 callback. This callback will provide your implementation with a change token to use when notifying HAP of changes to the corresponding value. To notify HAP of changes to a value, pass this change token to the HAP.value_changed/1 function. HAP will then query your value store for the new value of the corresponding characteristic and notify any HomeKit controllers of the change.

There are a number of things to be aware of when using Asynchronous Notifications:

• Your value store must be prepared to answer calls to get_value/1 with the updated value before calling HAP.value_changed/1.
• Do not call HAP.value_changed/1 to notify HAP of changes which come from HAP itself (ie: do not call it in the course of implementing put_value/2). Use it only for notifying HAP of changes which are truly asynchronous.
• If you have not yet received a set_change_token/2 call, then you should not call HAP.value_changed/1; HAP will only provide you with a change token for characteristics which a HomeKit controller has requested notifications on. Specifically, do not retain change tokens between runs; they should maintain the same lifetime as the underlying HAP process.
• The call to HAP.value_changed/1 is guaranteed to return quickly. It does no work beyond casting a message to HAP to set the notification process in motion.