State

A driver should be built to represent the state of the device or service it is abstracting.

• User interfaces can bind to state to provide visual feedback

• Logic can query state and subscribe to state changes to trigger further actions

Driver state is should always be considered public and as such, should never contain any sensitive information (auth details, access tokens etc).

• Any authenticated user can read the state

• Do not store sensitive material in state

Exposing State

A driver quacks like a hash

All state keys must be Symbols, Strings or respond to #to_sym

• Setting state self[:state_key] = 'value'

• Reading state self[:stafe_key] # => 'value'

Logic can access driver state in the same way

• Reading state system[:Display][:power] # => true

• Setting state system[:Display][:not_recommended] = true

Logic might trigger a useful action by changing the state of other drivers directly, however generally it is recommended to use a mutator method.

All state values should be limited to objects that can be converted to JSON for transmitting over the API.

• JSON compatible classes: nil, true, false, Hash, String, Integer, Array, Float, Symbol

• Objects that respond to: #to_json or failing that #to_s will be called when sending over the API

• Objects that don’t meet these requirements can be used server side and are sent as nil over the API

Subscribing to State

All drivers can subscribe to their own state.

# Subscribe to internal state

ref = subscribe(:state_variable) do |notify|
    notification.value     # => value of the status variable that triggered this notification
    notification.old_value # => the value of the variable before this change

    # Also comes with the subscription information
    notify.sys_name # => The system name
    notify.sys_id   # => The system ID this value originated from
    notify.mod_name # => The generic module name 
    notify.mod_id   # => The module database ID
    notify.index    # => The device index
    notify.status   # => the name of the status variable

    # And a reference to the subscription should you want to unsubscribe
    unsubscribe(notification.subscription)
end

# Optionally unsubscribe
unsubscribe(ref)

Logic can additionally subscribe to state of other drivers.

Change detection

When state is applied it is checked against the existing value and subscribers are only notified if the value has changed.

self[:power] = On  # => subscribers are notified of the change
self[:power] = On  # => no change detected, no action taken
self[:power] = Off # => subscribers are notified of the change

Change detection doesn’t work if you mutate a variable.

self[:my_array] = [1, 2, 3] # => subscribers are notified of the change
self[:my_array] << 4        # => no action taken (change detection isn't run)

@my_copy = self[:my_array]
@my_copy << 5
self[:my_array] = @my_copy  # => no change detected (change detection did run)

# Only if you are really sure you know what you are doing!
signal_status(:my_array)    # => forces a change notification to subscribers

Mutating complex status variables is not recommended as the variables might be being acted upon on another thread. This can lead to race conditions or worse.

The recommended method for updating complex state is:

1. Duplicate my_array = ['my', 'array'].dup or {complex:['hash']}.deep_dup

   • .deep_dup when in doubt

2. Update my_array << 8

3. Apply self[:my_array] = my_array

This can be achieved by using operations that create a new object

self[:my_array] = [1, 2, 3] # => subscribers are notified of the change
self[:my_array] += [4]      # => subscribers are notified of the change
self[:my_array]             # => [1, 2, 3, 4]

# These will both trigger notifications
self[:my_hash] = { example: 1 }
self[:my_hash] = self[:my_hash].merge({ update: true })
self[:my_hash] # => { example: 1, update: true }