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   1 # Property Accessors Mixin [![Build Status](https://travis-ci.org/atom/property-accessors.svg?branch=master)](https://travis-ci.org/atom/property-accessors)
   2
   3 A mixin for defining dynamic properties.
   4
   5 ## Basic Usage
   6
   7 To define a basic property accessor, use the `accessor` declaration. If you've
   8 included the mixin into a class, you define a prototype property by calling
   9 `@::accessor` on its prototype.
  10
  11 ```coffee
  12 PropertyAccessors = require 'property-accessors'
  13
  14 class Vehicle
  15   PropertyAccessors.includeInto(this)
  16
  17   @::accessor 'type',
  18     get: ->
  19       switch @doorCount
  20         when 4 then 'sedan' # i know this isn't strictly accurate
  21         when 2 then 'coupe'
  22     set: (type) ->
  23       switch type
  24         when 'sedan' then @doorCount = 4
  25         when 'coupe' then @doorCount = 2
  26
  27 car = new Vehicle
  28 car.doorCount = 2
  29 car.type # => 'coupe'
  30 ```
  31
  32 You can define a class-level property by *extending* with the mixin rather than
  33 including it (which extends the prototype).
  34
  35 ```coffee
  36 class Vehicle
  37   PropertyAccessors.extend(this)
  38
  39   @accessor 'vehicleCount', get: -> @allVehicles.length
  40 ```
  41
  42 You can just pass a single function if you only want to define a getter:
  43
  44 ```coffee
  45 class Vehicle
  46   PropertyAccessors.includeInto(this)
  47
  48   @::accessor 'type', -> # ...
  49 ```
  50
  51 ## Fancy Usage
  52
  53 ### Lazy Accessors
  54
  55 Lazy accessors call a function the first time a property is accessed. You are
  56 still free to overwrite this value by assigning the property explicitly.
  57
  58 ```coffee
  59 class ScienceLab
  60   PropertyAccessors.includeInto(this)
  61
  62   @::lazyAccessor 'crazyComputation', -> computeCrazyComputation()
  63 ```
  64
  65 ### Advised Accessors
  66
  67 Advised accessors allow you to call code before the reading or writing of a
  68 property value. If a property is being assigned, your advice function is called
  69 with the value being assigned and the old value.
  70
  71 ```coffee
  72 class SpyStation
  73   @advisedAccessor 'online',
  74     get: -> @ensureAllSystemsNominal()
  75     set: -> @ensureUserIsSpy()
  76
  77 station = new SpyStation
  78 station.online = true # ensures user is a spy, then assigns true
  79 station.online # ensures all systems are nominal, then returns true
  80 ```