promises.lua

   1 local PromisePrototype = {}
   2
   3 function PromisePrototype:__run_handler(func, ...)
   4         local values = {pcall(func, ...)}
   5
   6         if table.remove(values, 1) then
   7                 self:__resolve_raw(unpack(values))
   8         else
   9                 self:__reject_raw(values[1])
  10         end
  11 end
  12
  13 function PromisePrototype:__add_child(promise)
  14         if self.state == "resolved" then
  15                 promise:__resolve(unpack(self.values))
  16         elseif self.state == "rejected" then
  17                 promise:__reject(self.reason)
  18         else
  19                 table.insert(self.__children, promise)
  20         end
  21 end
  22
  23 function PromisePrototype:__resolve_raw(...)
  24         self.state = "resolved"
  25         self.values = {...}
  26         self.reason = nil
  27
  28         for _, child in ipairs(self.__children) do
  29                 child:resolve(...)
  30         end
  31 end
  32
  33 function PromisePrototype:__reject_raw(reason)
  34         self.state = "rejected"
  35         self.values = nil
  36         self.reason = reason
  37
  38         local any_child = false
  39
  40         for _, child in ipairs(self.__children) do
  41                 child:reject(reason)
  42         end
  43
  44         assert(any_child, "Uncaught (in promise): " .. reason)
  45 end
  46
  47 function PromisePrototype:resolve(...)
  48         assert(self.state == "pending")
  49
  50         if self.__on_resolve then
  51                 self:__run_handler(self.__on_resolve, ...)
  52         else
  53                 self:__resolve_raw(...)
  54         end
  55 end
  56
  57 function PromisePrototype:reject(reason)
  58         assert(self.state == "pending")
  59
  60         if self.__on_reject then
  61                 self:__run_handler(self.__on_reject, reason)
  62         else
  63                 self:__reject_raw(reason)
  64         end
  65 end
  66
  67 function PromisePrototype:then_(func)
  68         local promise = Promise()
  69         promise.__on_resolve = func
  70
  71         self:__add_child(promise)
  72
  73         return promise
  74 end
  75
  76 function PromisePrototype:catch(func)
  77         local promise = Promise(function() end)
  78         promise.__on_reject = func
  79
  80         self:__add_child(promise)
  81
  82         return promise
  83 end
  84
  85 Promise = setmetatable({}, {
  86         __call = function(_, resolver)
  87                 local promise = {
  88                         state = "pending",
  89                         __children = {},
  90                 }
  91
  92                 setmetatable(promise, {__index = PromisePrototype})
  93
  94                 if resolver then
  95                         resolver(
  96                                 function(...)
  97                                         promise:resolve(...)
  98                                 end,
  99                                 function(...)
 100                                         promise:reject(...)
 101                                 end
 102                         )
 103                 end
 104
 105                 return promise
 106         end
 107 })
 108
 109 function Promise.resolve(...)
 110         local args = {...}
 111         return Promise(function(resolve)
 112                 resolve(unpack(args))
 113         end)
 114 end