Class: Promise

Inherits:
Object show all
Defined in:
opal/stdlib/promise.rb

Overview

Promise is used to help structure asynchronous code.

It is available in the Opal standard library, and can be required in any Opal application:

require 'promise'

Basic Usage

Promises are created and returned as objects with the assumption that they will eventually be resolved or rejected, but never both. A Promise has a #then and #fail method (or one of their aliases) that can be used to register a block that gets called once resolved or rejected.

promise = Promise.new

promise.then {
  puts "resolved!"
}.fail {
  puts "rejected!"
}

# some time later
promise.resolve

# => "resolved!"

It is important to remember that a promise can only be resolved or rejected once, so the block will only ever be called once (or not at all).

Resolving Promises

To resolve a promise, means to inform the Promise that it has succeeded or evaluated to a useful value. #resolve can be passed a value which is then passed into the block handler:

def get_json
  promise = Promise.new

  HTTP.get("some_url") do |req|
    promise.resolve req.json
  end

  promise
end

get_json.then do |json|
  puts "got some JSON from server"
end

Rejecting Promises

Promises are also designed to handle error cases, or situations where an outcome is not as expected. Taking the previous example, we can also pass a value to a #reject call, which passes that object to the registered #fail handler:

def get_json
  promise = Promise.new

  HTTP.get("some_url") do |req|
    if req.ok?
      promise.resolve req.json
    else
      promise.reject req
    end

  promise
end

get_json.then {
  # ...
}.fail { |req|
  puts "it went wrong: #{req.message}"
}

Chaining Promises

Promises become even more useful when chained together. Each #then or #fail call returns a new Promise which can be used to chain more and more handlers together.

promise.then { wait_for_something }.then { do_something_else }

Rejections are propagated through the entire chain, so a "catch all" handler can be attached at the end of the tail:

promise.then { ... }.then { ... }.fail { ... }

Composing Promises

Promise.when can be used to wait for more than one promise to resolve (or reject). Using the previous example, we could request two different json requests and wait for both to finish:

Promise.when(get_json, get_json2).then |first, second|
  puts "got two json payloads: #{first}, #{second}"
end

Direct Known Subclasses

Trace, When

Defined Under Namespace

Classes: Trace, When

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(action = {}) ⇒ Promise

Returns a new instance of Promise



115
116
117
118
119
120
121
122
123
124
125
126
 
# File 'opal/stdlib/promise.rb', line 115

def initialize(action = {})
  @action = action

  @realized  = false
  @exception = false
  @value     = nil
  @error     = nil
  @delayed   = false

  @prev = nil
  @next = []
end

Instance Attribute Details

#error ⇒ Object (readonly)

Returns the value of attribute error



113
114
115
 
# File 'opal/stdlib/promise.rb', line 113

def error
  @error
end

#next ⇒ Object (readonly)

Returns the value of attribute next



113
114
115
 
# File 'opal/stdlib/promise.rb', line 113

def next
  @next
end

#prev ⇒ Object (readonly)

Returns the value of attribute prev



113
114
115
 
# File 'opal/stdlib/promise.rb', line 113

def prev
  @prev
end

Class Method Details

.error(value) ⇒ Object 



105
106
107
 
# File 'opal/stdlib/promise.rb', line 105

def self.error(value)
  new.reject(value)
end

.value(value) ⇒ Object 



101
102
103
 
# File 'opal/stdlib/promise.rb', line 101

def self.value(value)
  new.resolve(value)
end

.when(*promises) ⇒ Object 



109
110
111
 
# File 'opal/stdlib/promise.rb', line 109

def self.when(*promises)
  When.new(promises)
end

Instance Method Details

#<<(promise) ⇒ Object 



167
168
169
170
171
 
# File 'opal/stdlib/promise.rb', line 167

def <<(promise)
  @prev = promise

  self
end

#>>(promise) ⇒ Object 



173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
 
# File 'opal/stdlib/promise.rb', line 173

def >>(promise)
  @next << promise

  if exception?
    promise.reject(@delayed[0])
  elsif resolved?
    promise.resolve(@delayed ? @delayed[0] : value)
  elsif rejected?
    if !@action.has_key?(:failure) || Promise === (@delayed ? @delayed[0] : @error)
      promise.reject(@delayed ? @delayed[0] : error)
    elsif promise.action.include?(:always)
      promise.reject(@delayed ? @delayed[0] : error)
    end
  end

  self
end

#^(promise) ⇒ Object 



160
161
162
163
164
165
 
# File 'opal/stdlib/promise.rb', line 160

def ^(promise)
  promise << self
  self    >> promise

  promise
end

#act? ⇒ Boolean

Returns:



136
137
138
 
# File 'opal/stdlib/promise.rb', line 136

def act?
  @action.has_key?(:success) || @action.has_key?(:always)
end

#action ⇒ Object 



140
141
142
 
# File 'opal/stdlib/promise.rb', line 140

def action
  @action.keys
end

#always(&block) ⇒ Object Also known as: finally, ensure 



293
294
295
 
# File 'opal/stdlib/promise.rb', line 293

def always(&block)
  self ^ Promise.new(always: block)
end

#always!(&block) ⇒ Object Also known as: finally!, ensure! 



297
298
299
300
 
# File 'opal/stdlib/promise.rb', line 297

def always!(&block)
  there_can_be_only_one!
  always(&block)
end

#exception!(error) ⇒ Object 



261
262
263
264
265
 
# File 'opal/stdlib/promise.rb', line 261

def exception!(error)
  @exception = true

  reject!(error)
end

#exception? ⇒ Boolean

Returns:



144
145
146
 
# File 'opal/stdlib/promise.rb', line 144

def exception?
  @exception
end

#fail(&block) ⇒ Object Also known as: rescue, catch 



279
280
281
 
# File 'opal/stdlib/promise.rb', line 279

def fail(&block)
  self ^ Promise.new(failure: block)
end

#fail!(&block) ⇒ Object Also known as: rescue!, catch! 



283
284
285
286
 
# File 'opal/stdlib/promise.rb', line 283

def fail!(&block)
  there_can_be_only_one!
  fail(&block)
end

#inspect ⇒ Object 



322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
 
# File 'opal/stdlib/promise.rb', line 322

def inspect
  result = "#<#{self.class}(#{object_id})"

  if @next.any?
    result += " >> #{@next.inspect}"
  end

  if realized?
    result += ": #{(@value || @error).inspect}>"
  else
    result += ">"
  end

  result
end

#realized? ⇒ Boolean

Returns:



148
149
150
 
# File 'opal/stdlib/promise.rb', line 148

def realized?
  !!@realized
end

#reject(value = nil) ⇒ Object 



224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
 
# File 'opal/stdlib/promise.rb', line 224

def reject(value = nil)
  if realized?
    raise ArgumentError, 'the promise has already been realized'
  end

  if Promise === value
    return (value << @prev) ^ self
  end

  begin
    if block = @action[:failure] || @action[:always]
      value = block.call(value)
    end

    if @action.has_key?(:always)
      resolve!(value)
    else
      reject!(value)
    end
  rescue Exception => e
    exception!(e)
  end

  self
end

#reject!(value) ⇒ Object 



250
251
252
253
254
255
256
257
258
259
 
# File 'opal/stdlib/promise.rb', line 250

def reject!(value)
  @realized = :reject
  @error    = value

  if @next.any?
    @next.each { |p| p.reject(value) }
  else
    @delayed = [value]
  end
end

#rejected? ⇒ Boolean

Returns:



156
157
158
 
# File 'opal/stdlib/promise.rb', line 156

def rejected?
  @realized == :reject
end

#resolve(value = nil) ⇒ Object 



191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
 
# File 'opal/stdlib/promise.rb', line 191

def resolve(value = nil)
  if realized?
    raise ArgumentError, 'the promise has already been realized'
  end

  if Promise === value
    return (value << @prev) ^ self
  end

  begin
    if block = @action[:success] || @action[:always]
      value = block.call(value)
    end

    resolve!(value)
  rescue Exception => e
    exception!(e)
  end

  self
end

#resolve!(value) ⇒ Object 



213
214
215
216
217
218
219
220
221
222
 
# File 'opal/stdlib/promise.rb', line 213

def resolve!(value)
  @realized = :resolve
  @value    = value

  if @next.any?
    @next.each { |p| p.resolve(value) }
  else
    @delayed = [value]
  end
end

#resolved? ⇒ Boolean

Returns:



152
153
154
 
# File 'opal/stdlib/promise.rb', line 152

def resolved?
  @realized == :resolve
end

#then(&block) ⇒ Object Also known as: do 



267
268
269
 
# File 'opal/stdlib/promise.rb', line 267

def then(&block)
  self ^ Promise.new(success: block)
end

#then!(&block) ⇒ Object Also known as: do! 



271
272
273
274
 
# File 'opal/stdlib/promise.rb', line 271

def then!(&block)
  there_can_be_only_one!
  self.then(&block)
end

#there_can_be_only_one! ⇒ Object 



316
317
318
319
320
 
# File 'opal/stdlib/promise.rb', line 316

def there_can_be_only_one!
  if @next.any?
    raise ArgumentError, 'a promise has already been chained'
  end
end

#trace(depth = nil, &block) ⇒ Object 



307
308
309
 
# File 'opal/stdlib/promise.rb', line 307

def trace(depth = nil, &block)
  self ^ Trace.new(depth, block)
end

#trace!(*args, &block) ⇒ Object 



311
312
313
314
 
# File 'opal/stdlib/promise.rb', line 311

def trace!(*args, &block)
  there_can_be_only_one!
  trace(*args, &block)
end

#value ⇒ Object 



128
129
130
131
132
133
134
 
# File 'opal/stdlib/promise.rb', line 128

def value
  if Promise === @value
    @value.value
  else
    @value
  end
end