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


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# 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

#errorObject (readonly)

Returns the value of attribute error


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

def error
  @error
end

#nextObject (readonly)

Returns the value of attribute next


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

def next
  @next
end

#prevObject (readonly)

Returns the value of attribute prev


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

def prev
  @prev
end

Class Method Details

.error(value) ⇒ Object


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

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

.value(value) ⇒ Object


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

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

.when(*promises) ⇒ Object


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

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

Instance Method Details

#<<(promise) ⇒ Object


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# File 'opal/stdlib/promise.rb', line 167

def <<(promise)
  @prev = promise

  self
end

#>>(promise) ⇒ Object


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# 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


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# File 'opal/stdlib/promise.rb', line 160

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

  promise
end

#act?Boolean

Returns:


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

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

#actionObject


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

def action
  @action.keys
end

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


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

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

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


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

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

#exception!(error) ⇒ Object


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# File 'opal/stdlib/promise.rb', line 261

def exception!(error)
  @exception = true

  reject!(error)
end

#exception?Boolean

Returns:


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

def exception?
  @exception
end

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


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

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

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


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

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

#inspectObject


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# 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:


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

def realized?
  !!@realized
end

#reject(value = nil) ⇒ Object


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# 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


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# 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:


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

def rejected?
  @realized == :reject
end

#resolve(value = nil) ⇒ Object


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# 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


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# 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:


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

def resolved?
  @realized == :resolve
end

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


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

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

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


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# 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


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# 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


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

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

#trace!(*args, &block) ⇒ Object


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

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

#valueObject


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# File 'opal/stdlib/promise.rb', line 128

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