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(success = nil, failure = nil) ⇒ Promise

Returns a new instance of Promise



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

def initialize(success = nil, failure = nil)
  @success = success
  @failure = failure

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

  @prev = nil
  @next = nil
end

Instance Attribute Details

#error ⇒ Object (readonly)

Returns the value of attribute error



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

def error
  @error
end

#next ⇒ Object (readonly)

Returns the value of attribute next



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

def next
  @next
end

#prev ⇒ Object (readonly)

Returns the value of attribute prev



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

def prev
  @prev
end

#value ⇒ Object (readonly)

Returns the value of attribute value



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

def value
  @value
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 156

def <<(promise)
  @prev = promise

  self
end

#>>(promise) ⇒ Object 



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# File 'opal/stdlib/promise.rb', line 162

def >>(promise)
  @next = promise

  if exception?
    promise.reject(@delayed)
  elsif resolved?
    promise.resolve(@delayed || value)
  elsif rejected? && (!@failure || Promise === (@delayed || @error))
    promise.reject(@delayed || error)
  end

  self
end

#^(promise) ⇒ Object 



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

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

  promise
end

#act? ⇒ Boolean

Returns:



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# File 'opal/stdlib/promise.rb', line 129

def act?
  @success != nil
end

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



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# File 'opal/stdlib/promise.rb', line 277

def always(&block)
  if @next
    raise ArgumentError, 'a promise has already been chained'
  end

  self ^ Promise.new(block, block)
end

#exception!(error) ⇒ Object 



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# File 'opal/stdlib/promise.rb', line 250

def exception!(error)
  @exception = true

  reject!(error)
end

#exception? ⇒ Boolean

Returns:



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# File 'opal/stdlib/promise.rb', line 133

def exception?
  @exception
end

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



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# File 'opal/stdlib/promise.rb', line 266

def fail(&block)
  if @next
    raise ArgumentError, 'a promise has already been chained'
  end

  self ^ Promise.new(nil, block)
end

#inspect ⇒ Object 



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# File 'opal/stdlib/promise.rb', line 296

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

  if @next
    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 137

def realized?
  @realized != nil
end

#reject(value = nil) ⇒ Object 



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# File 'opal/stdlib/promise.rb', line 211

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

  if Promise === value
    value << @prev

    return value ^ self
  end

  @realized = :reject
  @error    = value

  begin
    if @failure
      value = @failure.call(value)

      if Promise === value
        reject!(value)
      end
    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 242

def reject!(value)
  if @next
    @next.reject(value)
  else
    @delayed = value
  end
end

#rejected? ⇒ Boolean

Returns:



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

def rejected?
  @realized == :reject
end

#resolve(value = nil) ⇒ Object 



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# File 'opal/stdlib/promise.rb', line 176

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

  if Promise === value
    value << @prev

    return value ^ self
  end

  @realized = :resolve
  @value    = value

  begin
    if @success
      value = @success.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 203

def resolve!(value)
  if @next
    @next.resolve(value)
  else
    @delayed = value
  end
end

#resolved? ⇒ Boolean

Returns:



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

def resolved?
  @realized == :resolve
end

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



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# File 'opal/stdlib/promise.rb', line 256

def then(&block)
  if @next
    raise ArgumentError, 'a promise has already been chained'
  end

  self ^ Promise.new(block)
end

#trace(depth = nil, &block) ⇒ Object 



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# File 'opal/stdlib/promise.rb', line 288

def trace(depth = nil, &block)
  if @next
    raise ArgumentError, 'a promise has already been chained'
  end

  self ^ Trace.new(depth, block)
end