Class: PromiseV2

Inherits:
Object show all
Defined in:
opal/stdlib/promise/v2.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/v2'

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 = PromiseV2.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 = PromiseV2.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 = PromiseV2.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 PromiseV2 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

PromiseV2.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:

PromiseV2.when(get_json, get_json2).then |first, second|
  puts "got two json payloads: #{first}, #{second}"
end

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize {|_self| ... } ⇒ PromiseV2

Returns a new instance of PromiseV2.

Yields:

  • (_self)

Yield Parameters:

  • _self (PromiseV2) —

    the object that the method was called on



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# File 'opal/stdlib/promise/v2.rb', line 344

def initialize(&block)
  yield self if block_given?
end

Instance Attribute Details

#next ⇒ Object (readonly)

Returns the value of attribute next.



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# File 'opal/stdlib/promise/v2.rb', line 166

def next
  @next
end

#prev ⇒ Object (readonly)

Returns the value of attribute prev.



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# File 'opal/stdlib/promise/v2.rb', line 166

def prev
  @prev
end

Class Method Details

.all_resolved(*promises) ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 122

def all_resolved(*promises)
  promises = Array(promises.length == 1 ? promises.first : promises)
  `Promise.allResolved(#{promises})`.tap do |prom|
    prom.instance_variable_set(:@type, :all_resolved)
  end
end

.allocate ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 105

def allocate
  ok, fail = nil, nil

  prom = `new self.$$constructor(function(_ok, _fail) { #{ok} = _ok; #{fail} = _fail; })`
  prom.instance_variable_set(:@type, :opal)
  prom.instance_variable_set(:@resolve_proc, ok)
  prom.instance_variable_set(:@reject_proc, fail)
  prom
end

.any(*promises) ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 129

def any(*promises)
  promises = Array(promises.length == 1 ? promises.first : promises)
  `Promise.any(#{promises})`.tap do |prom|
    prom.instance_variable_set(:@type, :any)
  end
end

.error ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 162

def reject(value = nil)
  `Promise.reject(#{value})`.tap do |prom|
    prom.instance_variable_set(:@type, :reject)
    prom.instance_variable_set(:@realized, :reject)
    prom.instance_variable_set(:@value_set, true)
    prom.instance_variable_set(:@value, value)
  end
end

.race(*promises) ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 136

def race(*promises)
  promises = Array(promises.length == 1 ? promises.first : promises)
  `Promise.race(#{promises})`.tap do |prom|
    prom.instance_variable_set(:@type, :race)
  end
end

.reject(value = nil) ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 152

def reject(value = nil)
  `Promise.reject(#{value})`.tap do |prom|
    prom.instance_variable_set(:@type, :reject)
    prom.instance_variable_set(:@realized, :reject)
    prom.instance_variable_set(:@value_set, true)
    prom.instance_variable_set(:@value, value)
  end
end

.resolve(value = nil) ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 143

def resolve(value = nil)
  `Promise.resolve(#{value})`.tap do |prom|
    prom.instance_variable_set(:@type, :resolve)
    prom.instance_variable_set(:@realized, :resolve)
    prom.instance_variable_set(:@value_set, true)
    prom.instance_variable_set(:@value, value)
  end
end

.value ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 163

def resolve(value = nil)
  `Promise.resolve(#{value})`.tap do |prom|
    prom.instance_variable_set(:@type, :resolve)
    prom.instance_variable_set(:@realized, :resolve)
    prom.instance_variable_set(:@value_set, true)
    prom.instance_variable_set(:@value, value)
  end
end

.when(*promises) ⇒ Object Also known as: all 



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# File 'opal/stdlib/promise/v2.rb', line 115

def when(*promises)
  promises = Array(promises.length == 1 ? promises.first : promises)
  `Promise.all(#{promises})`.tap do |prom|
    prom.instance_variable_set(:@type, :when)
  end
end

Instance Method Details

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



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# File 'opal/stdlib/promise/v2.rb', line 259

def always(&block)
  prom = nil
  blk = gen_tracing_proc(block) do |val|
    prom.instance_variable_set(:@realized, :resolve)
    prom.instance_variable_set(:@value_set, true)
    prom.instance_variable_set(:@value, val)
  end
  prom = `self.finally(function() { return blk(self.$value_internal()); })`
  prom.instance_variable_set(:@prev, self)
  prom.instance_variable_set(:@type, :always)
  (@next ||= []) << prom
  prom
end

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



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# File 'opal/stdlib/promise/v2.rb', line 273

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

#and(*promises) ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 331

def and(*promises)
  promises = promises.map do |i|
    if PromiseV2 === i
      i
    else
      PromiseV2.value(i)
    end
  end
  PromiseV2.when(self, *promises).then do |a, *b|
    [*a, *b]
  end
end

#error ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 326

def error
  light_nativity_check!
  @value if rejected?
end

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



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# File 'opal/stdlib/promise/v2.rb', line 240

def fail(&block)
  prom = nil
  blk = gen_tracing_proc(block) do |val|
    prom.instance_variable_set(:@realized, :resolve)
    prom.instance_variable_set(:@value_set, true)
    prom.instance_variable_set(:@value, val)
  end
  prom = `self.catch(#{blk})`
  prom.instance_variable_set(:@prev, self)
  prom.instance_variable_set(:@type, :fail)
  (@next ||= []) << prom
  prom
end

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



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# File 'opal/stdlib/promise/v2.rb', line 254

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

#gen_tracing_proc(passing, &block) ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 193

def gen_tracing_proc(passing, &block)
  proc do |i|
    res = passing.call(i)
    yield(res)
    res
  end
end

#inspect ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 356

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

  if @type
    result += ":#{@type}" unless %i[opal resolve reject].include? @type
  else
    result += ':native'
  end

  result += ":#{@realized}" if @realized
  result += "(#{object_id})"

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

  result += ": #{value.inspect}"
  result += '>'

  result
end

#light_nativity_check! ⇒ Object

Raise an exception when a non-JS-native method is called on a JS-native promise but permits some typed promises

Raises:

  • (ArgumentError) 


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# File 'opal/stdlib/promise/v2.rb', line 181

def light_nativity_check!
  return if %i[reject resolve trace always fail then].include? @type
  raise ArgumentError, 'this promise is native to JavaScript' if native?
end

#native? ⇒ Boolean

Is this promise native to JavaScript? This means, that methods like resolve or reject won't be available.

Returns:



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# File 'opal/stdlib/promise/v2.rb', line 170

def native?
  @type != :opal
end

#nativity_check! ⇒ Object

Raise an exception when a non-JS-native method is called on a JS-native promise

Raises:

  • (ArgumentError) 


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# File 'opal/stdlib/promise/v2.rb', line 175

def nativity_check!
  raise ArgumentError, 'this promise is native to JavaScript' if native?
end

#realized? ⇒ Boolean

Returns:



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# File 'opal/stdlib/promise/v2.rb', line 315

def realized?
  light_nativity_check!
  !@realized.nil?
end

#reject(value = nil) ⇒ Object Also known as: reject!

Raises:

  • (ArgumentError) 


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# File 'opal/stdlib/promise/v2.rb', line 211

def reject(value = nil)
  nativity_check!
  raise ArgumentError, 'this promise was already resolved' if @realized
  @value_set = true
  @value = value
  @realized = :reject
  @reject_proc.call(value)
  self
end

#rejected? ⇒ Boolean

Returns:



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# File 'opal/stdlib/promise/v2.rb', line 310

def rejected?
  light_nativity_check!
  @realized == :reject
end

#resolve(value = nil) ⇒ Object Also known as: resolve!

Raises:

  • (ArgumentError) 


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# File 'opal/stdlib/promise/v2.rb', line 201

def resolve(value = nil)
  nativity_check!
  raise ArgumentError, 'this promise was already resolved' if @realized
  @value_set = true
  @value = value
  @realized = :resolve
  @resolve_proc.call(value)
  self
end

#resolved? ⇒ Boolean

Returns:



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# File 'opal/stdlib/promise/v2.rb', line 305

def resolved?
  light_nativity_check!
  @realized == :resolve
end

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



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# File 'opal/stdlib/promise/v2.rb', line 221

def then(&block)
  prom = nil
  blk = gen_tracing_proc(block) do |val|
    prom.instance_variable_set(:@realized, :resolve)
    prom.instance_variable_set(:@value_set, true)
    prom.instance_variable_set(:@value, val)
  end
  prom = `self.then(#{blk})`
  prom.instance_variable_set(:@prev, self)
  prom.instance_variable_set(:@type, :then)
  (@next ||= []) << prom
  prom
end

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



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# File 'opal/stdlib/promise/v2.rb', line 235

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

#there_can_be_only_one! ⇒ Object

Allow only one chain to be present, as needed by the previous implementation. This isn't a strict check - it's always possible on the JS side to chain a given block.

Raises:

  • (ArgumentError) 


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# File 'opal/stdlib/promise/v2.rb', line 189

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

#to_v1 ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 348

def to_v1
  v1 = PromiseV1.new

  self.then { |i| v1.resolve(i) }.rescue { |i| v1.reject(i) }

  v1
end

#trace(depth = nil, &block) ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 278

def trace(depth = nil, &block)
  prom = self.then do
    values = []
    prom = self
    while prom && (!depth || depth > 0)
      val = nil
      begin
        val = prom.value
      rescue ArgumentError
        val = :native
      end
      values.unshift(val)
      depth -= 1 if depth
      prom = prom.prev
    end
    yield(*values)
  end

  prom.instance_variable_set(:@type, :trace)
  prom
end

#trace!(*args, &block) ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 300

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

#value ⇒ Object 



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# File 'opal/stdlib/promise/v2.rb', line 320

def value
  if resolved?
    value_internal
  end
end