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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 339
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 162
def next
 @next
end

#prev ⇒ Object (readonly)

Returns the value of attribute prev.

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# File 'opal/stdlib/promise/v2.rb', line 162
def prev
 @prev
end

Class Method Details

.all_resolved(*promises) ⇒ Object 

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# File 'opal/stdlib/promise/v2.rb', line 120
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 103
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 127
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 158
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, value)
 end
end

.race(*promises) ⇒ Object 

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# File 'opal/stdlib/promise/v2.rb', line 134
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 149
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, value)
 end
end

.resolve(value = nil) ⇒ Object 

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# File 'opal/stdlib/promise/v2.rb', line 141
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, value)
 end
end

.value ⇒ Object 

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# File 'opal/stdlib/promise/v2.rb', line 159
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, value)
 end
end

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

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# File 'opal/stdlib/promise/v2.rb', line 113
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 251
def always(&block)
 prom = nil
 blk = gen_tracing_proc(block) do |val|
 prom.instance_variable_set(:@realized, :resolve)
 prom.instance_variable_set(:@value, val)
 end
 prom = `self.finally(#{blk})`
 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 264
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 326
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 321
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 233
def fail(&block)
 prom = nil
 blk = gen_tracing_proc(block) do |val|
 prom.instance_variable_set(:@realized, :resolve)
 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 246
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 189
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 351
def inspect
 result = "#<#{self.class}"
 if @type
 result += ":#{@type}" unless %i[opalresolvereject].include? @type
 else
 result += ':native'
 end
 result += ":#{@realized}" if @realized
 result += "(#{object_id})"
 if @next && @next.any?
 result += " >> #{@next.inspect}"
 end
 result += ": #{@value.inspect}" if @value
 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 177
def light_nativity_check!
 return if %i[rejectresolvetracealwaysfailthen].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 166
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 171
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 306
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 206
def reject(value = nil)
 nativity_check!
 raise ArgumentError, 'this promise was already resolved' if @realized
 @value = value
 @realized = :reject
 @reject_proc.call(value)
 self
end

#rejected? ⇒ Boolean

Returns:

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# File 'opal/stdlib/promise/v2.rb', line 301
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 197
def resolve(value = nil)
 nativity_check!
 raise ArgumentError, 'this promise was already resolved' if @realized
 @value = value
 @realized = :resolve
 @resolve_proc.call(value)
 self
end

#resolved? ⇒ Boolean

Returns:

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# File 'opal/stdlib/promise/v2.rb', line 296
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 215
def then(&block)
 prom = nil
 blk = gen_tracing_proc(block) do |val|
 prom.instance_variable_set(:@realized, :resolve)
 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 228
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 185
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 343
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 269
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 291
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 311
def value
 if resolved?
 if PromiseV2  === @value
 @value.value
 else
 @value
 end
 end
end

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