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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

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