class Garcon::Delay

Lazy evaluation of a block yielding an immutable result. Useful for expensive operations that may never be needed. It may be non-blocking, supports the ‘Obligation` interface, and accepts the injection of custom executor upon which to execute the block. Processing of block will be deferred until the first time `#value` is called. At that time the caller can choose to return immediately and let the block execute asynchronously, block indefinitely, or block with a timeout.

When a ‘Delay` is created its state is set to `pending`. The value and reason are both `nil`. The first time the `#value` method is called the enclosed opration will be run and the calling thread will block. Other threads attempting to call `#value` will block as well. Once the operation is complete the value will be set to the result of the operation or the reason will be set to the raised exception, as appropriate. All threads blocked on `#value` will return. Subsequent calls to `#value` will immediately return the cached value. The operation will only be run once. This means that any side effects created by the operation will only happen once as well.

‘Delay` includes the `Garcon::Dereferenceable` mixin to support thread safety of the reference returned by `#value`.

@!macro [attach] delay_note_regarding_blocking

@note The default behavior of `Delay` is to block indefinitely when
  calling either `value` or `wait`, executing the delayed operation on
  the current thread. This makes the `timeout` value completely
  irrelevant. To enable non-blocking behavior, use the `executor`
  constructor option. This will cause the delayed operation to be
  execute on the given executor, allowing the call to timeout.

@see Garcon::Dereferenceable

Public Class Methods

new(opts = {}, &block) click to toggle source

Create a new ‘Delay` in the `:pending` state.

@yield the delayed operation to perform

@raise [ArgumentError] if no block is given

# File lib/garcon/task/delay.rb, line 68
def initialize(opts = {}, &block)
  raise ArgumentError, 'no block given' unless block_given?
  init_obligation
  set_deref_options(opts)
  @task_executor = get_executor_from(opts)
  @task          = block
  @state         = :pending
  @computing     = false
end

Public Instance Methods

reconfigure(&block) click to toggle source

Reconfigures the block returning the value if still ‘#incomplete?`

@yield the delayed operation to perform

@return [true, false] if success

# File lib/garcon/task/delay.rb, line 158
def reconfigure(&block)
  mutex.lock
  raise ArgumentError.new('no block given') unless block_given?
  unless @computing
    @task = block
    true
  else
    false
  end
ensure
  mutex.unlock
end
value(timeout = nil) click to toggle source

Return the value this object represents after applying the options specified by the ‘#set_deref_options` method. If the delayed operation raised an exception this method will return nil. The execption object can be accessed via the `#reason` method.

@param [Numeric]

Timeout the maximum number of seconds to wait.

@return [Object] the current value of the object.

Calls superclass method Garcon::Obligation#value
# File lib/garcon/task/delay.rb, line 88
def value(timeout = nil)
  if @task_executor
    super
  else
    mutex.synchronize do
      execute = @computing = true unless @computing
      if execute
        begin
          set_state(true, @task.call, nil)
        rescue => e
          set_state(false, nil, e)
        end
      end
    end
    if @do_nothing_on_deref
      @value
    else
      apply_deref_options(@value)
    end
  end
end
value!(timeout = nil) click to toggle source

Return the value this object represents after applying the options specified by the ‘#set_deref_options` method. If the delayed operation raised an exception, this method will raise that exception (even when) the operation has already been executed).

@param [Numeric]

Timeout the maximum number of seconds to wait.

@raise [Exception] when ‘#rejected?` raises `#reason`.

@return [Object]

The current value of the object.
Calls superclass method Garcon::Obligation#value!
# File lib/garcon/task/delay.rb, line 123
def value!(timeout = nil)
  if @task_executor
    super
  else
    result = value
    raise @reason if @reason
    result
  end
end
wait(timeout = nil) click to toggle source

Return the value this object represents after applying the options specified by the ‘#set_deref_options` method.

@param [Integer]

Timeout (nil) the maximum number of seconds to wait for the value to be
computed. When `nil` the caller will block indefinitely.

@return [Object] self

Calls superclass method Garcon::Obligation#wait
# File lib/garcon/task/delay.rb, line 142
def wait(timeout = nil)
  if @task_executor
    execute_task_once
    super(timeout)
  else
    value
  end
  self
end

Private Instance Methods

execute_task_once() click to toggle source

@!visibility private

# File lib/garcon/task/delay.rb, line 174
def execute_task_once
  mutex.lock
  execute = @computing = true unless @computing
  task    = @task
  mutex.unlock

  if execute
    @task_executor.post do
      begin
        result  = task.call
        success = true
      rescue => e
        reason = e
      end
      mutex.lock
      set_state(success, result, reason)
      event.set
      mutex.unlock
    end
  end
end