Class: DelayedValue

• Inheritance
• Description
• Class protocol
  • instance creation
• Instance protocol
  • initialization
  • printing
  • private
  • providing value
  • synchronising
  • testing
• Examples

Inheritance:

   ProtoObject
   |
   +--DelayedValue
      |
      +--Future

Package:

stx:libbasic2

Category:

Kernel-Processes

Version:

rev: 1.7 date: 2023/06/08 13:58:50

user: cg

file: DelayedValue.st directory: libbasic2

module: stx stc-classLibrary: libbasic2

Description:

I am similar to a Future, in that I represent an execution in progress, 
whose value will be required some time in the future.
In contrast to a future, which itself spawns a thread to perform the computation,
my value comes from an external source (typically, an event or incoming message from an
interprocess communication channel).
In contrast to a Promise, instances of me can be used interchangable with the promised value
(i.e. I will catch DNU, wait for the value and forward the message automatically)

I will delay any messages sent to me, until the execution has completed (i.e. the value was provided).
This is useful to return values from external sources (print jobs, compile jobs etc.),
which can be done in the background and the user can do something else
in the meantime. 
If the computation is finished before the user needs its value, he is not forced to wait.
If the computation is unfinished, he has to wait for the remaining time only.

copyright
COPYRIGHT (c) 2019 by eXept Software AG
             All Rights Reserved

This software is furnished under a license and may be used
only in accordance with the terms of that license and with the
inclusion of the above copyright notice.   This software may not
be provided or otherwise made available to, or used by, any
other person.  No title to or ownership of the software is
hereby transferred.

Class protocol:

 new

return an initialized instance

Instance protocol:

 initialize

Invoked when a new instance is created.

 initializeSemaphore

Invoked when a new instance is created.

 displayOn: aGCOrStream

notice: displayString and displayOn: will not wait for the value (they are for developers and inspectors),
whereas printString and printOn: will wait (they are for the program to print data).

 displayString

notice: displayString and displayOn: will not wait for the value (they are for developers and inspectors),
whereas printString and printOn: will wait (they are for the program to print data).

 onValueRequested

nothing done here, but needed in subclass

 errorValueAlreadyProvided

 value: anyObject

value is now provided.
If anyone is waiting on me, signal the semaphore

 doesNotUnderstand: aMessage

(Almost) any message to a Future will end up here.

 perform: aSelector withArguments: argArray

send the message aSelector with all args taken from argArray
to the receiver.

 value

retrieve the value, possibly waiting for the result to arrive

 valueOnTimeout: secondsOrNilOrTimeDuration do: exceptionalValue

retrieve the value, possibly waiting for the result to arrive;
if a timeout happens, return the value from exceptionalValue.

 valueWithTimeout: secondsOrNilOrTimeDuration

retrieve the value, possibly waiting for the result to arrive;
if a timeout happens, return nil.

 hasValue

true if I have already a value
(i.e. would not block when sending me a message)

 isLazyValue

true if I have no value yet
(i.e. would block when sending me a message)

Examples:

  | delayedValue |

  delayedValue := DelayedValue new.
  [
      Transcript showCR: 'evaluating factorial...'.
      Delay waitForSeconds:2.
      delayedValue value:(100 factorial).
      Transcript showCR: 'done...'.
  ] fork.    
  Transcript showCR:'The value is: %1' with:delayedValue