Class: QuerySignal

• Inheritance
• Description
• Instance protocol
  • answering queries
  • exception creation
  • initialization
  • queries
  • raising
• Examples

Inheritance:

   Object
   |
   +--Signal
      |
      +--QuerySignal

Package:

stx:libbasic

Category:

Kernel-Exceptions

Version:

rev: 1.54 date: 2024/03/26 11:36:18

user: cg

file: QuerySignal.st directory: libbasic

module: stx stc-classLibrary: libbasic

Description:

Notice: 
    Signals have been replaced by class based Exceptions.
    They are provided for compatibility, but new code should use Exceptions
    (in this case: Query)

QuerySignals are like signals, except that they are not accepted
by handlers for ordinary signals.
I.e. a signal handler for a normal signal will not handle a query
signal. Thus, these bypass anySignal handlers.

However, if unhandled, no error is raised, instead they are simply ignored
and nil is returned from the raise
(as opposed to normal signals, which raise an unhandled signal exception).
QuerySignals are also ignored, if a handler exists, but rejects.

Their main use is to implement upQueries via signals, that work even 
if intermediate errorSignal handlers are present 
(which is not possible with ordinary signals, since errorSignal handlers 
 would catch those signals).

Code deep down in the calling hierarchy can post such an up-Query to ask
for some information or to pass some information upward. 

For example, the activityNotification mechanism is built on top of this:
everyone can send such a notification which is either handled by someone
up in the hierarchy (to show it in the windows info area) or simply
ignored.

Using QuerySignals for this (instead of regular Signals) helps in documenting
the intended usage of those signals.

Another use of querySignals is to provide additional information to
deeply nested methods, which is only required in the uncommon case;
or if another parameter is required by some method, which was not planned
for in the beginning, and you do not want to hand this value (via an
additional argument) through all intermediate levels.
A highly elegant solution to this problem is to provide a handler somewhere
at the top of the calling hierarchy, and raise an upQuery from whereever
that value is required.
A concrete application can be found in the windowGroup-lastEvent
queries. If anyone is interested in the windowEvent which was responible for 
being invoked, all he needs to do is to raise the lastEventQuerySignal, 
which returns that event.
No intermediate methods are required to know anything about that.
Another example is found in the way Metaclass asks for the nameSpace
when new classes are to be installed. A Browser may simply answer such
a query and provide a namespace (no need to pass that information down
the calling chain).

A final note (to C++ and Java fans):
    such upQueries are only possible, if the exception handling mechanism
    does not automatically unwind the stack for the handler invokation.
    Since the handler must be able to proceed the execution and return
    a value to the raiser ....
    ... another demonstration of why ST's exception mechanisms are superior.

copyright
COPYRIGHT (c) 1995 by Claus Gittinger
             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.

Instance protocol:

 answer: someAnswer do: aBlock

evaluate the argument, aBlock.
If the receiver is queried during evaluation, answer and proceed with someAnswer.
This is a wrapper for #handle:do: for lazy typists; no new functionality.

Usage example(s):

|q| q := QuerySignal new. q answer:true do:[ Transcript showCR:'query answers: ' , (q query printString). ]

Usage example(s):

|q| q := QuerySignal new. q answer:false do:[ Transcript showCR:'first query answers: ' , (q query printString). q answer:true do:[ Transcript showCR:'second query answers: ' , (q query printString). ] ]

 newException

answer a new exception object for this signal

 defaultAnswer: someValue

define the query's defaultAnswer to be someValue.
This is the same as defining an appropriate handlerBlock.

Usage example(s):

QuerySignal new query (QuerySignal new defaultAnswer:true) query (QuerySignal new defaultAnswer:false) query

Usage example(s):

|sig rslt| sig := QuerySignal new. sig defaultAnswer:false. rslt := sig query. Transcript showCR:rslt.

Usage example(s):

|sig rslt| sig := QuerySignal new. sig defaultAnswer:false. sig answer:true do:[ rslt := sig query ]. Transcript showCR:rslt.

 accepts: aSignalOrExceptionClass

return true, if the receiver accepts the argument, aSignal.
(i.e. the receiver is aSignal or a parent of it). False otherwise.

 defaultAnswer

 handlerForSignal: signal context: theContext originator: originator

answer the handler block for the signal from originator.
The block is retrieved from aContext.
Answer nil if the signal is not handled

 handles: anException

return true, if the receiver handles the argument, anException.
(i.e. the receiver is anExceptions signal or a parent of it)

 isQuery

return true, if this is a query - always return true here

 isQuerySignal

return true, if this is a querySignal - always return true here

 query

raise the query - return the handler's value, or the default
value, if there is no handler.

 queryWith: aParameter

raise the query passing a parameter
- return the handler's value, or the default value, if there is no handler.

 raise

QuerySignals are proceedable by definition,
so they should be raised with #query or #raiseRequest

** This is an obsolete interface - do not use it (it may vanish in future versions) **

 raiseRequest

raise the query - return the handler's value, or the default
value, if there is no handler.
Invoking the handler is exactly the functionality of Signal>>raiseRequest,
but we can do it faster here, by avoiding the creation of a full-blown
exception object in case we find an answer:do: context.

Examples:

  |querySignal zero|

  zero := 0.
  querySignal := QuerySignal new.
  querySignal handle:[:ex |
      Transcript showCR:'query'.
      ex proceedWith:true
  ] do:[
      'nesting'.
      [
          [
              Object errorSignal handle:[:ex |
                  Transcript showCR:'some error: ' , ex errorString.
                  ex proceed
              ] do:[
                  [
                      1 // zero.  'an error which is caught in the handler'.
                      (querySignal query) == true ifTrue:[
                          Transcript showCR:'query says: ok'.
                      ] ifFalse:[
                          Transcript showCR:'query says: no'
                      ]
                  ] value
              ]
          ] value
      ] value
  ]

  |querySignal|

  querySignal := QuerySignal new.
  querySignal answer:true do:[
      'nesting'.
      [
          [
              (querySignal query) == true ifTrue:[
                  Transcript showCR:'query says: ok'.
              ] ifFalse:[
                  Transcript showCR:'query says: no'
              ]
          ] value
      ] value
  ]

  |querySignal zero|

  zero := 0.
  querySignal := QuerySignal new.

  [
      'nesting'.
      [
          [
              Object errorSignal handle:[:ex |
                  Transcript showCR:'some error: ' , ex errorString.
                  ex proceed
              ] do:[
                  [
                      1 // zero.  'an error which is caught in the handler'.
                      (querySignal raise) == true ifTrue:[
                          Transcript showCR:'query says: ok'.
                      ] ifFalse:[
                          Transcript showCR:'query says: no'
                      ]
                  ] value
              ]
          ] value
      ] value
  ] value

  |signal|

  signal := Signal new.
  'nesting deeply'.
  [
      [
          [
              [
                  [
                      (signal raise) == true ifTrue:[
                          Transcript showCR:'query says: ok'.
                      ] ifFalse:[
                          Transcript showCR:'query says: no'
                      ]
                  ] value
              ] value
          ] value
      ] value
  ] value

  |signal|

  signal := Signal new.
  'nesting deeply'.
  [
      [
          [
              [
                  [
                      Signal noHandlerSignal handle:[:ex |
                          ex proceedWith:nil
                      ] do:[
                          (signal raise) == true ifTrue:[
                              Transcript showCR:'query says: ok'.
                          ] ifFalse:[
                              Transcript showCR:'query says: no'
                          ]
                      ]
                  ] value
              ] value
          ] value
      ] value
  ] value