"

 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.

"

Signal subclass:#QuerySignal

	instanceVariableNames:'defaultAnswer hasDefault'

	classVariableNames:''

	poolDictionaries:''

	category:'Kernel-Exceptions'

!

!QuerySignal class methodsFor:'documentation'!

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.

"

!

documentation

"

    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.

    [see also:]

        Signal SignalSet Exception

        Object

        (``Exception handling and signals'': programming/exceptions.html)

    [author:]

        Claus Gittinger

"

!

examples 

"

  an up-query from a deeply nested operation to a higher level:

                                                                        [exBegin]

    |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'.

                            Transcript showCR:'query says: ok'.

                        ] ifFalse:[

                            Transcript showCR:'query says: no'

                        ]

                    ] value

                ]

            ] value

        ] value

    ]

                                                                        [exEnd]

  for lazy typists, a more compact interface is also provided

  (which is also easier to read):

                                                                        [exBegin]

    |querySignal|

    querySignal := QuerySignal new.

    querySignal answer:true do:[

        'nesting'.

        [

            [

                    Transcript showCR:'query says: ok'.

                ] ifFalse:[

                    Transcript showCR:'query says: no'

                ]

            ] value

        ] value

    ]

                                                                        [exEnd]

  an up-query from a deeply nested operation, for which there

  is no handler:

  (notice, this would not work with normal signals, which would raise

   another unhandled exception-exception;

   also notice the == check #raise's return value being true,

   instead of a simple ifTrue; this handles a nil-value from

   the unhandled query)

                                                                        [exBegin]

    |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

                                                                         [exEnd]

  counter-example, just to show that things would not work this way

  with regular signals:

                                                                        [exBegin]

    |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

                                                                         [exEnd]

   except, by handling the unhandled exception

   (but we think, that querySignals are easier to use and

    better document the intent):

                                                                        [exBegin]

    |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

                                                                         [exEnd]

"

! !

!QuerySignal methodsFor:'answering queries'!

answer:someAnswer do:aBlock

     "evaluate the argument, aBlock.

      If the receiver is queried during evaluation, answer with someAnswer.

      This is a wrapper for #handle:do: for lazy typists; no new functionality."

      ^ self handle:[:ex | ex proceedWith:someAnswer] do:aBlock.

      "

       |q|

       q := QuerySignal new.

       q answer:true do:[

       ]

      "

      "

       |q|

       q := QuerySignal new.

       q answer:false do:[

          q answer:true do:[

          ]

       ]

      "

! !

!QuerySignal methodsFor:'initialization'!

defaultAnswer:someValue

    "define the queries defaultAnswer to be someValue.

     This is the same as defining an appropriate handlerBlock."

    |handler|

    defaultAnswer := someValue.

    hasDefault := true.

    "/ avoid creating a fullBlock, in the most common cases

    someValue == true ifTrue:[

        handler := [:ex | ex proceedWith:true]

    ] ifFalse:[

        someValue == false ifTrue:[

            handler := [:ex | ex proceedWith:false]

        ] ifFalse:[

            someValue isNil ifTrue:[

                handler := [:ex | ex proceedWith:nil]

            ] ifFalse:[

                handler := [:ex | ex proceedWith:someValue].

        ]

    ].

    self handlerBlock:handler.

    "

    "

    "

     |sig rslt|

     sig := QuerySignal new.

     sig defaultAnswer:false.

     Transcript showCR:rslt.

    "

    "

     |sig rslt|

     sig := QuerySignal new.

     sig defaultAnswer:false.

     sig answer:true 

     do:[

     ].

     Transcript showCR:rslt.

    "

! !

!QuerySignal methodsFor:'queries'!

accepts:aSignal

    "return true, if the receiver accepts the argument, aSignal.

     (i.e. the receiver is aSignal or a parent of it). False otherwise."

    |s|

    aSignal isQuerySignal ifFalse:[^ false].

    s := aSignal.

    [s notNil] whileTrue:[

	self == s ifTrue:[^ true].

	s := s parent

    ].

    ^ false

!

isQuerySignal

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

    ^ true

    "Modified: 22.4.1996 / 13:45:10 / cg"

!

    "raise the query - return the handlers value, or the default

     value, if there is no handler.

     but we can do it faster here."

    |con|

    con := thisContext sender.

    [con notNil] whileTrue:[

        con := con findNextContextWithSelector:#doRaise 

                                            or:#handle:do:

                                            or:#handle:from:do:.

        con notNil ifTrue:[

            (con selector == #handle:do:) ifFalse:[

            ].

            (con receiver == self) ifTrue:[

                "/ found a non-busy handler ...

                "/ if its sender is a #answer context,

                "/ fetch its value quickly from it.

                con := con sender.

                con selector == #answer:do: ifFalse:[

                    con receiver == self ifFalse:[

                    ]

                ].

                ^ con argAt:1

            ]

        ]

    ].

    "/ no handler found - return the default value

    hasDefault == true ifTrue:[

    ].

    handlerBlock isNil ifTrue:[

        ^ nil

    ].

! !

!QuerySignal class methodsFor:'documentation'!

version

! !