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Class: CascadingNotification


Inheritance:

   Object
   |
   +--GenericException
      |
      +--Notification
         |
         +--CascadingNotification

Package:
stx:libbasic
Category:
Kernel-Exceptions
Version:
rev: 1.3 date: 2013/08/24 12:24:42
user: cg
file: CascadingNotification.st directory: libbasic
module: stx stc-classLibrary: libbasic
Author:
Claus Gittinger

Description:


A CascadingNotification behaves much like a regular notification,
in that if no handler is present, execution proceeds after the raise.
However, iff a handler is present, all other outer handlers are still called,
if the handles simply fall through
I.e. the default behavior of a handler is not to proceed or return, but to reject,
iff there is another outer handler. 
For the very outermost handler, the default is to proceed.

This is used to allow for notifier-hooks to be stacked upon each other.
For now, the one and only use is the aboutToOpenBox notifier, which can be used
as a hook to add additional widget components to a standard dialog box.


Related information:

    Exception
    Query
    Notification

Class protocol:

initialization
o  initialize


Instance protocol:

default actions
o  doCallHandler: aHandlerBlock
call the handler proper - if the handler falls through, reject and try more handlers.
- an extra method is needed to have a raise-marked context around.
(see implementation of #reject and #proceed).


Examples:


an up-notification from a deeply nested operation to a higher level. Notice, that in contrast to a regular Notification, all outer handlers are called:
  |zero|

  zero := 0.
  CascadingNotification handle:[:n |
      Transcript showCR:'h1: ' , n description.
      n proceedWith:1.    
  ] do:[
      CascadingNotification handle:[:n |
          Transcript showCR:'h2: ' , n description.
      ] do:[
          CascadingNotification handle:[:n |
              Transcript showCR:'h3: ' , n description.
          ] do:[
              'nesting'.
              [
                  [
                      Error handle:[:ex |
                          Transcript showCR:'some error: ' , ex errorString.
                          ex proceed
                      ] do:[
                          [
                              |answer|

                              1 // zero.  'an error which is caught in the handler'.
                              answer := CascadingNotification notify:'hello world'.
                              Transcript show:'after notification 1: '; showCR:answer.
                          ] value
                      ]
                  ] value
              ] value.
              Transcript showCR:'after notification 2'.
          ].
          Transcript showCR:'after notification 3'.
      ].
      Transcript showCR:'after notification 4'.
  ].
  Transcript showCR:'after notification 5'.
however, if any handler proceeds or returns, this is not the case:
  |zero|

  zero := 0.
  CascadingNotification handle:[:n |
      Transcript showCR:'h1: ' , n description.
  ] do:[
      CascadingNotification handle:[:n |
          Transcript showCR:'h2: ' , n description.
          n proceedWith:2.    
      ] do:[
          CascadingNotification handle:[:n |
              Transcript showCR:'h3: ' , n description.
          ] do:[
              'nesting'.
              [
                  [
                      Error handle:[:ex |
                          Transcript showCR:'some error: ' , ex errorString.
                          ex proceed
                      ] do:[
                          [
                              |answer|

                              1 // zero.  'an error which is caught in the handler'.
                              answer := CascadingNotification notify:'hello world'.
                              Transcript show:'after notification 1: '; showCR:answer.
                          ] value
                      ]
                  ] value
              ] value.
              Transcript showCR:'after notification 2'.
          ].
          Transcript showCR:'after notification 3'.
      ].
      Transcript showCR:'after notification 4'.
  ].
  Transcript showCR:'after notification 5'.
however, if any handler proceeds or returns, this is not the case:
  |zero|

  zero := 0.
  CascadingNotification handle:[:n |
      Transcript showCR:'h1: ' , n description.
  ] do:[
      CascadingNotification handle:[:n |
          Transcript showCR:'h2: ' , n description.
          n return.    
      ] do:[
          CascadingNotification handle:[:n |
              Transcript showCR:'h3: ' , n description.
          ] do:[
              'nesting'.
              [
                  [
                      Error handle:[:ex |
                          Transcript showCR:'some error: ' , ex errorString.
                          ex proceed
                      ] do:[
                          [
                              |answer|

                              1 // zero.  'an error which is caught in the handler'.
                              answer := CascadingNotification notify:'hello world'.
                              Transcript show:'after notification 1: '; showCR:answer.
                          ] value
                      ]
                  ] value
              ] value.
              Transcript showCR:'after notification 2'.
          ].
          Transcript showCR:'after notification 3'.
      ].
      Transcript showCR:'after notification 4'.
  ].
  Transcript showCR:'after notification 5'.


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