1.  Tree Trace/Notify

A tree trace is used primarily to get control during accesses to tree key values. A notify gets control for other types tree operations. Following are some examples.

1.1  Readonly keys.

The following example can be used to make a field read-only.

proc Readonly {t node key ops} {
    if {[string match *c* $ops]} return; # CREATE
    $t set $node $key [$t oldvalue]
    return -code error "attempt to modify read only: $key"
}

set t [tree create]
$t trace create all a w Readonly
$t insert end -data {a 1 b 2}

catch {$t update 1 a 9} ;# kicks an error.
puts [$t get 1 a]       ;# "1"

Note: if the trace is set after node data is initialized, the first line in Readonly can be ommited.

1.2  Tag Trace

Tag uniqueness is a requirement for single-node commands such as get. We can detect duplicate tags with a t trace, forcing certain tag patterns to be unique. eg,

 proc ::TagTrace {t id key op} {
    error "duplicate tag: $key"
 }
 $t trace create all .* t ::TagTrace

This trace generates an error for any duplicate tag begining with a period. The check is very efficient, since the t trace fires only when an exception occurs (ie. multiple nodes try to share a tag).

1.3  Unique Labels Without Tags

Reliable label addressing requires that duplicate labels be avoided. One way to prevent them is with a notifier such as:

 proc DupLab {t id ops} {
     set l [$t label $id]
     if {[string match {[0-9]*} $l]} {
          return -code error "numeric label"
     }
     set labs [$t children -labels $id->parentnode]
     if {[llength [lsearch -all -exact $labs $l]]>1} {
          return -code error "label already exists: $l"
     }
 }
 $t notifier create -create DupLab $t

1.4  Demand Loading Data

Data can be demand loaded into empty nodes upon access with a -get notify. This is invoked whenever a node with no keys is accessed via get or with. The notify call occurs before the read, eg.

 set t [tree create]
 $t insert end
 $t insert end
 $t insert end
 proc FillMe {t id args} {
 puts "FF [info level 0]"
   $t set $id A 1 B 2
 }
 $t notify create -get FillMe $t
 puts [$t get 1]
 puts [$t get 2 B]
 $t with p all {
   parray p
 }

Also see TreeView demand loading.

© 2008 Peter MacDonald