How to received proc return data in fileevent - tcl

Question

I am calling a proc through fileevent. that proc returns a line od data.
how to receive this data?
the following code I have written to receive data from pipe when ever data is available. I dont want to block by using direct gets.
proc GetData1 { chan } {
if {[gets $chan line] >= 0} {
return $line
}
}
proc ReadIO {chan {timeout 2000} } {
set x 0
after $timeout {set x 1}
fconfigure $chan -blocking 0
fileevent $chan readable [ list GetData1 $chan ]
after cancel set x 3
vwait x
# Do something based on how the vwait finished...
switch $x {
1 { puts "Time out" }
2 { puts "Got Data" }
3 { puts "App Cancel" }
default { puts "Time out2 x=$x" }
}
# How to return data from here which is returned from GetData1
}
ReadIO $io 5000
# ** How to get data here which is returned from GetData1 ? **

Answer 1

There are probably as many ways of doing this as there are Tcl programmers. Essentially, you shouldn't use return to pass the data back from your fileevent handler as it isn't called in the usual way so you can get at what it returns.
Here are a few possible approaches.
Disclaimer None of these is tested, and I'm prone to typing mistakes, so treat with a little care!
1) Get your fileevent handler to write to a global veriable:
proc GetData1 {chan} {
if {[gets $chan line]} >= 0} {
append ::globalLine $line \n
}
}
.
.
.
ReadIO $io 5000
# ** The input line is in globalLine in the global namespace **
2) Pass the name of a global variable to your fileevent handler, and save the data there
proc GetData2 {chan varByName} {
if {[gets $chan line]} >= 0} {
upvar #0 $varByName var
append var $line \n
}
}
fileevent $chan readable [list GetData1 $chan inputFromChan]
.
.
.
ReadIO $chan 5000
# ** The input line is in ::inputFromChan **
A good choice for the variable here might be an array indexed by $chan, e.g. fileevent $chan readable [list GetDetail input($chan)]
3) Define some kind of class to look after your channels that stashes the data away internally and has a member function to return it
oo::class create fileeventHandler {
variable m_buffer m_chan
constructor {chan} {
set m_chan $chan
fileevent $m_chan readable [list [self object] handle]
set m_buffer {}
}
method data {} {
return $m_buffer
}
method handle {} {
if {[gets $m_chan line]} >= 0 {
append m_buffer $line \n
}
}
}
.
.
.
set handlers($chan) [fileeventHandler new $chan]; # Save object address for later use
ReadIO $io 5000
# Access the input via [handlers($chan) data]