CODEWITHSUNDEEP
tcl
Dcode
Dcode

Reputation: 223

perform certain action when variable changes in TCL

In TCL do we have any mechanism that will keep polling for the variable change and perform certain action after that.

I have read about vwait but it is pausing the script. I want the script to be running and in between if the variable value changes, perform certain action.

Kind of asynchronous mode of vwait.

Upvotes: 2

Views: 1140

Answers (2)

stark
stark

Reputation: 2256

You could also use a recursive procedure to keep polling for the current value of a variable at a particular interval and bow out of the recursion once a specific condition for the variable is met.

For Example :

set x 1
proc CheckVariableValue {
    global x
    if { $x >= 5 } {
        puts "end"
        return 1;
    }
    else{
        incr x
        puts $x
        after 1000 CheckVariableValue
    }
}
CheckVariableValue

Upvotes: 0

Donal Fellows
Donal Fellows

Reputation: 137567

You can attach a trace to a variable so that you can do something immediately whenever the variable is changed (or, depending on flags, read from or deleted). Try out this example:

set abc 123
proc exampleCallback args {
    global abc
    puts "The variable abc is now $abc"
}
trace add variable abc write exampleCallback
incr abc
incr abc
incr abc

It's possible to trace local variables, but not recommended. Also, internally, the vwait command sets a trace that just trips a flag when the variable is written to; that flag signals the wait to end when the event loop is returned to. It just happens that that trace is set using Tcl's C API, not its script-level API…

Upvotes: 5

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