Callback

Callback procedures

Description:

From Wikipedia: A callback, also known as a "call-after" function, is any executable code that is passed as an argument to other code; that other code is expected to call back (execute) the argument at a given time. This execution may be immediate as in a synchronous (or blocking) callback, or it might happen at a later time as in an asynchronous (or deferred) callback.

Examples:

Example where a mathematical function is an argument in plot function call:
(the invocation is implemented by calling the 'PlotF()' procedure, and the callback function is the 'Linear()', 'Sinusoidal()', or 'Quadratic()' function)

Type MathFunction As Function( ByVal x As Double ) As Double

Function Linear( ByVal x As Double ) As Double
Return x
End Function

Function Quadratic( ByVal x As Double ) As Double
Return x * x
End Function

Function Sinusoidal( ByVal x As Double ) As Double
Return Sin(x)
End Function

Sub PlotF( ByVal f As MathFunction )
PSet( -15, f(-15) )
For x As Double = -15 To 15 Step 0.1
Line -( x, f(x) )
Next
End Sub

Screen 19
Window (-15,-10)-(15,10)

PlotF( @Linear )
PlotF( @Sinusoidal )
PlotF( @Quadratic )

Sleep

The three callback processes are synchronous because each callback function is completely executed before 'PlotF()' returns to the caller code.

Similar plotting example (as above) but with a asynchronous callback. Here an artificial delay is added. A more realistic case would be if the data to be plotted is retrieved form a web-server with a response time larger than acceptable for the main loop of our program (prevent blocking behavior):

#Include "fbthread.bi"

Type MathFunction As Function( ByVal x As Double ) As Double

Sub ThreadPlot( ByVal p As Any Ptr )
Sleep 1500, 1 '' sleep added only to check the asynchronous way of the callback
Dim f As MathFunction = p
Window (-15,-10)-(15,10)
PSet( -15, f(-15) )
For x As Double = -15 To 15 Step 0.1
Line -( x, f(x) )
Next
End Sub

Function PlotF( ByVal f As MathFunction ) As String
Print "Plotting requested"
Threaddetach( ThreadCreate( @ThreadPlot, f ) )
Return "Plotting request taken into account"
End Function

Function Linear( ByVal x As Double ) As Double
Return x
End Function

Function Quadratic( ByVal x As Double ) As Double
Return x * x
End Function

Function Sinusoidal( ByVal x As Double ) As Double
Return Sin(x)
End Function

Screen 19

Print PlotF( @Linear )
Print PlotF( @Sinusoidal )
Print PlotF( @Quadratic )

'' following code added only to check the asynchronous way of callbacks
Print "Main program continues ";
For I As Integer = 1 To 15
Print ".";
Sleep 200, 1
Next I
Print
Print "Main program finished"

Sleep

The three callback processes are asynchronous because the 'PlotF()' function returns immediately, and each callback function is executed in parallel with the caller's next code, from a 'ThreadPlot()' thread created (previously) by 'PlotF()' for each of them.

Example of a callback function for comparing values in a array to be sorted by the qsort algorithm:
(the invocation is implemented by calling the 'qsort()' procedure, and the callback function is the 'CmpVarLenStr()' function)

#include "crt/stdlib.bi"

Function CmpVarLenStr CDecl( ByVal p1 As Any Ptr, ByVal p2 As Any Ptr ) As Long '' compare 2 var-len strings
Dim As String Ptr ps1 = p1
Dim As String Ptr ps2 = p2
If *ps1 < *ps2 Then
Return -1
ElseIf *ps1 > *ps2 Then
Return 1
Else
Return 0
End If
End Function

Sub PrintList( array() As String) '' print a var-len string list
For I As Integer = LBound(array) To UBound(array)
Print array(I)
Next I
Print
End Sub

Dim forename(1 To 12) As String = {"Madison", "Emily", "Hailey", "Sarah", "Kaitlyn", "Hannah", _
"Jacob", "Christopher", "Nicholas", "Michael", "Matthew", "Joshua" }

Print "LIST OF UNSORTED FORENAMES:"
PrintList( forename() )

qsort( @forename(LBound(forename)), UBound(forename) - LBound(forename) + 1, SizeOf(String), @CmpVarLenStr )

Print "LIST OF SORTED FORENAMES:"
PrintList( forename() )

Sleep

The callback process is synchronous because the callback function is completely executed (several times), (successively) called by 'qsort()', before 'qsort()' returns to the caller code.

See also:

Back to Programmer's Guide

Callback - FreeBASIC Wiki Manual | FBWiki

Callback