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.

Example

Example where a mathematical function is an argument in plot function call:
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
        

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
        

Example of a callback function for comparing values in a array to be sorted by the qsort algorithm:
#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
        

See also