NSIS write to a file on APPDATA inside a subdir - nsis

Question

I'm trying to append 1 line of text into a file on the $APPDATA folder which is inside of a folder that's generated randomly, so I don't know it's full path like:
C:\Users\MyUser\AppData\Roaming\MyApp\RANDOM_CRAP\config.json
While RANDOM_CRAP looks like some random string for a folder, like G4F6Hh3L.
What are my options here? Do I need to use either Search For a File or Search for a File or Directory (Alternative) ? It's a given that the only subfolder of MyApp folder is the RANDOM_CRAP folder, that contains the file I want to edit.
If there's no other way to access this file without searching for it, I've tried doing so but couldn't get this to work. (I'm very new to NSIS)
This is what I've tried (With the alternative approach):
Push "config.json"
Push "$APPDATA"
Push $0
GetFunctionAddress $0 "myCallback"
Exch $0
Push "1" ; include subfolders because my desired file is in the random folder
Push "0" ; no need the . option
Call SearchFile
Than I've copied the SearchFile code from this post and put a callback:
Function myCallback
Exch 3
Pop $R4
MessageBox MB_OK "Callback executing!"
MessageBox MB_OK "File is at : $R4"
FunctionEnd
I know that SearchFile is running (I've put a MessageBox inside) but myCallback isn't seemed to be called.
Many thanks.

Answer 1

If you are looking for a known file and only one directory in the path is unknown then you can probably just do a basic FindFirst search:
Section
; Create "random" folders:
CreateDirectory "$temp\MyApp\foo"
System::Call kernel32::GetTickCount()i.r1 ; random enough
CreateDirectory "$temp\MyApp\bar$1"
FileOpen $0 "$temp\MyApp\bar$1\config.json" a
FileWrite $0 '{bogus:"data"}$\n'
FileClose $0
CreateDirectory "$temp\MyApp\baz"
!include LogicLib.nsh
; Do the actual search:
StrCpy $9 "$temp\MyApp" ; The folder we are going to search in
FindFirst $0 $1 "$temp\MyApp\*"
loop:
StrCmp $1 "" done
${If} ${FileExists} "$9\$1\config.json"
DetailPrint "Found: $9\$1\config.json"
${EndIf}
FindNext $0 $1
Goto loop
done:
FindClose $0
SectionEnd