!define MUI_PAGE_DIRECTORY_VARIABLE does not create variable - NSIS - nsis

Question

I'm trying to create an NSIS script which copies various files to two different locations that can be specified.
I have checked the documentation and other StackOverflow entries but do not come up with a solution.
My Problem is that I want to define a second variable for a directory. In this directory sample files and projects shall be copied.
When trying to compile the NSI I get the following error/warnings:
3 warnings:
unknown variable/constant "APPDIR" detected, ignoring (C:\Users\max\Desktop\PortablePlayer\so.nsi:29)
unknown variable/constant "APPDIR" detected, ignoring (C:\Users\max\Desktop\PortablePlayer\so.nsi:33)
unknown variable/constant "APPDIR\Testfile.txt" detected, ignoring (C:\Users\max\Desktop\PortablePlayer\so.nsi:43)
Here is my script that I'm using:
!include "MUI.nsh"
Name "MyApp"
OutFile "MyApp-Installer.exe"
InstallDir "$PROGRAMFILES\My App"
; Installation Directory for the App
!insertmacro MUI_PAGE_DIRECTORY
; Installation Directory for the samples and projects
!define MUI_PAGE_HEADER_SUBTEXT "Choose your custom Samples Folder"
!define MUI_DIRECTORYPAGE_TEXT_TOP "To separate your App and your samples and projects you can choose a different folder than the installation directory"
!define MUI_PAGE_DIRECTORY_VARIABLE $APPDIR
!insertmacro MUI_PAGE_DIRECTORY
!insertmacro MUI_PAGE_INSTFILES
!insertmacro MUI_PAGE_FINISH
!insertmacro MUI_UNPAGE_CONFIRM
!insertmacro MUI_UNPAGE_INSTFILES
!insertmacro MUI_LANGUAGE "English"
Section ""
SetOutPath $INSTDIR
File Testfile.txt
SetOutPath $APPDIR
File Testfile.txt
FileOpen $0 "$DESKTOP\Hello_world.txt" w
FileWrite $0 $APPDIR
FileClose $0
WriteUninstaller "$INSTDIR\MyApp-Uninstaller.exe"
SectionEnd
Section "Uninstall"
Delete "$INSTDIR\Testfile.txt"
Delete "$INSTDIR\MyApp-Uninstaller.exe"
RMDir $INSTDIR
Delete "$APPDIR\Testfile.txt"
SectionEnd
For debugging I tried to write the value of $APPDIR into a textfile but it only writes $APPDIR instead of the value.
If I change it to $INSTDIR it prints the correct path for $INSTDIR.
How can I initialize the variable for $APPDIR?
If I try to initialize it on the top, it is there but empty, so the installer fails after succesful compilation of the script.
Running Win 7 with NSIS 2.5.1
Can anybody please shed some light on what is wrong?
Thanks!

Answer 1

The MUI_DIRECTORYPAGE_VARIABLE define tells MUI that you have a custom variable that you want to use but MUI does not create the variable, you have to do that yourself near the top of your script:
Var APPDIR
So it should look something like
Var APPDIR
!include "MUI.nsh"
!define MUI_DIRECTORYPAGE_VARIABLE $APPDIR
!insertmacro MUI_PAGE_DIRECTORY
!insertmacro MUI_PAGE_INSTFILES
!insertmacro MUI_LANGUAGE "English"
Section
DetailPrint $APPDIR
SectionEnd
Edit: The name of the define is MUI_DIRECTORYPAGE_VARIABLE not MUI_PAGE_DIRECTORY_VARIABLE like you have in your example...