How to set the installation directory depending on a choice user made earlier ?
 

A user can select either installation locally or on a network.

When the user selects local, the install dir should be by default eg C:\Install.
When the user selects network, the install dir should be by defalt eg F:\Install.

When I call the Directory page however, I can't get to set the right default installation directory.

The directory page uses InstallDir and I can't change the value of that InstallDir from inside a section/function.

You can't do that. InstallDir means the suggested default directory that installer adds on directory page.
It'd change once users hit the browse button and specify another directory.

I think I got it.
There was already a topic with title : "overwrite default InstallDir"

I will use the same technique.
Sorry for asking the same question twice.

If you want to show a suggested dir on directory page different than the one specified with InstallDir then you should do something like this:

code:
Name "My Application"

OutFile test.exe

LicenseData '${NSISDIR}\License.txt'

LicenseBkColor 0xFFFFFF

InstallDir '$PROGRAMFILES\$(^name)'



!include winmessages.nsh



Page License

Page Components

Page Directory '' dir_show

Page InstFiles



Section "boo"

######

SectionEnd



Function dir_show



FindWindow $R1 "#32770" "" $HWNDPARENT

GetDlgItem $R2 $R1 '1019'

ExpandEnvStrings $R0 '%USERPROFILE%'

SendMessage $R2 ${WM_SETTEXT} 1 'STR:$R0\$(^name)'

GetDlgItem $R2 $HWNDPARENT 1

System::Call "user32::SetFocus(i R2)i"



FunctionEnd

You're able to achieve this only in Function .onInit.
However you must use messageboxes to interact with users since there is no GUI yet.
EXAMPLE:

code:
Name "My Application"

OutFile test.exe

LicenseData '${NSISDIR}\License.txt'

LicenseBkColor 0xFFFFFF

InstallDir '$PROGRAMFILES\$(^name)'



Page License

Page Components

Page Directory

Page InstFiles



Section "boo"

;;;;;;;;;

SectionEnd



Function .onInit

  StrCpy $0 0

  EnumRegKey $1 HKCU Network $0

  StrCmp $1 "" end ;there is no network drive mapped

  ;found mapped drive(s) ask user if install either on local or network

  MessageBox MB_ICONQUESTION|MB_YESNO \

  "Install on local or Network drive?$\n$\n\

  Either select Yes for local or No for network" IDYES end ;user selection is local installation



  EnumRegKey $1 HKCU Network $0

  StrCpy '$R0' '$1:'

  IntOp $0 $0 + 1

  EnumRegKey $1 HKCU Network $0

  StrCmp $1 "" done ;there is only one mapped net drive we shall use this



  StrCpy $0 0



loop: ;find all available network drives and promt user to select one

  EnumRegKey $1 HKCU Network $0

  StrCpy '$R0' '$1:'

  IntOp $0 $0 + 1

  EnumRegKey $1 HKCU Network $0

  StrCmp $1 "" last

  MessageBox MB_YESNO|MB_ICONQUESTION \

  "Install on $R0 network drive?$\n$\n\

  Select Yes to install on $R0, No to select another drive" IDYES done

  goto loop



last:

  MessageBox MB_YESNO|MB_ICONQUESTION \

  "Last network drive found $R0 install here?$\n$\n\

  Select Yes to install on $R0, No to start over" IDYES done

  StrCpy $0 0

  goto loop



done:

  StrCpy '$INSTDIR' '$R0\$(^name)'



end:

FunctionEnd