How to set registry key (environment variable) with value from [Code] section of Inno Setup?

Question

I'm creating an installer with Inno Setup, and I need to set the Environment Variable for the user if they didn't set yet.

What I exactly want to achieve is as follows :

1. Detect whether the system has the Environment Variable called "JAVA_HOME" or not.

2. If it already has, then skip the remaining steps. If not, then I will show a page to prompt the user to input the path where they install the JDK, and set to the value of the Environment Variable.

And my problem is : How would I set the Environment Variable according to the value input by user? I've searched some articles, and the only way I found to set Environment Variable is like this

[Registry]
; set PATH
Root: HKLM; Subkey: "Environment"; ValueType:string; ValueName:"VARIABLE_NAME"; \
    ValueData:"new_value" ; Flags: preservestringtype ;

But obviously, that couldn't achieve what I want to do, because the value must be written "Before" the installer been built. So could any one has any device? Thanks!

Answer 1

You do not need a code to set the registry key (= environment variable). You just need to get the registry key value (= environment variable value) from the code.

You are looking for a scripted constant :

[Registry]
Root: HKLM; Subkey: "Environment"; ValueType: string; ValueName: "VARIABLE_NAME"; \
    ValueData: "{code:GetJDKPath}"; Flags: preservestringtype;

[Code]

var
  { A global variable that contains the path provided by the user in step 2 }
  JDKPath: string;

function GetJDKPath(Param: string): string;
begin
  Result := JDKPath;
end;