Sstrhsrtj

I have no idea, why this test fails and how to get it to work:

documentclass{article}



usepackage{filecontents}

begin{filecontents}{myTestSty.sty}

NeedsTeXFormat{LaTeX2e}[1994/06/01]

ProvidesPackage{myTestSty}[2018/11/29 v. 1.0 myTestSty]



RequirePackage{kvoptions}

SetupKeyvalOptions{family=@myTestSty,prefix=@myTestSty@}



DeclareStringOption{myKey}



defmyKey{@myTestSty@myKey}

ProcessKeyvalOptions*

end{filecontents}



usepackage{etoolbox}

usepackage[%

    %myKey={Hello world!},

    myKey={} %<-- blank?

    ]{myTestSty}



begin{document}



    myKey: --{myKey}--



    myKey: ifblank{myKey}{blank}{not-blank}

end{document}

Three errors:

1. you have to expand @myTestSty@myKey when defining myKey;


2. you have to expand myKey to test whether its replacement text is empty;


3. the definition of myKey should go after processing the options.

documentclass{article}



usepackage{filecontents}

begin{filecontents}{myTestSty.sty}

NeedsTeXFormat{LaTeX2e}[1994/06/01]

ProvidesPackage{myTestSty}[2018/11/29 v. 1.0 myTestSty]



RequirePackage{kvoptions}

SetupKeyvalOptions{family=@myTestSty,prefix=@myTestSty@}



DeclareStringOption{myKey}



ProcessKeyvalOptions*



edefmyKey{expandonce{@myTestSty@myKey}} % <--- expand



end{filecontents}



usepackage{etoolbox}

usepackage[%

    %myKey={Hello world!},

    myKey={} %<-- blank?

    ]{myTestSty}



begin{document}



myKey: X{myKey}X



myKey: expandafterifblankexpandafter{myKey}{blank}{not-blank}



end{document}

You can shorten the test by defining

newcommand{ifblanke}[1]{%

  expandafterifblankexpandafter{#1}%

}

and call

ifblanke{myKey}{blank}{not~blank}

A different approach with l3keys2e:

documentclass{article}



usepackage{filecontents}



begin{filecontents}{myTestSty.sty}

NeedsTeXFormat{LaTeX2e}[1994/06/01]

ProvidesPackage{myTestSty}[2018/11/29 v. 1.0 myTestSty]



RequirePackage{expl3,l3keys2e,xparse}



ExplSyntaxOn



keys_define:nn { mytest/main }

 {

  myKey .tl_set:N = myKey,

 }



ProcessKeysPackageOptions { mytest/main }



NewExpandableDocumentCommand{IfBlankTF}{smmm}

 {

  IfBooleanTF{#1}

   {

    tl_if_blank:VTF #2 { #3 } { #4 }

   }

   {

    tl_if_blank:nTF #2 { #3 } { #4 }

   }

 }



ExplSyntaxOff

end{filecontents}



usepackage[%

    %myKey={Hello world!},

    myKey={} %<-- blank?

    ]{myTestSty}



begin{document}



myKey: X{myKey}X



myKey: IfBlankTF*{myKey}{blank}{not-blank}



end{document}