How to make hook in custom module to execute only on condition

Question

How to make contributed module hook execute only if conditions are met. Those functions seem to get automatically invoked. Environment is a custom module.

examples: 1.

if ($condition) {

   function mymodule_modulename_alter ($parameters) {
     //altering code here
   }

}

2.

    if ($condition) {
           //if condition is met, i call the function included in this custom module
           mymodule_modulename_alter ($parameters)
      }

  function mymodule_modulename_alter ($parameters) {
         //but this function will get auto-invoked! I want it run only if $condition is met.
       }

Thanks

Answer 1

Let the hook be called, but put your operations(that your hook executes) inside your required condition.

function mymodule_modulename_alter($parameters) {
  if ($condition) {
    // Your code.
  }  
}

Answer 2

I don't see a need to use variable_get() or variable_set(), rather, IMHO, I'd set this up somewhat like this inside your foo.module file:

<?php

function foo_is_whatever_condition() {

   return true or false based on whatever logic you define

}

function foo_WHATEVER_alter($parameters...) {

   if (foo_is_whatever_condition()) {

      // code for when the condition is true here

   }

}

Now, if I am missing out on why or how "conditions are not known to function inside" please add some more specific details here, because, unless it is some really odd edge case, few things are ever not known or knowable :)